Metric VS: Imperial Measurement Systems

I’m curious as how do PCB designers route metric pitch BGA’s on a mil grid system or gridless (which consumes memory and CPU)? I can’t figure why anyone would use a mil grid system for any PCB CAD library construction or PCB design layout when all the SMT component manufacturers are only producing metric pin pitch packages. This is where we note that all chip resistors and capacitors are dimensioned using whole metric numbers. So by design all of them are considered metric based components. i.e.: 1206 = 0.125″ x 0.062″ or 3216 = 3.2mm x 1.6mm for the same component. Which is easier to understand? To me it’s interesting why some PCB designers still use the Imperial measurement system. I’d like to hear from designers and engineers who use the technology combination of metric pin pitch components and mil based part placement and trace sizes & routing grids. How do you do it?

My big secret was to teach all the mechanical engineers and EE engineers the metric measurement system by providing equivalent charts. Even purchasing had to have a metric to Imperial chart for chip components because the BOM had all metric land pattern names but the buyers had to have the inch equivalent names to place an order.

So we had to do for Chips. Notice that 0603 and 0402 appear in both columns –

Metric = Inch

3216 = 1206

2012 = 0805

1608 = 0603

1005 = 0402

0603 = 0201

0402 = 01005

Here is what I’m talking about. This is a table from a component manufacturer that lists the dimensions of a series of resistors in hard metric but all of the component names in the far left column are Imperial units (Type Inches). i.e. the first part is a metric 0402 (0.4mm x 0.2mm) but Panasonic calls it a 01005 (the inch equivalent 0.4mm = 0.0015748″ x 0.2mm = 0.007874). Why doesn’t Panasonic call it for what it really is, 0402?

After successfully transitioned all the engineers then the PCB design process got really easy, faster and simpler. Then when I give the PCB manufacturer all my nice clean metric drawings and metric Gerber and Drill data the first thing they do is convert all units to Imperial to panelize and CAM the job with their mil based DRC rules. Wow, it took longer to CAM my job because of the translation.

Inch Names but Metric Dimensions

Inch Names but Metric Dimensions

When I dig deeper to find out “Why do they do that”, I discover that all the materials like core, copper and prepreg all come in mil based thicknesses and shapes. So that’s where the buck stops! I mean that if overnight all the material providers only produced metric thickness and metric sizes that all of the PCB fabrication shops would be doing metric based manufacturing. The next thing you know they’d be recommending all their customers to produce metric based Gerber and drill data for their new machines made in Europe. (The reason why they’re made in Europe is because there’s no way in this lifetime that any USA manufacturer is going to create a metric based machine. Maybe in another lifetime…) and I digress.

When the PCB fabrication shop starts recommending metric units preferred to their customers that’s when true electronic product development automation will really kick in and maybe we’ll start creating faster, better, more accurate, cheaper products or rather products that today cost $100,000 will only cost $1000. I hope to see the revelation in my lifetime where all design units are the same regardless if it’s Imperial or metric, just pick one and make life easy. 99 nations voted for metric. USA voted to gradually adopt with full conversion by 2015. At least that’s the 1985 Omnibus Act signed by Ronald Regan defines to extend Jimmy Carter’s 1975 Metric Conversion Act which was to convert USA in 10 years and adds another 30 years for the gradual metric conversion one industry at a time. Note that the EU Metric Directive went into effect January 1, 2010 for all weights & measures throughout Europe. The worldwide PCB design industry is 50% converted on the front end and we’re waiting on the back end. The PCB component manufacturers have semi successfully converted to metric units. At least they dimension everything in metric units but some still refer to the Imperial (Inch) name to order the component (like the Panasonic chart above). Very confusing!

