What Does Mil Mean in Plastic Sheeting? Understanding 20 Mil Thickness

A “mil” in plastic sheeting is one-thousandth of an inch, so 20 mil sheeting measures 0.020 inch thick — roughly half a millimeter.

Order a roll of 20 mil plastic sheeting expecting something like a heavy trash bag and the box that arrives will surprise you. This is pond-liner territory, not drop-cloth territory. The confusion starts with the word itself: “mil” sounds metric, and plenty of buyers read it as millimeter. It isn’t. In plastic sheeting, a mil is a thousandth of an inch, a unit the industry borrowed from wire and film manufacturing, and that single letter changes a spec by roughly 25 times.

Here’s what the number actually means, where 20 mil lands in the real world of liners and barriers, and how to tell it apart from the sheets sold right next to it.

Mil vs. Millimeter: What Does Mil Mean In Plastic Sheeting?

A mil is a thickness unit equal to 0.001 inch, so plastic sheeting sold as 20 mil is 0.020 inch thick. That is not the same as 20 millimeters, which would be nearly an inch of plastic — no flexible roll works that way. Americover’s thickness guide and Interplas both spell out the distinction because the mix-up costs buyers real money.

The metric conversion is where the confusion gets practical. One mil equals about 0.0254 mm, so 20 mil works out to roughly 0.508 mm, or about half a millimeter. Global Plastic Sheeting rounds it to 0.5 mm in its conversion chart.

A quick thickness comparison

Notice the pattern: each step up roughly doubles the material. A 20 mil sheet carries about five times the plastic of a 4 mil drop cloth, which is why the two feel nothing alike in the hand.

What 20 Mil Plastic Sheeting Is Actually Used For

Suppliers describe 20 mil as heavy-duty sheeting built for containment and long-term ground contact, not quick cover-ups. Americover, Global Plastic Sheeting, and Interplas all list overlapping jobs for the gauge.

• Pond and aquaponics liners — the thickness resists punctures from roots and rock.
• Vapor retarders and barriers — under slabs and inside crawl spaces.
• Landfill covers and caps — large containment projects.
• Earthen liners and grain storage — agricultural ground protection.
• Construction enclosures and outdoor storage covers — temporary weather walls.

Some 20 mil products are reinforced rather than plain film. Americover notes that reinforced sheeting is built around a polyester scrim grid — one product page lists scrim spacing at 3/8 inch — and at least one reinforced 20 mil sheet carries up to 5 years of UV resistance and FDA compliance for food-adjacent use.

Material matters just as much as the number. Global Plastic Sheeting describes one 20 mil liner as high-density polyethylene (HDPE) geomembrane made from virgin polyethylene resin, with a composition of roughly 97.5% polyethylene, 2.5% carbon black, and trace antioxidants and heat stabilizers, meeting or exceeding the GRI GM13 specification used for geomembranes.

Why The Same “20 Mil” Label Can Mean Different Things

Two rolls both stamped 20 mil can behave very differently, because thickness alone doesn’t set performance. Reinforcement, resin type, and UV additives all change how a sheet holds up, and suppliers measure reinforced sheeting at the cross-section of its internal grid, not at a smooth single layer.

That’s the practical warning: treat “20 mil” as the starting spec, not the whole story. A plain 20 mil film and a scrim-reinforced 20 mil sheet are built for different jobs, and neither one beats the other across the board. Check the construction, the UV rating, and any compliance claims before you commit — those details decide whether the sheet survives a season or a decade.

If you’re weighing specific rolls for a liner or barrier project, our roundup of tested 20 mil sheeting options compares what each construction handles best. For the underlying unit definitions, the mil-to-mm conversion reference from Global Plastic Sheeting lays out the math cleanly.