Choosing wetsuit thickness for local water temperatures
Use water temperature as the starting point for wetsuit thickness, then adjust for fit, layering and dive length, using a rough range table to plan from.
Water temperature is the main factor in choosing wetsuit thickness, because the gas cells in the neoprene insulate, and a snug fit limits how much cold water flushes through the suit. Thickness is given in millimeters, often as two numbers for the torso and the limbs, and thicker neoprene holds more heat. Air temperature, wind, the length of the dive and your own tolerance for cold all adjust the result, so a temperature range is a starting point rather than a rule.
For most divers the short answer is to choose the suit for the coldest water they expect to meet, then adjust for the day.
Starting from water temperature
Begin by checking the typical water temperature for the site and season, since the water can be colder at depth or in certain months. The table gives rough ranges that many divers use as a starting point. Boundaries differ between manufacturers and between people, and conditions on the day can shift them, so treat the figures as approximations.
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| Water temperature | Common choice |
|---|---|
| Above about 26 °C (79 °F) | A light rash guard, or no suit |
| About 24 to 26 °C (75 to 79 °F) | A shorty or a suit of about 1 to 2 mm |
| About 20 to 24 °C (68 to 75 °F) | A 3 mm full suit or a shorty |
| About 16 to 20 °C (60 to 68 °F) | A 5 mm full suit, with a hood often advised |
| About 12 to 16 °C (54 to 60 °F) | A 5 to 7 mm suit with a hood and gloves |
| Below about 12 °C (54 °F) | A 7 mm suit or thicker, or a drysuit |
Use the table to decide what to bring, then confirm with the local operator what the water usually feels like. Operators know how the temperature changes across the season and how cold the water gets on the deeper parts of a site.
Suit design changes the warmth as well. A full suit covers the torso, arms and legs, a shorty leaves the arms and upper legs open, and a vest or two-piece layer can be easier to adjust during a day of changing conditions. Choose the design by the expected water temperature and by how much time you spend out of the water, since exposed skin loses heat to air and wind as well as to the sea.
Fit and layering
A thicker suit only keeps you warm if it fits closely. Water that flushes through gaps at the neck, wrists and ankles removes the warm layer, so a loose 5 mm suit can feel colder than a snug 3 mm suit. Try the suit on, then bend, kneel and reach. Check that it closes without strain and without gaps at the cuffs and collar. A suit that restricts breathing or limits shoulder movement will make a dive harder, whatever its thickness. Suits that have stretched out or sagged also let water flush more freely, so check an older suit before a cold trip. A suit can feel snug on land and still let water in once you are kneeling, kicking or swimming, so a short session in shallow, calm water is the best test of fit before you rely on the suit in colder conditions.
Layering can extend the range of a suit. A rash guard under a wetsuit adds a little warmth and reduces chafing, while a hood, gloves or boots cover the places that lose heat fastest. Hands and feet become uncomfortable quickly in cold water, and numb fingers make it harder to operate equipment, so these accessories often help more than moving up a full thickness. Thicker is not always better in warm water, though. A thick suit can overheat during surface intervals. Its neoprene also adds buoyancy, so a thicker suit needs more lead to get you down, and that buoyancy shrinks as the neoprene compresses with depth. Re-check your weighting whenever you change suit thickness. Two suits for two water temperatures may serve you better than one compromise.
When a thicker wetsuit is not enough
Some trips call for more than a thicker wetsuit. Multiple dives per day, long surface intervals in cold air and boat trips where wind chill is strong all increase heat loss, and a suit that works for one dive may not be enough for the whole day. In very cold water, a drysuit with suitable undergarments keeps the body dry, and many divers find it more comfortable than a thick wetsuit in those conditions. Drysuit use is a separate skill, so learn it from a qualified instructor before you depend on one.
Whatever the suit, shivering, numbness or difficulty concentrating during a dive are signs to end the dive and warm up. Cold can affect judgment and make controlled breathing harder, so a suit that looks adequate on paper may not be adequate in practice. Ask the operator what they recommend for the water and the season, and bring a backup layer such as a hood or an extra top in case conditions turn colder than expected. The table is a sensible place to start, but the final choice should match your body and the trip in front of you.