Come on PCB suppliers. The electronics industry needs all of you to transition to hard metric to complete the entire loop in the PCB design engineering and manufacturing industry. You are directly responsible for holding the electronics industry back from achieving the highest productivity levels obtainable and standardizing on a single unit system. I do not think the PCB material suppliers have a clue on how much of an impact they have on every electronic product being developed today. I beg you; please make our job easier and put an end to the dual measurement system chaos that EE engineers, mechanical engineers and PCB designers have to deal with every day until you transition. It’s time to stop using the Imperial unit system in the electronics industry and use one measurement system. Every Standards organization in the world has referred to the metric unit system as the “vastly superior” alternative. So why isn’t everyone listening?

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Commented on July 8, 2010 at 11:46 pm
By Robin Bornoff

Despite the US’s gradual adoption of SI it is in the less than enviable company of Liberia and Burma as the only countries not to have adopted it as standard. A bit farcical really.

Commented on July 13, 2010 at 7:08 pm
By Keith Ackermann

Metric vs. imperial…

I can only speak for myself but the resistance to metric… or more accurately, the appeal of imperial was ingrained years ago. I intuitively knew how big 0.025 inches was, but not 0.635 millimeters. It sounds stupid, but that’s the way it was.

One thing I did to solve this in my system was to allow a suffix on the end of numbers, so the system might be set to display in metric, but it would allow 0.025in to be entered, but that value would be converted to internal units and redisplayed as 0.635mm.

In fact, all numbers could be entered as expressions.

The internal units were 1/10,000,000th of an inch which made the irrational rounding insignificant over the entire physical domain.

Commented on July 13, 2010 at 7:46 pm
By Tom Hausherr


Did you look at the component dimension numbers in the Panasonic Chip Resistor chart?

Most of the values are rounded off to 0.1mm increments and the only variances are 0.05mm which is the global Universal Grid System increments for every day PCB layout in 2010. Why doesn’t Panasonic provide alternate Imperial units in their latest component datasheets? You can’t live in the past any longer. The metric unit system is calling on all members of the electronic industry to join together to form a single standard that will greatly simplify the processes for everyone in the supply chain.

I guarantee that if you did 5 PCB layouts using a metric PCB library and a metric placement and route grid system that you would never go back to Imperial units. I cannot say the same for metric only people (try the Imperial unit system, you’ll like it). There’s no turning back once you learn metric units. Metric units are such a superior measurement system that I can’t figure out why some people are still lingering in the Imperial world. I just don’t get it and I wish for anyone who has converted to metric to please speak up and give us your testimony.

Commented on July 13, 2010 at 9:58 pm
By Keith Ackermann

Oh, I agree. I eventually got used to thinking in both terms.

My point was that I did my best to accomodate both units.

There are terrible inconsistencies even within the same part vendor’s datasheets. There are some terrible parts out there in general.

I’ll speak up for metric right now and say I love it. How can you not?

The question is, why the resistance to change? I think it’s because some people have imprinted on a certain unit, and unless they have to use an alternative unit constantly, they find the mapping difficult.

This whole business should be run as a dictatorship. Even writing code… I’ve wasted so much time porting stuff at the margin that I don’t even want to think about it.

Commented on July 13, 2010 at 10:03 pm
By Keith Ackermann

Also, you touched upon PCB fab as a sorce of imperial usage, and I can tell that is very much true. I would guess that most of the domestic (US) manufacturers you call to ask about minimum geometries will answer in mils… maybe inches if you are a bit luckier.

I’m just glad there are not 1200 mils in an inch 🙂

Commented on July 13, 2010 at 10:09 pm
By Keith Ackermann

1000 mils to an inch
12 inches to a foot
3 feet to a yard
1760 yards to a mile

Such a smooth progression. Such nice round numbers for those unencumbered by the confines of sanity.

Commented on July 14, 2010 at 8:17 am
By Tom Hausherr

Most Americans think that our involvement with metric measurement is relatively new. In fact, the United States has been increasing its use of metric units for many years, and the pace has accelerated in the past three decades. In the early 1800’s the U.S. Coast and Geodetic Survey (the government’s surveying and map-making agency) used meter and kilogram standards brought from France. In 1866, Congress authorized the use of the metric system in this country and supplied each state with a set of standard metric weights and measures.

