The International Unit of Mystery

By The Metric Maven

I have had the pleasure of visiting a number of technology and manufacturing firms over the years. But whenever I question them about if and how they use the metric system, I often feel I’ve been thrust into an Austin Powers movie. Because Austin was frozen in the 1960s and thawed in the 1990s he had not kept up with the times. His use of groovy is clearly anachronistic.  Dr. Evil’s plan to hold the world hostage for ONE MILLION DOLLARS shows that he has not been out to purchase a Bob’s Big Boy Burger in a while. His cryogenic nap has left him  culturally temporally challenged. Here is a version of the Austin Powers conversation I have about metric:

MM: “So, do you use metric here at your company”

Proud Company Representative: “Yes we do, we use microns and millimicrons.”

My mind halts and is transported back to the time many moons ago, when I was employed at my first engineering position. This position is also where I produced my first drawings in metric, and was told in no uncertain terms that they would not be accepted. Not long after I had been told that metric was not acceptable in aerospace, I had a dimension described in microns. “Micron?, what the hell is a micron?” I thought. I’d never heard of it when I was in college or doing amateur projects. I opened up my copy of Reference Data for Radio Engineers, which was published in 1982. There was no entry for micron in the index. Another engineer who also had a copy said, “look on page 1-1” which is the very first page of the reference. When I did, the book did not exactly define the micron, but it did have this footnote: (Note that the term “micrometer” has superseded the term “micron.”). Micron was only mentioned as a term which should not be used in engineering work, which I was now told to use in my non-metric aerospace engineering environment. Now that’s groovy baby! Just as groovy, was the use of the term mil or 1/1000th of an inch. This was the aerospace default measurement “unit,” but the way micron and mil were used, one might never realize that one was Ye Old English, and the other a poor usage of the metric system. Microns were welcome, but no micrometers please, this is American aerospace, only proven “heritage” is welcome.

In fact I seldom heard micron used, so mil became the preferred default feral non-unit of choice. Then when discussing some coatings with a researcher one day, he said “yeah, these are really thin” and then quoted a value in millimicrons. Millimicrons? Ok, so we’ve proudly added a metric prefix to a non descriptive archaic metric moniker of French origin which has lost its prefix. In other words it’s a millimicrometer,  which is of course a nanometer for those of you out there who are numerate.

Wikipedia confirms my experience with a micron in industry:

The term micron representing the micrometre, was officially accepted between 1879 and 1967, but officially revoked by the International System of Units (SI) in 1967.[2]

Nevertheless, in practice, “micron” remains a widely used term in preference to “micrometre” in many English-speaking countries, both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing).[citation needed] Additionally, in American English the use of “micron” helps differentiate the unit from the micrometer, a measuring device, because the unit’s name in mainstream American spelling is a homograph of the device’s name.

In engineering it is common to use the micron as a replacement for the traditional thou or mil, each of which represent a thousandth of an inch. So a bin bag may be originally specified as 0.35 mil thick, but stated as 8.89 microns.

Part of this Wikipedia entry is laughable: “the use of “micron” helps differentiate the unit from the micrometer, a measuring device.” I used a micrometer when I worked as a printer, and experienced a bit of machining. Every US technical person I’ve met calls the measurement device a My-crom-et-er (accent on the first syllable). The measurement unit is pronounced mike-Crow-meter with an emphasis on the second syllable. The word micron is pronounced mike-ron, and the instrument for measurement, the micrometer, is proverbially called a “mike.” When I worked as an offset pressman, all other printers called a micrometer (the instrument) a mike. We generally shared one mike, and I never recall a single pressman requesting that he borrow the “micrometer.” This explanation appears to be yet another strange after-the-fact rationalization for strange versions of US metric non-usage. The micrometer-micron usage explanation also appears similar to folk etymology. There is no excuse other than the irrational urge for US engineers to continue Living in The Past.

Wikipedia also “weighs in” (I could not help myself) on the millimicron:

The nanometre was formerly known as the millimicrometre – or, more commonly, the millimicron for short – since it is 1/1000 of a micron (micrometre), and was often denoted by the symbol or (more rarely) µµ.[1][2][3] In 1960, the U.S. National Bureau of Standards adopted the prefix “nano-” for “a billionth”.[4] The nanometre is often associated with the field of nanotechnology. Since the late 1980s, it has also been used to describe generations of the manufacturing technology in the semiconductor industry.

I’m not knowledgeable enough of the current workings of the semiconductor industry in the US to know if they use nanometers or not. I do know one of the largest  semiconductor corporations uses it as a company name. The irony of using micron is that it appears it was coined by the French in 1880 for one millionth of a meter, and became accepted in 1892. Apparently the tonality of a French term for a micrometer is so alluring that Americans just can’t give up sonorous franophonic sound of the word micron.

One would generally never get a clue that the micron is French, or a metric measurement when looking through a US technology catalog. Here is an advertisement for a micromachining product:

One of the most curious of articles I have come across, is in a trade magazine. The article has five authors, and is about a new glass material which allows one to create through glass vias (TGV). Vias are just methods of connecting electric circuits together. The entire article is nothing but a discussion of micron dimensions. The  caption for figure 2 is all microns:

The next figure has descriptions using microns, but also reminds the reader that it is a millimeter wave circuit.

The final figure in the paper is even more “descriptive,” it has millimeters, microns, and micrometers:

The mad desire for microns is not confined to engineers. Caleb Scharf in his 2012 book Gravity’s Engines on page 52 describes interstellar dust:

This is not the same kind of dust you find under your bed. This is far finer and very different in composition. A typical grain of interstellar dust is only about 0.001 millimeters (one micron) across….

It’s not Austin Powers fault that he had not kept up with the times, he never experienced them. We in the US have no excuse when it comes to metric usage, other than we refuse to experience change, and try to preserve the perfected world of 1789 forever.


I thought the use of the micron was exclusively a US problem, but apparently it exists in Europe and the UK. A blog from the UKMA describes the problem: The Reports of the Death of The Micron are Greatly Exaggerated.

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 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.

3 thoughts on “The International Unit of Mystery

  1. I have a minor disagreement with the Wikipedia post on the nanometer. Nano-, as a prefix, was adopted not so much by NBS, but by the BIPM in the International System of Units in 1960. Prior to that, it was not an approved prefix which resulted in the need, prior to that time, for millimicrons (or millimicrometers). Double prefixes were relatively common prior to the expansion of prefixes (but have been “prohibited” since).

    Microns do remain common in semiconductors, probably more common than the correct micrometer. However, I haven’t heard millimicrons in ages, the semiconductor industry uses nanometers instead. People pretty quickly moved away from double prefixes as the BIPM expanded the list of prefixes.

  2. I can report that, at least when spoken, micron is still alive and well in my industry in Australia. For example when discussing filter media, I would usually write 10um (preferably with a mu not a u), but in conversation say “10 micron”. I would not think it odd to see the full word micron used in written material.
    In speaking there is a parallel when discussing millimeters. I would use the normal 50mm when writing, but if in conversation it would be common to shorten to “50 mil”.

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