Category Archives: Centimeters or Millimeters

The American “Metric” Ruler

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By The Metric Maven

Bulldog Edition

Early in my metric-only odyssey I learned some metric lessons the “hard way.” Because of my  inexperience, I had a naïve view of metric implementation that caused me to be incautious. My better half decided she would try using metric for some of her craft work. She flipped her ruler over to its metric side and began using it. I thought nothing of leaving her alone to work, after all metric is simpler—right?  Within a few minutes she was complaining that the pattern she was working on was 1.5 mm long and she needed it to be larger.  I had seen the pattern and it was way larger than 1.5 mm. Incredulously I walked into her study, looked at the pattern, looked at the ruler and realized it was more like 15 mm in length. I inquired how on earth she measured 1.5 mm, she quickly showed me the marking on the ruler, and I realized that according to the ruler, she was indeed measuring 1.5 mm. Here is the end of that ruler.

Ruler which indicates mm graduations — But cm are numbered

One notes that it is labeled millimeters, and therefore one would expect, that like inches, each numbered mark, 1, 2, 3 and so on would be in millimeters!  I looked at it in astonishment, and realized that anyone not familiar with the metric system could easily make this mistake. I explained the situation that the large marks are centimeters and the small divisions
are millimeters. It was my first realization that American “Metric” Rulers (AMR) actually had two units, and the imperial side only used inches. Until I actually started using metric exclusively, I had never noticed this. One of my biggest complaints is the common mixing of units in imperial. For instance a person’s height might be 1 yard, 2 feet, 10 inches and 1 barleycorn tall. Ok, we have simplified this to just 5′ 10 1/3″ but that’s still two units–designated with quotation marks. The use of multiple units is a clear violation of Naughtin’s Laws and only serve to obfuscate the experience of a direct cognitive understanding of magnitude.

Centimeter label with cm designations

I realized that the  ruler should say cm instead of mm. This caused me to start looking at rulers offered in office supply stores. It is there, one can see that the metric side of a dual-scale ruler gets little respect. For instance, I went to an office supply store recently and purchased a green ruler, fabricated by a company which boasts they have made rulers since 1872. You can see that the designation is cm this time, but the origin of the scale is on the end which is rounded and used to hang the ruler on a wall. The inch side gets the square end which is psychologically more appealing. You don’t think this is a big deal? When you pick up a slice of pizza, how often do you begin eating the wide, crusted end?—rather than starting with the point? Simple ergonomics matter.

A ruler that acknowledges both cm and mm units are on it

But in reality, both millimeters and centimeters exist on this ruler. To my surprise, the store brand rulers have both mm and cm designated at the end. Someone actually thought about this and marked both! Unfortunately, it now gives one two possibilities for what the numerical marks might designate, instead of resolving the situation. Assuming one knows which are cm and mm,  a person can, for example, measure something with an American “Metric” Ruler, which is say 1.5 cm long. So how is this done? One first looks at the 1 cm designation, then counts the millimeters, and concatenates them with a decimal point to obtain the answer. You are always holding the first digit in your mind 1 and then counting to the next digit, which is 5 and then creating the answer of 1.5 cm. This is true even if you designate the millimeter graduations as tenths of centimeters. When I first received an Australian metric ruler, it had nothing but millimeters, and my mind quickly responded to its ease of use. The Australian who sent it to me said “I learned early on that centimeters cause mistakes, and in my business this cost me money, and I stopped using them.” He told me it took Australians a while to realize this, but more and more, he was seeing centimeters, less and less.

Custom metric rulers given to Metric Maven Clients

In my engineering work, I have never seen a metric drawing in centimeters. After I had used a metric ruler marked only in millimeters, I hoped I would never encounter one. A client who is an imperial die-hard informed me that there is no use using metric, you have to buy meat in pounds, and lengths in yards, or feet or inches in this country. I was told to “get real.” My reaction to his statement was not what I expected, my mind wandered back to my youth, when lumber yards, car dealers and others would hand out free yardsticks on the Fourth of July, Labor Day and on other important dates for advertising. I realized that I needed to hand out mm only rulers  to my clients, as I suspected many of my clients, despite all being engineering firms, had never seen such a thing as a mm ruler. I first tried to get wooden millimeter rulers from wooden yardstick suppliers. One of the vendors informed me, in no uncertain terms, that this is America and we use inches. Finally I came across a supplier who would make custom metal mm-only rulers for me. I decided to go with a 600 mm long ruler to hand out to my clients and their Engineers. I reasoned 600 mm would be long enough to measure most common designs found in my industry, which is microwave and antennas. The rulers are metric-only (no inches), have my company logo, name and information, just like the old wooden ones, which are now collected as antiques. The end of one of my mm-only metric rulers is shown in the inset above.

