Gauge

Gauge describes how thick a film is. In the United States it's almost always expressed in mils, where one mil equals one thousandth of an inch (0.001 inch), while much of the rest of the world uses microns, where one mil equals roughly 25.4 microns. A typical produce bag might run around 0.75 to 1.5 mils, while a heavy-duty industrial liner could run 4 to 10 mils or more.

Gauge is rarely perfectly uniform across an entire roll or down its length; instead it's controlled within a tolerance band, often expressed as a percentage of the target gauge, such as plus or minus 10 percent. Consistent gauge matters because thickness directly affects strength, cost (since resin is sold and consumed by weight, not area), seal performance, and how a film runs on a customer's packaging equipment. Gauge bands, meaning visible or measurable variation in thickness running in narrow strips down the length of a roll, are a common quality issue that extrusion technicians monitor and correct using die bolt adjustments and air ring tuning.

Gauge is typically measured with a micrometer at multiple points across the web width, and many film producers also use in-line laser or capacitance gauge-measuring systems that scan continuously and feed data back to automated die controls to keep thickness within spec in real time.

A customer's purchase order specifying 1.5 mil plus or minus 10 percent is defining both the target gauge and the acceptable tolerance band the finished film needs to fall within across the roll.

A common point of confusion is mixing up mils and millimeters, which are not remotely the same; a film described as 0.001 inch thick is 1 mil, while 1 millimeter would be roughly 39 mils, a massively thicker and impractical film for almost any flexible packaging use. Always confirm whether a spec is in mils, microns, or gsm (grams per square meter, a weight-based measure sometimes used instead of true thickness).