Very simply, a mil is one-thousandth of an inch: 1 mil = 0.001 inch. It is not the same as a millimeter; 1 mil = 0.0254 millimeter. Mil is commonly used to describe the thickness of materials such as plastic sheeting, geomembranes and barriers. Understanding this measurement makes it easier to read product specifications and compare material thicknesses.
A higher mil number means a thicker material. For example, a 60 mil sheet is twice as thick as a 30 mil sheet. The table below gives some examples of thickness conversions numbers, for reference.
| Mils | Inches | Millimeters |
|---|---|---|
| 1 | 0.001 | 0.0254 |
| 10 | 0.010 | 0.254 |
| 20 | 0.020 | 0.508 |
| 30 | 0.030 | 0.762 |
| 60 | 0.060 | 2.032 |
| 100 | 0.100 | 2.540 |
To convert mils to inches, divide by 1,000. To convert mils to millimeters, multiply by 0.0254. These conversions use the exact relationships listed in NIST’s conversion tables.
How Do You Measure Mil Thickness?
For sheet materials, a micrometer is a common tool for measuring thickness accurately. Both digital and mechanical models can measure small dimensions that are difficult to resolve with a standard ruler. Select an instrument appropriate for the material and the precision required.
For a basic thickness check:
- Clean the measuring surfaces and check the instrument’s zero setting.
- Place a single, flat layer of material between the measuring faces.
- Apply the instrument’s specified measuring force. Excess pressure can compress softer materials and affect the reading.
- Record the measurement and repeat at several locations to check for variation.
Many micrometers use a ratchet or friction mechanism to provide consistent measuring force; some offer adjustable force for different materials. Follow the instrument manufacturer’s instructions. If the instrument reads in inches, multiply the reading by 1,000 to obtain mils. A reading of 0.030 inch, for example, equals 30 mils.
Reading Product Specifications
DeepRoot’s barrier products use thickness specifications to help describe their materials. Thickness is one part of product selection; material properties, intended use and installation requirements also matter. When checking a product against its specification, confirm whether the stated thickness is nominal or minimum and what manufacturing tolerances apply.
Good article and well explained, as you say Wiki can be a little “heavy”.
what is full name of MIL?
Mil is not short for anything – it is its own unit of measurement and stands for one-thousandth of an inch.
How does one measure the thickness of a membrane while applying it? This is a membrane which is applied with a squeegee.
After it is applied can it be accurately measured and if so how?
thanks, Brian
Why rename a thou (thousandth of an inch) a mil?
A mil could easily be interpreted as the SI unit, millimetre, or as a micron (a thousandth of a millimetre). In the UK, Europe and most of the world, commercial membrane thickness is stated in millimetres for all practical purposes.
I feel sure some young people could be confused by this nonsense.
Roland, THAT is why ‘Merica is ‘exceptional’. We left the old world sometime ago and ain’t looked back since. And TRUST me there ain’t no fear that anyone on this side of the pond is gonna confuse a thou with anything called “SI Units”.
Could mil be from the French, “mille” for thousand? Isn’t that where cent weight came from? Cent for a hundred from the French, “cente”?
I hate to ask but is 30 mil thinner than 60 mil? I am wanting to purchase some magnetic sheets and I want the thicker sheets. It seems in some calculation the higher the number the smaller the material. I have to ask before I buy.
come on …. “mil” did not just pop up one day. mil is short for mili-inch. and it became popularized as a mil. it is easy to see that the term is related to the way mil is used in the if you were to look at the metric system, you will see that mili-meter is 1/1000th of a meter, mili-liter is 1/1000th of a liter and so on.
Er – you will see that [a] millimetre is 1/1000th of a metre, [a] millilitre is 1/1000th of a litre and so on. A metre is a unit of length; a meter is a measuring device.
We in the U.S. seem stuck with “standard” measures, using inches. It’s slight improvement to divide inches by tenths, hundredths, and thousandths, instead of going with 1/2 inch, 1/4 inch, 1/8 inch, 1/16 inch, etc. But clearly it would be better to switch entirely to the metric system, where everything is in units of 10.
Your mic’s aren’t reading the same. About 1.5 “mils” off.
Our imperial system of measurement has its benefits. Especially for those in construction. Base 10 is only divisible by 1,2,5,10. Base 12 (imperial or standard) is divisible by 1,2,3,4,6,12. 8 is also a handy # in this system as it divides cleanly at 24, 48, etc. We do have reasons for keeping our standard of measurement.