In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Treaty of the Meter. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Treaty of the Meter, also know as the “Metric Convention,” established the International Bureau of Weights and Measures (BIPM) in Sèvres, France, to provide standards of measurement for worldwide use.

In 1893, metric standards, developed through international cooperation under the auspices of BIPM, were adopted as the fundamental standards for length and mass in the United States. Our customary measurements — the foot, pound, quart, etc. — have been defined in relation to the meter and the kilogram ever since. The General Conference of Weights and Measures, the governing body that has overall responsibility for the metric system, and which is made up of the signatory nations to the Treaty of the Meter, approved an updated version of the metric system in 1960. This modern system is called Le Système International d’Unités or the International System of Units, abbreviated SI.

The United Kingdom, began a transition to the metric system in 1965 to more fully mesh its business and trade practices with those of the European Common Market. The conversion of the United Kingdom and the Commonwealth nations to SI created a new sense of urgency regarding the use of metric units in the United States.

In 1968, Congress authorized a three-year study of systems of measurement in the U.S., with particular emphasis on the feasibility of adopting SI. The detailed U.S. Metric Study was conducted by the Department of Commerce. A 45-member advisory panel consulted with and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials.

The final report of the study, “A Metric America: A Decision Whose Time Has Come,” concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement. The study found that measurement in the United States was already based on metric units in many areas and that it was becoming more so every day. The majority of study participants believed that conversion to the metric system was in the best interests of the Nation, particularly in view of the importance of foreign trade and the increasing influence of technology in American life.

The study recommended that the United States implement a carefully planned transition to predominant use of the metric system over a ten-year period. Congress passed the Metric Conversion Act of 1975 “to coordinate and plan the increasing use of the metric system in the United States.” The Act, however, did not require a ten-year conversion period. A process of voluntary conversion was initiated, and the U.S. Metric Board was established. The Board was charged with “devising and carrying out a broad program of planning, coordination, and public education, consistent with other national policy and interests, with the aim of implementing the policy set forth in this Act.” The efforts of the Metric Board were largely ignored by the American public, and, in 1981, the Board reported to Congress that it lacked the clear Congressional mandate necessary to bring about national conversion. Due to this apparent ineffectiveness, and in an effort to reduce Federal spending, the Metric Board was disestablished in the fall of 1982.

The Board’s demise increased doubts about the United States’ commitment to metrication. Public and private sector metric transition slowed at the same time that the very reasons for the United States to adopt the metric system — the increasing competitiveness of other nations and the demands of global marketplaces — made completing the conversion even more important.

Congress, recognizing the necessity of the United States’ conformance with international standards for trade, included new encouragement for U.S. industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designates the metric system as the Preferred system of weights and measures for United States trade and commerce.” The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement.

Federal agencies were required by this legislation, with certain exceptions, to use the metric system in their procurement, grants and other business-related activities by the end of 1992. While not mandating metric use in the private sector, the Federal Government has sought to serve as a catalyst in the metric conversion of the country’s trade, industry, and commerce.

The current effort toward national metrication is based on the conclusion that industrial and commercial productivity, mathematics and science education, and the competitiveness of American products and services in world markets, will be enhanced by completing the change to the metric system of units. Failure to complete the change will increasingly handicap the Nation’s industry and economy.

Commented on July 14, 2010 at 10:28 am
By Keith Ackermann

It’s obvious you are very passionate about the subject, and that will help your goal. That same passion can be a source of frustration when others don’t “get it”.

One can start brushing up against irrational resistance and, in this case, even nationalistic pride that can blind. I remember the push in the 70’s was greeted with “we don’t need no stinking metric…” or some such thing.

