Wetsuit Thickness, by Water Temperature

Neoprene thickness is usually the first spec people look at, and it's a reasonable starting point, but it's not the whole story. Fit, your own cold tolerance, exposure time, and whether you're working hard (spearfishing) or drifting (macro photography) all shift the right answer in either direction.

As a rough baseline: above roughly 25°C/77°F, a rash guard or thin 2mm suit is often enough for most divers. From about 20-24°C/68-75°F, a 3mm full suit is the common choice. From 15-20°C/59-68°F, most divers move to 5-7mm, often with a hooded vest layered underneath. Below that, most divers are in drysuits rather than wetsuits at all -- neoprene alone stops being a comfortable (or safe, for long dives) solution.

If you run cold, size up in thickness before you size up in exposure time. A slightly-too-warm 5mm suit on a 45-minute dive beats being cold and cutting the dive short in a 'technically correct' 3mm.

One thing thickness charts never tell you: a wetsuit that doesn't fit snugly won't perform at its rated thickness at all, because loose neoprene lets water flush through instead of forming the thin insulating layer it's designed around. Fit first, thickness second.