What is a Mil? A mil is defined as one-thousandth of an inch. Easy enough, right? How many times have you read a technical data sheet only to see square footage measured in mil thickness? What exactly is a mil?

For those who use the same products over and over, it may be easy enough just to guesstimate how much material to use, but is that really a wise thing to do? Some materials require a very specific thickness, so let’s jump right in and gain some knowledge together.

When we use the term mil thickness we’re thinking of how many square feet we can get out of a coating at a desired thickness. Mils can be defined as 1,604 square feet at 1 mil thick. There are several formulas that can help us figure the square footage of a project.

First off, there is 1,604 sq. ft. at 1 mil thick, in a US gallon of any liquid.  With that being said we can use the following formulas to determine mil thickness and coverage.

1,604 divided by mil recommended thickness = sq.ft. Coverage

1,604 divided by sq. ft. recommended coverage = mil thickness

For example, the manufacturer requires a coating to be applied at 20 mils. So by using the first formula and dividing 1,604 by 20, we get an approximate coverage rate of 80 square feet per gallon.

We realize this is a bunch of numbers and can be difficult to remember and calculate on the go.  We always use the term 4 by 400.  Which tells us at 4 mils thick coverage equals 400 square feet.  You can then figure the approximate square footage by doubling the mils and halving the square footage.  So 8 mils would be 200 square feet and 16 mils would be 100 square feet.  It is an approximate number but will help you determine the needed product on the fly. Below is an actual mil and square footage chart for your viewing pleasure.

 Coating Mil Thickness Coating Coverage Sq. Feet Per Gallon 1 1604 2 802 4 401 6 267.33 8 200.5 10 160.4 12 133.66 15 106.93 16 100.25 20 80.2 30 53.46 40 40.1 50 32.08 100 16.04