There are many things that are killing us right now. We are being beat over the head with the free market mantra even as we see companies such as Applied Materials moving to China to get closer to their customers. That, of course, is a byproduct of labor arbitrage that sent their customers overseas, as if labor arbitrage is trade.

TSMC has a low-key service that encourages companies here to fire their engineering departments and use them for all phases of engineering because they know the process, and they have the cheap labor – they outsource it themselves.

I’ve got successful products in the field, and my technology is everywhere, but my old company only dribbles me work, and I can’t even get anyone to look at what is certainly the fastest autorouter in the world, but then again, I haven’t tried overseas yet.

Commented on July 16, 2010 at 9:44 am
By Tom Hausherr

A Metric America:
“A Decision Whose Time Has Come” – For Real

The Metric Conversion Act of 1975 (amended in 1988) and a 1991 Presidential Executive Order provide both the rationale and the mandate for a transition to the use of metric units. Federal agencies are developing and implementing metric transition plans, cooperating on mutual concerns, and working with industry and user groups to establish realistic schedules for change.

Metric; metrication; metric system; metric transition

Dr. Gary P. Carver is chief of the Metric Program at the National Institute of Standards and Technology in Gaithersburg, MD. Prior to assuming responsibility for the Commerce Department’s leading role in federal agency metrication, Dr. Carver was a research physicist in integrated circuit technology and a manager in the semiconductor and manufacturing engineering areas at NIST His goal is to successfully eliminate the need for his office.

Thirty-seven years ago, a National Bureau of Standards1 report to Congress, A Metric America: A Decision Whose Time Has Come, described the United States as an island in a metric world. The report’s recommendations contributed to passage of the 1975 Metric Conversion Act. This Act raised the expectation of a 10-metric system. However, the Act lacked a clearly stated objective and a timetable for implementation. Possibly as a result, the voluntary metric transition process eventually lost momentum.

In 1988, the growing influence of the metric system of units as an international standard and the increasing competitive importance of metric specifications for products in international commerce caused Congress to respond by including “metric usage” provisions in the Omnibus Trade and Competitiveness Act. The Omnibus Trade Act amendments strengthened the Metric Conversion Act of 1975. The amendments make each federal agency responsible for implementing metric usage in grants, contracts and other business-related activities, to the extent economically feasible, by the end of fiscal year (FY) 1992. However, no statutory provision was made for leadership and coordination of the overall effort.

On July 25, 1991, President Bush acted to fill the federal metric transition leadership void by issuing Executive Order 12770, “Metric Usage in Federal Government Programs.” It gives specific direction and new management authority to the Secretary of Commerce to lead and coordinate implementation of the metric-usage provisions of the Omnibus Trade Act. This responsibility was delegated to the Under Secretary for Technology, with staff support to be provided by the Office of Metric Programs at the Department of Commerce. The Department of Commerce has long been concerned with the technical aspects of metric usage through NIST’s role as the nation’s science and engineering laboratory for measurement technology and research on standards. Since 1901, NIST has played a major role in the evolution of a national measurement system policy by providing the measurements, calibrations, data, and quality assurance that are vital to U.S. commerce and industry. NIST also provides technical support to the National Conference on Weights and Measures, an organization of state, county, and city weights and measures enforcement officials and associated business and consumer representatives.

A Rationale and a Mandate
The amended Metric Conversion Act of 1975 and the 1991 Executive Order provide both the rationale and the mandate for a transition to the use of metric units. The rationale is the need to remove a trade impediment to U.S. products, as well as to improve our efficiency and competitive edge, since the modern metric system is now the international standard of measurement. The metric system, for purposes of international trade, is more than just the International System of Units (SI). It includes the product standards and preferred sizes that are accepted by industries and governments throughout the world. “World class products” must be built to metric specifications to be competitive in the international marketplace.

