A mil is a unit of thickness equal to one thousandth of an inch (0.001 inch). If you’ve seen it on packaging for trash bags, plastic sheeting, or screen protectors, that’s what it refers to. One mil equals 0.0254 millimeters for those working in metric, and the term is used across industries from construction to electronics to paint coatings.
How a Mil Relates to Inches and Millimeters
The math is straightforward: divide mils by 1,000 to get inches. A 4 mil plastic bag is 0.004 inches thick. A 60 mil sheet of construction plastic is 0.060 inches, or roughly 1/16 of an inch. In metric, multiply mils by 0.0254 to get millimeters, so that same 60 mil sheet is about 1.5 mm.
To put this in perspective, a standard sheet of copy paper is about 3 mils thick. A human hair averages around 4 mils (roughly 100 microns). So when you pick up a 1 mil trash bag, you’re holding something thinner than a single strand of hair.
Mil, Thou, and Thousandths Are the Same Thing
You’ll sometimes hear machinists and engineers say “thou” or “thousandths” instead of mil. These are all the same measurement: 0.001 inch. The terminology shifts depending on the industry. Electronics and plastics manufacturers tend to say “mil.” Machinists working with shims and dial indicators typically say “thou” or just drop the unit entirely. A machinist asking for “a 25 shim” means a shim that’s 25 thousandths of an inch (0.025 inches), or 25 mils.
Mil vs. Gauge in Plastics
If you’re shopping for plastic bags or poly sheeting, you’ll run into both mil and gauge numbers, which causes confusion. The conversion is simple: gauge equals mil multiplied by 100. A 2 mil bag is 200 gauge. A 4 mil bag is 400 gauge. Mil is the more intuitive measurement because it directly tells you the thickness in thousandths of an inch, while gauge is an older, less transparent numbering system. Most manufacturers are moving toward listing thickness in mils for this reason.
Common Mil Thicknesses for Plastic Sheeting
For construction film, vapor barriers, and protective sheeting, mil thickness determines how tough the material is and what jobs it can handle:
- 30 to 40 mil: Flexible and lightweight. Suitable for general protection where the sheet won’t take much abuse, like temporary dust barriers during renovation.
- 50 to 60 mil: The workhorse range. Strong puncture resistance while still flexible enough to handle by hand. A 50 mil sheet works well for moisture control; 60 mil holds up to repeated cleaning and regular contact with equipment.
- 80 mil: Reinforced and very durable, built for high-traffic or high-abuse areas.
- 100 mil: Maximum toughness. Used when long-term durability and puncture resistance are the top priorities.
For everyday products, the numbers are much smaller. Standard kitchen trash bags run about 0.9 to 1 mil. Heavy-duty contractor bags are 3 to 6 mil. Ziplock-style storage bags fall around 1.5 to 2.5 mil.
Mils in Electronics and Circuit Boards
The mil is a standard unit in electronics manufacturing, particularly for printed circuit boards (PCBs). Trace widths (the thin copper lines that carry electrical signals) and copper thickness are both specified in mils. Standard copper on a circuit board is 1.4 mils thick, which corresponds to what the industry calls “1 ounce copper” because a square foot of it weighs one ounce. Thinner boards use 0.7 mil copper (half-ounce), while heavier-duty boards carrying more current use 2.8 mil copper (2 ounce). Wire sizes and probe calibrations in electronics also use mils as their go-to unit.
How Mil Thickness Is Measured
The standard tool for verifying mil thickness on a physical product is a micrometer. For coatings like paint, varnish, and lacquer, industrial testing follows formal procedures using either stationary or hand-held micrometers. The micrometer’s jaws close gently on the material and give a reading in thousandths of an inch. One thing to keep in mind: soft or spongy coatings can compress under the micrometer’s pressure, which throws off the reading. For rigid materials like metal or hard plastic, micrometer readings are highly reliable.
Digital calipers also measure in mils and are a more affordable option for home use. If you’re checking whether a product matches its advertised thickness, a decent digital caliper will get you close enough for practical purposes.