When I handed out the mm-only rulers, I was pleasantly surprised by the reactions. One Engineer said “Oh, thank heaven’s it’s 600 mm, that’s actually a useful length.”  Two others at another company took the opportunity to thank me for suggesting the centimeter be eschewed, they really found it simplified matters. One older Engineer said “It’s amazing, I’m starting to lose my feel for inches, and think in millimeters now. This ruler is great.” I was floored. I was used to a much more visceral and negative reaction, or one of dismissive semi-contempt. I later had lunch with two Engineers, one of which has seemed to enjoy his attempts at devil’s advocate for centimeters. He continued trying to needle me, and then quickly moved the ruler I’d given him closer to his person, and said “Now you aren’t thinking of taking this back—are you?” I replied “I can see that a ruler like this desperately needs to be with someone like you.” When I took a ruler to the Engineer who had told me to “get real,” his countenance was that of a person who had asked me to find a Unicorn, and I had obtained one, broke and saddled it for his riding pleasure, then delivered it to his door.

This is the first time, that I felt I might have made a small difference in American metrication, but why was this time different? My conclusion is the ubiquitous presence of American “Metric” Rulers. They offer very little obvious advantage, and probably are viewed as an equivalent to decimalized inches, or are seen as even more complicated with their mixture of cm and mm scales. I’ve come to believe more and more that American “Metric” Rulers are a big obstacle to Americans adopting metric for common everyday use. With millimeters, one almost never needs a decimal point for common measurements, but this simplicity is never experienced, or realized, because all that is around us are American “Metric” Rulers. A small step which might help metrication in this country, would be to mandate that ruler manufacturers change the “metric” side of our dual-scale rulers to millimeter only. It’s a minute change, but I think it might cause people to view the metric system more favorably, and even possibly use that side of the ruler more often. Dual scales (mm and inches) clearly violate Naughtin’s Laws, and conspire to put off metric use, but it would be some small victory toward mandatory metrication.

I have wondered many times since, that if, many years ago, I had attended local Fairs in the Midwest, where the metric side of the wooden yardsticks was in millimeters,  if we might not be much further down the path to metrication, and not continuing to stand still as we have been for over 150 years.

Related essays:

Stickin’ it to Yardsticks

The Design of Everyday Rulers

America’s Fractional Mind

If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:

The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays  that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website,  but the book has them all in one place in print. The book may be purchased from Amazon here.


The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.


The third book is not of direct importance to metric education. It is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.

The Pseudo-Inch

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By The Metric Maven

Bulldog Edition

I have a friend, Pierre, who has a passion for woodworking, but a passion for metric?—not so much. My knowledge of woodworking is at best minimal, but Pierre does his best to enlighten me. One evening I received an email from him after he had watched a woodworking program by Roy Underhill.  Roy is an American, and a bit of legend in woodworking as I understand it. During the program, Roy explained a Swedish method of teaching called sloyd. My friend Pierre then related:

The original Swedish drawings in the book of exercises he used were measured in metric. Underhill says, “I took up woodworking so I wouldn’t have to learn metric.” Thought you’d want to know that.

Next, Underhill was showing how to measure 2 cm over on a board using a Swedish wooden folding rule. He said he got it from some visiting Swedish woodworking friends, because his American one doesn’t have centimeters on the back. He said, they said, and this is really the part I wanted you to know, that in Sweden, his woodworking friends use only inches.

Then, he flipped the Swedish folding rule over, and it showed inches. Here’s another interesting part, I hope you aren’t asleep yet.

The Swedish “inch” is bigger than our inch. Holding the rules one over the other, you could clearly see the Swedish “inch” is about 1/16 bigger.

WTF, man?

Well, the problem is, there is really no such thing as “an inch.”  Why do we think that an inch exists?—well, therein lies the tale. The Wikipedia entry for “the inch” has an enlightening “inch converter” which was used before the age of the metric system. Here is the illustration:

19th Century Inch Converter

19th Century Inch Converter — Wikipedia Commons

One can see that “the inch” has many different lengths in the 19th century. Many of them are considerably different in length. Note the Moscow and Russian inch are not even close to one another. Here’s what I have surmised from this converter about inches:

