The conversion
A mil is one thousandth of an inch. It is not an abbreviation of millimetre, and that is the whole problem, because the two look alike in a hurried email.
1 mil = 0.0254 mm. So 60 mil is 1.524 mm, which the industry rounds to 1.50 mm. Conversely 1 mm is about 39.4 mil.
Microns appear too, mostly in packaging and agricultural film. A micron is one thousandth of a millimetre, so 1.00 mm liner is 1000 micron. Where you see a geomembrane quoted at 500 micron, that is 0.5 mm.
Convert a figure
The grades you will actually meet
| Mil | Millimetres | Microns | Stocked grade | Typical use |
|---|---|---|---|---|
| 20 mil | 0.51 mm | 508 | Below range | Light duty and temporary work |
| 30 mil | 0.76 mm | 762 | 0.75 mm | Farm ponds, water pans, aquaculture |
| 40 mil | 1.02 mm | 1016 | 1.00 mm | Irrigation reservoirs, lagoons |
| 60 mil | 1.52 mm | 1524 | 1.50 mm | Landfill, canals, containment |
| 80 mil | 2.03 mm | 2032 | 2.00 mm | Leach pads, tailings |
| 100 mil | 2.54 mm | 2540 | 2.50 mm | Mining and severe duty |
| 120 mil | 3.05 mm | 3048 | 3.00 mm | Specialist heavy gauge |
The stocked grade column shows the metric gauge normally supplied against each imperial call-out. Full properties for each are in the specification table.
Why two systems still exist
Geomembrane grew up in North America, where thickness has always been quoted in mil, and the ASTM test methods that govern the material came from the same place. The rest of the world works in millimetres. Both conventions survived, and international projects routinely carry documents in each.
It shows up most across our export markets in Africa and the Gulf, where a design may be produced by a European consultant in mm, tendered against an American specification in mil, and quoted by an Asian manufacturer in either. Nobody is wrong and the numbers still have to reconcile.
Where it goes wrong
Reading mil as millimetres. Someone sees 60 mil and orders 60 mm, or more often sees 40 mil and assumes 4.0 mm. The first is absurd enough to be caught; the second has been ordered.
Assuming 60 mil equals 1.5 mm exactly. It is 1.524 mm. The industry supplies 1.50 mm against a 60 mil call-out and that is normally accepted, but a specification enforcing 60 mil as a minimum average value is asking for slightly more material than 1.50 mm delivers. If the contract is strict, raise it before you order rather than at conformance testing.
Comparing quotations in different units. Two prices per square metre for what looks like the same liner, one written in mil and one in mm, are not comparable until both are converted. Convert first, then compare.
Forgetting minimum average value. Whatever the unit, a nominal thickness is a minimum average, with a lowest individual value ten percent below it. A 1.00 mm sheet must average 1.00 mm with no reading under 0.90 mm.
Textured sheet is measured differently again
On textured geomembrane, thickness means core thickness measured to ASTM D5994, not a caliper reading over the asperities. A textured sheet quoted at 1.50 mm over the texture has considerably less structural section than one quoted on core. Combine that with a unit mix-up and two quotations can differ by more than they appear to. Once you know the gauge, the pond liner calculator turns it into a roll count.
Frequently asked questions
How many mm is 60 mil geomembrane?
60 mil is 1.524 mm, normally supplied as the 1.50 mm grade. The same conversion gives 30 mil as 0.76 mm (supplied as 0.75 mm), 40 mil as 1.02 mm (supplied as 1.00 mm) and 80 mil as 2.03 mm (supplied as 2.00 mm).
Is a mil the same as a millimetre?
No. A mil is one thousandth of an inch, which is 0.0254 mm. A millimetre is roughly 39.4 mil. They differ by a factor of about forty, so confusing them produces an order that is either absurdly heavy or dangerously thin.
What is 1000 micron in mm?
One millimetre. A micron is a thousandth of a millimetre, so 1000 micron is 1.00 mm and 500 micron is 0.5 mm. Microns appear mostly on agricultural film and packaging grades rather than on engineered geomembrane.
It belongs with the rest of the welding plant we supply to lining contractors.