What wetsuit thickness do you need?
Thickness is quoted as two numbers — 4/3 mm means 4 mm around the torso and 3 mm at the limbs, where flexibility matters more than warmth. Thicker is warmer and stiffer. Below, month by month for 906 coastal cities across 92 countries.
Swimming and surfing are not the same answer
Nearly every wetsuit chart online is a surfing chart, because surf brands are the ones who publish them. Surfing is close to the worst case: sessions run one to three hours and most of that is spent sitting still on the board, torso and head out of the water and in the wind. Very little heat is being generated and a great deal is being lost.
Swimming is the opposite case. A steady front crawl produces several hundred watts of heat continuously, and you are horizontal and fully immersed rather than sitting in a breeze. Open-water swimmers routinely wear a full step less neoprene than a surf chart calls for. Handing both groups the same number is the most common mistake in this, so both are given here and on every city page.
| Water | Swimming | Surfing | Notes |
|---|---|---|---|
| 24°C and above | nothing | boardshorts | A rash vest for sun, or against the board — not for warmth. |
| 22–24°C | nothing | boardshorts, or a rash vest against the board | Long swims without neoprene are comfortable. |
| 19–22°C | nothing, or a rash vest | shorty or a 2 mm top | The band where acclimatisation matters most. |
| 16–19°C | shorty, or skins if you are used to it | 3/2 mm full suit | Most people want neoprene beyond twenty minutes. |
| 13–16°C | 3/2 mm full suit | 4/3 mm full suit | Boots make more difference than most expect. |
| 10–13°C | 4/3 mm full suit | 5/4 mm full suit | Extremities fail first. Gloves stop being optional. |
| Below 10°C | 5/4 mm full suit | 6/5 mm full suit | Specialist territory. Never alone, never far from shore. |
Buy for the season you swim
The useful question is almost never "what does the coldest month need". It is "if I buy one suit, which one" — so each city page names the thickness covering the most months people actually swim there, rather than the one that would let you swim in February. Sizing for the coldest week is how people end up owning a 5/4 they wear twice a year and overheat in for the rest.
What thickness does not fix
- Fit beats millimetres. A suit works by holding a thin layer of water still against your skin. A loose 5/4 keeps flushing that layer and replacing it with cold sea, and will lose to a well-fitted 3/2.
- Extremities go first. Hands and feet have little muscle to warm them and a poor surface-area-to-volume ratio, so they lose heat far faster than the torso a suit covers. Boots below 14°C, gloves below 12°C, a hood below 10°C — they often add more usable time than a thicker suit would.
- Getting out is worse than getting in. Most cold injuries happen after the swim. Soaked neoprene in wind cools you faster than standing there in dry clothes. Change immediately; do not stand about in it.
736 of the coastal cities covered here never fall below 22°C, so they get no wetsuit page — the answer is simply no, and it appears on their sea temperature page instead.
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Thicknesses are retail and open-water convention rather than physics. Individual cold tolerance varies by a couple of degrees in both directions, and acclimatisation over a season moves it further. Full method