The mandates in the law and Executive Order call for the Federal Government to use the metric system in all of its business-related activities, unless it is not economically feasible or is likely to cause significant inefficiencies or loss of markets to U.S. firms. It is intended that the Federal Government set an example and use its influence to catalyze a transition to the metric system by U.S. industry. The Federal Government uses measurements in many ways that influence business, including regulation, data collection, publishing, and other services. The Federal Government also is the largest customer of U.S. industry. By offering to buy metric products and services, government can help industry make the transition to the use of metric units of measurement.

In addition, by requesting metric products, the government can demonstrate its commitment to the metric system of measurement

A Special Year
The metric program is taking an evolutionary path to the statutory goals. The federal agencies are developing and implementing transition plans, cooperating on mutual concerns, and working with industry and user groups to establish realistic schedules for change.

1992 was a special year in the implementation of the mandate. The law requires that “each federal agency, by a date certain and to the extent economically feasible by the end of FY 1992, use the metric system of measurement in its procurements, grants, and other business-related activities…” The Executive Order requires that agencies provide to the Secretary of Commerce, by June 30, 1992, “an assessment of agency progress and problems, together with recommendations for steps to assure successful implementation of the Metric Conversion Act.” The Executive Order also requires in 1992, as part of the annual report to the President by the Secretary of Commerce, “recommendation which the Secretary may have for additional measures, including proposed legislation, needed to achieve the full economic benefits of metric usage.”

This year is also special because more significant progress is being made by federal agencies in metric usage than has been made in any previous year. This will be viewed as a watershed year in the federal metrication process. In 1990 a report on the federal metric program by the Government Accounting Office was critical of the slow pace of progress. Then, in 1991, a Congressional Research Service report documented a low level of compliance with the metric usage mandate by the federal agencies. A report on federal agency plans and the status of their execution that will be issued this year by NIST, however, provides evidence that federal agencies’ metric transition progress is gaining great momentum.

Commented on July 27, 2010 at 8:32 pm
By Tom Hausherr

Proper Metric Etiquette. What PCB Designers need to know about the metric measurement system.

Metric is Not a Unit of Measure. Metric is a term that describes a measurement system. You design in millimeters or microns. The proper terminology to describe your working units when using the metric measurement system is millimeters or microns, NOT Metric. You do not refer to your current working units as “Imperial”. Manufacturers, CAD Vendors, and PCB Designers refer to it as Inches or Mils, not English or Imperial.

Designing PCB’s in Millimeters. Millimeters allow finer (and greater) granularity in the grid system to optimize the board real-estate, placement, via and routing grids. This will be very important in future PCB RF Micro-technology. Board impedance measurements should be done in Microns, not Ounces of Copper and Mil core/prepreg dielectric. Everything should be referred to in Microns to achieve the most accurate impedance results. Unfortunately, manufacturers are directly responsible for holding back the progress of metrication. When they make the transition then metrication of product development and the elimination of the English system will happen.

Designing PCB’s in Millimeters –
Every element in a PCB design should reside on a 0.05mm grid.

Optimal Via Padstacks:
0.1 Trace Width / 0.1 Route Grid
Pad Size: 0.5
Hole Size: 0.25
Anti-pad: 0.7 – Avoid Trace / Anti-pad Overlap

0.125 Trace Width / 0.25 Route Grid
Pad Size: 0.6
Hole Size: 0.3
Anti-pad: 0.8 – Avoid Trace / Anti-pad Overlap

0.15 Trace Width / 0.1 Route Grid
Pad Size: 0.55
Hole Size: 0.25
Anti-pad: 0.75 – Avoid Trace / Anti-pad Overlap

Metric trace width rules are in increments of 0.05mm with one exception 0.125mm (5 mils).
0.25mm = 10 mils
0.20mm = 8 mils
0.15mm = 6 mils
0.125mm = 5 mils
0.10mm = 4 mils
0.075mm = 3 mils
0.05mm = 2 mils

Trace Routing Grid System:
The ultimate metric routing grid is 0.05mm.

