Finding the right pool liner for your pool can be difficult enough without have to worry about the thickness of the vinyl that is being used.
Above ground pool liners typically are available in two thicknesses, 20 and 25. Some manufacturers call this “gauge” while others use “mil” but these terms are interchangeable. A 20 gauge pool liner is the same as a 20 mil pool liner. The advantage of the 25 gauge pool liners is the increased strength of the pool liner along with a longer warranty.
Keep in mind that the term "gauge" doesn't always mean the same thing from one industry to the next. For example, when measuring items like wire, shotgun barrels, sheet metal, and hypodermic needles, the thickness or diameter increases as the gauge decreases (they’re inversely proportionate). While for sheet vinyl, used for pool liners, and photographic film, the thickness or width of the material increases as the gauge increases (they’re directly proportional).