Thickness is the least important number on the box.

Thickness compares two bags only when they are made the same way. Change the resin and the comparison stops working.

The unit first. A mil is one thousandth of an inch, or 25.4 microns. In a Plastics Technology market snapshot, most consumer trash bags ran about 1 to 1.25 mils, and reported gauges ranged from 0.35 to 3.5 mils. That figure counts plastic. It says nothing about which plastic.

Three families of polyethylene end up in bags. Low density is the older material. High density makes thin liners measured in microns, good for paper and weaker on puncture. Linear low density is the one for sharp waste, and the one that changed the trade: it can be extruded thinner than low density film and gain strength, the same magazine has reported. Its first commercial line started in Corunna, Ontario, in 1960. In the United States, trash bags led its growth in blown film.

Inside that family, the chemistry still moves the result. A TAPPI paper compared film made with three comonomers (butene, hexene, octene) at five gauges. Butene gave the least tough film and octene the toughest. Same thickness, different bag.

Two tests say more than a ruler. The dart test (ASTM D1709) drops a dart with a rounded head 1.5 inches wide from 26 inches onto the film. The score is the weight at which half the samples break. The tear test (ASTM D1922) starts from a slit already cut and measures the force needed to keep the tear going. That second test is the bag that catches the edge of the can. Once the nick is there, the tear either stops or runs.

The making counts as well. On a blown film line, the film is stretched two ways, pulled by the rolls and blown out by the air inside the bubble. Out of balance, a film tears easily along one line and resists the other way.

Then the bottom. A flat seal leaves two corners that take the load and collect liquid. A star seal folds the bottom first, so there are no corners and the weight spreads. On a gusseted bag, the seal crosses four layers at the folds and two in the middle, and that change is a known weak point. From the plant floor: a seal made too cold peels apart clean, and one made too hot thins the film beside it.

None of this makes thin always better. With the same resin, the thicker bag is the stronger one.

What this changes for you. When you compare two bags or two films, read thickness as one line among several. Ask what the film is made of, how it scores on dart and tear, and how the bottom is sealed. Those answers tell you what the number on the box cannot.