High density liners, almost always have their thickness expressed in microns; low density liners  almost always have their thickness expressed in mils.

Why? Tradition, as far as we know.

How do you compare microns to mils? Well, first remember that the thickness isn't a full measure of the strength of the bag, just the thickness. But microns and mils are both actually 1/1000th of something, which is where it gets confusing.

One mil is equal to 1/1000th of an inch (.001" = 1 mil). As an example, 250 mils would equal 1/4" (.250").

A micron, on the other hand, is 1/1000th of a millimeter (.001 millimeter = 1 micron). Since a millimeter is a much smaller measurement than an inch, the micron measurement will always be a larger number for a similar-thickness bag.

The actual equation is: 1 mil = 25.4 microns (or, if you're converting from microns to mils, divide the microns by 25.4).

For example, a 16 micron high density bag is equivalent to .63 mils (to be precise, .62992 mils). A 2 mil low density bag would be the equivalent thickness of 50.8 microns.

Again, mere thickness doesn't tell the whole story, but now you know how to compare one to another.