Reference Designators and Text Grid System:
0.1mm is the common grid for placing Reference Designators and Text, but 0.05mm is used for tight spaces

Copper Pour and Plane Fill Grid System:
0.1mm is the common grid for Copper Pour outlines and snap grid, but 0.05mm can be used for high density part placement and trace routing.

Mounting Hole Size and Placement Grid System:
All mounting-hole padstacks are in increments of 0.05mm and the placement grid is in 0.05mm increments.

The Universal PCB Design Grid System is based on the 0.05mm unit. All shapes and sizes for every aspect of the PCB layout should be in increments of 0.05mm. Transitioning to the metric system for PCB layout is necessary to achieve electronic product development automation.

Part Placement Grid System:
If you build your CAD library parts in millimeter units, the best placement grid rule is to use numbers that can be evenly divided into 1mm and are one place to the right of the decimal point. Optimized metric placement grids include: 1mm, 0.5mm, 0.2mm and 0.1mm. To achieve the best results, no other part placement grids should be used unless absolutely necessary like for a fixed connector or switch on the PCB edge.

Via Size and Fanout Grid System:
Via padstack sizes are in increments of 0.05mm. This includes all via-hole sizes.
The best via padstack for overall trace routing is – 0.5mm pad, 0.25mm hole, 0.7mm plane anti-pad.

If every via in the PCB design was placed on a 1mm grid system, the traces can be routed across the design layout without unnecessary bends.

Commented on August 19, 2010 at 1:52 pm
By Larry Maier

All of the preceding comments have a valid, if biased opinion of the metric vs imperial system. When working in a military based industry most of the components we use are based in imperial units, all our mechanical drawings are in imperial units. A large part of our business is to retrofit or upgrade existing military hardware. All built 20-30 years ago. IE.. all imperial units. When designing a replacement board it is 0.062 material, not 1.5748mm Some of us just can’t change yet. Anyone ever mention nuts, bolts and screws, glaringly overlooked. I remeber using Wentworth bolts and nuts on some of my older sports cars. Now thats a mess. whole new tool box which was hard to find.

Commented on November 16, 2011 at 9:16 am
By Chris Elliott

To be honest, I’m sure some of the resistance to switching to metric is based on still using through-hole devices, specifically ICs. They use 0.1″ pitch and work well with a 50mil grid. Not everyone has to design boards with super high densities that benefit from 0.05mm grid spacing. (Working with 0.05mm grid when used to 0.05″ coarse & 0.025″ fine grids kinda fells like gridless in my experience…) 10mil traces are still standard here.

Commented on December 22, 2011 at 11:21 pm
By Yuchi

Hi Tom,

I’m a layout engineer. I have been using Imperial grid system for years. For example; placement ICs in 100mils. 50mils,25mils, and small discrete symbols in 5mils. When fanouts, the vias are placed at the 50 or 25 mils grids. After I read your blogs, I made some practise in my work. I found some issues and want to discuss with you.

In the metric grid system, I wold like to fanout the vias in the 0.5 mm or 1mm grid. Now there’s a question.

25 mils grid would automatically allow two traces (5 mils) between vias,
but 0.5mm grid could only go through one trace, and 1mm grid would too much for fanouts.

So what’s your suggestion when fanout using grid?


Commented on December 23, 2011 at 2:51 am
By Yuchi

Hi Tom

I found your answer is vias on 1mm grid and go through two traces in 0.1mm line width and spaces.

Commented on January 1, 2014 at 8:29 am
By Ametrica

You may find these two articles on the subject quite interesting:

It seems the battle between the ISO, IEC, JEDEC and other organisations now requiring metric, and the US standards bodies agreeing to comply, but industry refusing to comply is just causing American PCB houses to go out of business.

Just a side note, imperial is illegal in the US. The British created imperial in 1824 an the US never adopted it. The US instead uses a different version called USC for United States Customary.

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