What each of the three units measures
Each of the three fixes one property honestly. Millimetres and mils fix the material’s thickness; grams per square metre fixes its areal weight. Within one material and one maker’s range they rank products in the same order.
That is the whole of it. There is no published minimum for a domestic pool liner, no grade scale, and no body that certifies the figure.
What the units leave unsaid
They do not promise a service life. A liner fails at a seam, at a fitting cut-out, or from ultraviolet at the water line — none of which thickness alone predicts.
They do not promise interchangeability between units. A maker quoting g/m² and one quoting mm cannot be ranked against each other with confidence, and a page that converted between them would be publishing a figure nobody measured.
And they do not promise a fit. A liner is specified by its fitting type and its exact dimensions, and a thicker one in the wrong attachment is simply the wrong liner.
What has to be right before thickness matters
Fitting type — overlap, bead, J-hook — which decides compatibility before thickness is worth reading. Exact dimensions, because liners are cut to a pool and not to a size range. Material, since reinforced sheet behaves differently from plain PVC at the same thickness. And seam construction, which is what the figure conspicuously does not cover.
On covers the same units reappear with different companions: a woven winter cover publishes grams per square metre or denier, and a solar cover publishes microns, which is the same dimension at a thousandth of the scale.
Liners first, then everything that covers
Pool liners first, where it is the headline figure. Frame and steel wall pools, where the liner is part of the purchase and its thickness is what separates two otherwise identical sets. Ground cloths, which publish the same units for a sheet that never touches water. And pool covers, where the areal weight is the closest equivalent of the same idea.
What readers keep writing in to ask
How do mil and millimetre relate?
A mil is a thousandth of an inch, so 20 mil is 0.5 mm and 30 mil is 0.75 mm. The conversion is exact and this site still prints whichever unit the maker chose, because converting a figure makes it ours rather than theirs — and the two systems belong to two different catalogues.
Is g/m² the same as thickness?
No. Grams per square metre is areal weight, and it tracks thickness only if the material’s density is the same. Between two PVC liners the correspondence is close; between a PVC liner and a reinforced one it is not, and neither is between a liner and a woven cover.
Is a thicker liner always better?
It resists puncture better, which is real. It is also stiffer to fit, less willing to conform in cold weather, and sometimes only available in fewer sizes. And nothing about thickness addresses the seam, which is where a liner most often fails first.
What does this site do when no thickness is published?
It leaves the row empty and says in words that the maker published nothing — which on this shelf is itself informative, since thickness is the first thing a maker prints when it is a selling point.
Last reviewed 21 September 2026