Wetsuit thickness: how to choose for your next trip
What the millimeters mean
Wetsuits are described by the thickness of their neoprene in millimeters. A 3 mm suit is a thin, flexible suit for warm water; a 5 mm adds real insulation for temperate seas; a 7 mm is a cold-water suit, often paired with a hood and gloves. Numbers written as a pair, like 5/4 mm, mean the torso uses the thicker neoprene and the arms and legs the thinner one — warmth where your core needs it, flexibility where your limbs do.
Thicker neoprene keeps more of your body heat, but it also floats more (which changes how much weight you carry), restricts movement a little, and is bulkier to pack. Choosing thickness is a comfort trade-off, not a status contest.
Matching thickness to the water
Comfort in water is personal — two divers in the same sea on the same day can disagree completely about whether it is warm. Body composition, how many dives you do per day, and how easily you chill all move the answer. As a broad starting point:
- Tropical water feels comfortable to most divers in a 3 mm suit, a shorty, or even a rash guard on the warmest days.
- Temperate water — think spring and autumn rather than summer — is where 5 mm suits earn their keep, often with a hooded vest underneath.
- Cold water calls for 7 mm of neoprene or a drysuit, along with hood, gloves, and boots.
Repetitive diving shifts you one step warmer in that list: on a liveaboard doing four dives a day, many divers who would happily wear 3 mm on a single dive reach for a 5 mm by day two, because the body never fully rewarms between dives.
Practical trip advice
- Ask the local operator what they actually wear. Dive staff answer this question every day, for their specific site and season, and their answer beats any chart.
- When between two thicknesses, take the warmer one. You can vent warm; you cannot add neoprene you did not bring. Being cold ends dives early and makes them less safe as well as less fun.
- Fit beats thickness. A well-fitting 3 mm keeps you warmer than a baggy 5 mm, because a wetsuit works by holding a thin layer of water still against your skin. If water flushes through with every kick, the millimeters hardly matter.
- Remember your weighting changes. More neoprene means more buoyancy, which means more lead and a fresh buoyancy check at the start of the trip — a standard part of any properly briefed first dive.
None of this replaces the judgment of the people running your diving. Treat it as packing guidance, and let your operator and your own training make the on-site calls.