Mil
One thousandth of an inch. It is the unit almost every coating specification, data sheet and film gauge is written in, and it is not related to the millimeter despite the resemblance.
One thousandth of an inch
That is the whole definition. A mil is a unit of length equal to a thousandth of an inch, and the name comes from the Latin for a thousand, the same root that gives us millennium. It means a thousandth of an inch in exactly the way a millimeter means a thousandth of a meter.
In coatings it does one job. It is how thick the film is. Nearly every thickness a data sheet quotes, every setting on a gauge, and every number in a specification is written in mils, which makes it the one unit worth being completely certain about before anything else on the sheet matters.
Why thousandths, rather than inches
Because that is the scale a coating genuinely works at, and every alternative is worse to use.
A floor coating is thin. Written in inches, an ordinary build is a string of zeros before any digit that carries information, and fractions are no better, because the useful range does not divide conveniently into halves and quarters. Mils put the working range into small whole numbers, which is the entire purpose of choosing a unit.
It also keeps real differences legible. Two specifications a few mils apart describe meaningfully different floors, with different costs and different service lives. Expressed in inches, that same gap would look like a rounding artifact.
A mil is not a millimeter
This is the misreading worth guarding against, because the error is not small and it does not announce itself.
A mil is a thousandth of an inch. A millimeter is a thousandth of a meter. They sit roughly a factor of twenty five apart: one mil is 25.4 microns, which is a little over two hundredths of a millimeter. Reading a specification of 12 mils as 12 millimeters describes a floor about twenty five times thicker than anyone intended, at a material cost to match.
The abbreviations invite it. Mils are shortened to mil, millimeters to mm, and thousandths are called thou in some places, so the resemblance is close enough to slip past a quick read. If a figure is ambiguous, settle it before ordering rather than afterward.
The metric equivalent is microns
Not millimeters, and adopting that convention removes most of the confusion above on its own.
Coating thicknesses in metric markets are normally quoted in microns, for precisely the reason mils are used in imperial ones: microns put the working range into small whole numbers where millimeters would push it back into decimals. One mil is 25.4 microns.
Our own coverage arithmetic follows the same convention, working in square feet per mil on one side and square meters per micron on the other. Those are one calculation expressed in two unit systems rather than two different rules, and the coverage per mil chart shows the derivation instead of asserting the result.
Wet mils and dry mils
A mil on a data sheet can describe the coat as it is applied or the coat that is left once the solvent and any other volatile content have gone. They are different numbers for the same application, and they are equal only on a product that is entirely solids.
That distinction is covered properly, as one of six common misreadings, in our guide to reading a technical data sheet, and the relationship between the two is the subject of solids by volume and real coverage. The habit worth forming is simply knowing which of the two you are holding before you set a gauge or write a number into a specification.
Unlike the other entries in this glossary, this page does publish figures. It has to: a definition of a unit that declined to state the unit’s size would not be a definition. The two numbers here are fixed by definition rather than measured or calculated, so unlike a coverage rate or a cure time there is nothing about them that can change.
Frequently Asked Questions
What is a mil?
One thousandth of an inch.
The name comes from the Latin for a thousand, the same root as millennium, and it means a thousandth of an inch in exactly the way that a millimeter means a thousandth of a meter. It is a unit of length and nothing more.
In coatings it is used for one thing above all others: how thick the film is. Nearly every thickness a data sheet quotes, every gauge you set, and every figure in a specification is in mils.
Why are coatings measured in thousandths of an inch?
Because that is the scale a coating actually works at, and the alternatives are unusable.
A floor coating is thin. Written in inches, a normal build is a run of zeros before anything interesting happens, and fractions are worse, because the useful range does not divide tidily into halves and quarters. Mils put the working range into small whole numbers, which is what a unit is for.
It also makes the difference between specifications legible. Two products a few mils apart are meaningfully different floors, and in inches that gap would look like rounding.
Is a mil the same as a millimeter?
No, and this is the single most expensive misreading of the unit.
A mil is a thousandth of an inch. A millimeter is a thousandth of a meter. They are roughly a factor of twenty five apart: one mil is about 25.4 microns, or a little over two hundredths of a millimeter. Reading a spec of 12 mils as 12 millimeters is not a small error, it is asking for a floor about twenty five times thicker than anyone intended.
The abbreviations do not help. Some places write mils as "mil" and millimeters as "mm", and some write thousandths as "thou". If a figure is ambiguous, resolve it before ordering rather than after.
What is the metric equivalent?
Microns, not millimeters, and that is the convention worth adopting.
One mil is 25.4 microns. Coating thicknesses in metric markets are normally quoted in microns for the same reason mils are used in imperial ones: it puts the working range into small whole numbers, where millimeters would put it back into decimals.
Our own coverage arithmetic follows that convention, working in square feet per mil on one side and square meters per micron on the other. They are the same calculation in two unit systems, not two different rules.
Does a mil on a data sheet mean wet or dry?
It depends which figure you are reading, and the data sheet should say.
This matters enough that it is one of the common misreadings covered in full on our guide to reading a technical data sheet, so the short version is all that belongs here: wet and dry film are different numbers for the same coat, and they are only equal on a product that is entirely solids.
The practical habit is to know which one you are holding before you set a gauge or write a specification.