Mil Thickness: What Does It Mean, And How Do I Measure It?

Many of DeepRoot’s products are defined by their mil thickness. Our Geomembranes (Water Barrier and Bamboo Barrier), for example, have varying thicknesses of 30, 40 and 60 mil, while our root barriers are all 80 mil thick. But what is a mil?

A mil is a thousandth of an inch — .001 inch. It is a typical manufacturing dimension.  When we make a product we specify the thickness as a mil.

How do you visualize such a small dimension? In order to help people understand what the mil thickness of a product is, I try to give them a point of reference. For example, I always explain that the 30 mil (30 divided by 1,000 = .030) Geomembrane is about the thickness of a credit card.

(By the way, Wikipedia uses the same comparison! Well, actually it uses plastic ID cards, but it’s the same idea. I’ve been saying this for years without Wikipedia’s help – actually I stole, er, borrowed the line from DeepRoot owner Julian Ray. For those who are interested and maybe a bit geeky like me you may want to read more about mils on the Wikipedia page.)

In the pictures in this post, I’m measuring the thickness of my Clipper Card (for the San Francisco transit system, MUNI) with both an old school and a digital micrometer to demonstrate this comparison (did I mention I am a bit of a geek?). It is easy to tell with the digital one that the Clipper card is .03 thick. The measurement is a little bit trickier with the old school one – math is involved, plus it is hard to show the small marks of the micrometer in a photo. Even though I love my, er, DeepRoot’s new digital micrometer, I always back up my finding with Mr. Old School trusty non digital micrometer.

Okay, so by now in this post you are aware that the micrometer is used to measure thicknesses. Wikipedia says that micrometers “are used widely for precise measurement of small distances…. Micrometers use the principle of a screw to amplify small distances that are too small to measure directly into large rotations of the screw that are big enough to read from a scale.” That’s a little heavy for me. I basically know it as the sweet tool that I use to check quality on the items we manufacture.

So there you have it – a brief explanation of what a mil is and why DeepRoot uses the term to describe so many of its products. But, honestly, I just wanted to show off the sweet micrometers.