How this instrument works
The mil is a purely derived unit: one thousandth of an inch, exactly, with no independent standard of its own. Because 1 inch equals 25.4 mm by the 1959 international agreement, 1 mil works out to 0.0254 mm — a clean, terminating decimal, unlike many cross-system conversions. British and Commonwealth trades often say 'thou' instead of 'mil' for the same unit, treating it as a countable noun — 'knock off two thou' — while American shops overwhelmingly favour 'mil,' and the two words never refer to different lengths, only different regional habits.
Plastic sheeting is where most people encounter mil ratings without necessarily realising it. Painter's polyethylene drop cloths are commonly sold at 1-mil, 2-mil and 4-mil thicknesses for light protective use, while construction vapor barriers typically step up to 6-mil or 10-mil polyethylene for durability underground or under concrete slabs. Pond and landscape liners go heavier still — 20-mil and 30-mil EPDM or reinforced PVC are standard weights for anything expected to hold water or resist punctures for years outdoors.
The same unit shows up in everyday objects most people never think to measure. A standard ISO-format credit or ID card runs about 30 mil thick, roughly 0.76 mm, a figure fixed by international card standards rather than chosen arbitrarily. A single sheet of ordinary copy paper comes in around 4 mil, about a tenth of a millimetre — thin enough that a stack of ten sheets barely reaches the thickness of one credit card.
- Enter a thickness into the Mils (thou) (mil) field. It starts at 20, a common heavy-duty sheeting weight.
- Read the Millimetres (mm) field, recalculated instantly by multiplying by the exact factor 0.0254.
- Converting millimetres back to mils? Divide by 0.0254, or multiply by 39.3700787402 — both give the same exact result.
- For inches directly, divide your mil figure by 1,000 — a mil is one thousandth of an inch by definition.
Worked example — 20-mil pond liner
Enter 20 into Mils (thou) (mil) and Millimetres (mm) reads 0.508. Landscapers commonly specify 20-mil reinforced polyethylene or EPDM as a standard weight for garden pond liners, heavy enough to resist punctures from roots and rocks but still flexible enough to shape around curves and edges.
That 0.508 mm sits well above the 4-mil, roughly 0.1 mm poly sheeting sold for indoor painting protection — a fivefold difference in thickness for a very different job. Contractors choosing between a 6-mil vapor barrier and a 20-mil pond liner are really choosing between roughly 0.15 mm and 0.508 mm of material, a distinction that matters far more in millimetres than the bare mil numbers suggest at a glance.
Questions
What does 'mil' mean, and how is it different from a millimetre?
A mil is one thousandth of an inch, equal to 0.0254 mm — nearly forty times smaller than a millimetre despite the similar-sounding name. Confusing the two on a specification sheet can turn a 20-mil liner order into a request for something 0.02 mm thick, which is why careful documentation always spells out 'mil' or 'mm' explicitly.
Why do painters buy plastic sheeting rated in mils?
Mil ratings give a quick, standardised way to compare durability without needing metric conversion — a 1-mil drop cloth is disposable, light protection, while a 6-mil sheet holds up to repeated folding and reuse. The plastics and construction industries in the US settled on mil as the customary thickness unit for sheeting long before metric conversion became common practice there.
How thick is a standard credit card in mils?
About 30 mil, or roughly 0.76 mm, a thickness set by international card-format standards rather than chosen casually. That makes a credit card a handy physical reference for judging other mil ratings by feel — a 30-mil pond liner, for instance, feels close to card-stock stiffness.
Do other countries use 'mil,' or is it strictly an American term?
The word 'mil' is mostly American usage; British, Australian and other Commonwealth trades commonly say 'thou' for the identical unit instead — both mean exactly one thousandth of an inch, and neither refers to a different length. Countries that use millimetres for everyday measurement skip the unit entirely and simply specify sheeting or coating thickness directly in mm.
What's the exact conversion factor between mils and millimetres?
One mil equals exactly 0.0254 mm, because a mil is one thousandth of an inch and an inch equals exactly 25.4 mm by the 1959 international yard and pound agreement. Unlike many length conversions, this one terminates cleanly rather than running on forever.
How do I convert mils to inches directly?
Divide by 1,000 — a mil is defined as one thousandth of an inch, so 20 mil is simply 0.020 in, and 500 mil is 0.5 in. No separate conversion factor is needed since the relationship is built into the unit's name.