  • Hamburgh – Inch divided into 8 parts. 1 inch ≈ 23.2 mm
  • Austrian – Inch divided into 8 parts. 1 inch ≈ 25.8 mm
  • Itallian – Inch divided into 8 parts. 1 inch ≈ 28.3 mm
  • Bremen – Inch divided into 10 parts. 1 inch ≈ 23.7 mm
  • Swedish – Inch divided into 12 parts. 1 inch ≈ 24.3 mm
  • Turkish – Inch divided into 12 parts. 1 inch ≈ 31.3 mm
  • Bavarian – Inch divided into 12 parts. 1 inch ≈ 24.0 mm
  • Spanish – Inch divided into 12 parts. 1 inch ≈ 23.0 mm
  • Portuguese – Inch divided into 12 parts. 1 inch ≈ 27.0 mm
  • Moscow – Inch divided into 12 parts. 1 inch ≈ 27.7 mm
  • Russian – Inch divided into 8 parts. 1 inch ≈ 44.1 mm
  • Amsterdam – Inch divided into 12 parts. 1 inch ≈ 23.5 mm
  • Rhynland – Inch divided into 12 parts. 1 inch ≈ 26.1 mm
  • French – Inch divided into 12 parts. 1 inch ≈ 27.0 mm
  • Fr. Metre – Centimetres divided into millimetres
  • English – Inch divided into 32 parts. 1 inch ≈ 25.3 mm
The “inch” is a perfect example of what I call  a retread unit. This is a unit with a name that is used over and over for different quantities, which renders it meaningless as a standard.
This lack of a clear international definition of the inch, produced a strange misunderstanding in the early part of the Twentieth Century between Americans, British and Australians–who were the contractors:
In 1909 the American firm Pratt and Whitney was contracted to supply all the equipment for the Lithgow plant to specifications that  would ensure interchangeability of components with British rifles. But the parts would not fit because no one told the Americans that the British drawings used two different standards of length: dimensions above 2 inches were expressed in inches aligned with the imperial yard; but dimensions below 2 inches were based on the `Enfield inch’, a standard used by the Royal Small Arms Factory at Enfield. (For Good Measure Jan Todd 2004 pg 58)
Here is the history of the Swedish inch according to Wikipedia:
In Sweden, from 1855 to 1863, the existing Swedish “working inch” of ≈24.74 mm was replaced by a “decimal inch” of ≈29.69 mm which was one-tenth of the Swedish foot. Proponents argued that a decimal system would simplify calculations. However, having two different Swedish inch measures (and the English inch on top of that) proved to be complicated. So in a transition period between 1878 and 1889 the metric units were introduced as the overall standard measures. However, the various inches survived some time in building and construction trades.
click to enlarge image

Originally the Anglo-Saxons used their smallest standard, the barleycorn, to define the inch. We still use barleycorns to define shoe sizes in the US. The standard was three barleycorns in a row make an inch. I call this “The Barleycorn Inch.” The barleycorn is also called the grain. There are 7000 grains to a pound. In the 20th century, the British and Americans decided to define their inch as exactly 25.4 mm. It is sometimes called the “Industrial Inch.” This is what Americans call “The Inch.” This inch was decimalized in many industrial applications and is found on many, many US technical drawings. The decimalization of the inch is the basis for our machine tools, yet finding a ruler in the US with decimalized inches is almost impossible. Our educational system doesn’t even teach decimalized inch units and how they are used by industry. The fascination with fractions in this country is beyond my understanding.

I have converted my engineering work to be entirely metric, which can cause heartburn for some American vendors. Recently I finished a PCB design for a broadband microwave device. I sent a PCB fabricator gerber files and metric drawings of the device to be fabricated. They requested I give them drawings with dimensions in inches—I demurred. They asked again—again I told them no. Their third email to me pointed out that inches were what their equipment was calibrated to, and what they are trained to use, are used in the US, and they wanted inches.

Dimensional Standard for Barleycorn Inches

The PCB files I sent actually were in inches as forced by US industry, but the drawings were not–so why was the vendor so insistent that I provide inch drawings? Well, in Engineering, engineering drawings define the controlling dimension. The controlling dimension is the one to which the manufacturer is expected to conform. If I have a 100 mm drawing dimension, that is what the length is supposed to be, with a tolerance in mm, not 3.937 inches and a tolerance in mils. The conversion from inch to mm is exact, the conversion from mm to inches is inexact. The vendor did not want to be held to the accuracy required in mm when they had inches on their fabrication equipment.

The point I made to them is that the controlling dimension on all engineering drawings in the US is actually metric. This is because we use the Industrial inch in the US. The definition of the US inch is 25.4 mm exactly. All the “inches” of the vendor’s equipment are calibrated and controlled by the meter, so the controlling dimension of all US drawings in inches, is actually in terms of mm (i.e. meters), we just don’t acknowledge this.  The situation ended up resolving itself, and the boards were successfully fabricated. As a country we pretend that we use something called “the inch,” but it is derived from a metric standard. We use metric as our base standard, but do not adopt the convenience of the actual metric system, preferring to pretend we have “our own American/Standard” system. This is delusional.

The power the inch has over US citizens and others appears to have caused early users of metric to impose vacuous imperial conventions on the metric system. You will note that on the “inch converter” from Wikipedia the centimeter also appears in the list. As I have explained in detail in other blogs, the centimeter impedes the ease of use and soils the elegance the metric system offers. The use of millimeters only, allows for a simple and accurate implementation of metric—often without decimal points. This is experienced by Australian construction workers every day. The fact that Roy Underhill doesn’t understand the power of using millimeters, and slavishly uses centimeters instead, makes him less than a legend to me. He simply embraces folklore. The centimeter is the perfect example of an unnecessary division which appears to exist only to preserve an unnecessary and ill-defined unit of magnitude called “the inch.”. The centimeter is but a pseudo-inch, demanded by tradition and not by necessity. Its utility has proven to be non-existant in practice. Remember friends don’t let friends use centimeters. And The inch?—the definite article?—well, it’s much like fairies, and other mythical creatures, it only exists in our imagination.

Updated 2013-01-31

Related essays:

Metamorphosis and Millimeters

Building a Metric Shed

If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:

The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays  that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website,  but the book has them all in one place in print. The book may be purchased from Amazon here.


The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.


The third book is not of direct importance to metric education. It is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.