Wetsuit Thickness Guide: What to Wear at Each Water Temperature

Wetsuit thickness chart by water temperature

The middle column below is our starting point for one recreational dive. The right column shows what manufacturers actually publish, because no single chart is authoritative. Their ranges overlap, and at 20°C (68°F) the lightest and heaviest recommendations differ by 6 mm. Pick from the middle column, then adjust for the factors in the sections that follow. For suit types and other kit, see our gear guides.

Two reference points sit behind the table. European dive suits are tested against EN 14225-1, whose thermal classes (as printed in the Waterproof and BARE manuals) are D above 22°C, C at 16-24°C, B at 10-18°C and A at 7-12°C. And DAN says most divers benefit from thermal protection in water cooler than 27°C (80°F), with significant thermal stress expected below 24°C (75°F).

Why do wetsuit temperature charts disagree?

Because they answer different questions. Waterproof's manual says its ranges come from EN 14225-1, a laboratory measurement of the thermal resistance of the suit material. The same page says the test ignores fit, tight cuffs and gas-tight zips, and that there is no general rule for which suit to wear at which temperature, since physique, workload and conditions all change the answer.

O'Neill's chart is written for surfing and other watersports. It puts a 1mm suit at 21-25°C and a 2mm or shorty at 18-20°C, far lighter than any dive chart here. DAN explains most of the gap: surf suits are cut for overhead paddling, while diving suits are often thicker to make up for the compression that happens at depth. A surfer is also never 20 m down.

The Mares chart sits at the cold end: an 8mm or a 6mm semi-dry at 20-23°C and a drysuit under 20°C. It is a guest post by a dive-centre instructor who calls the figures 'my personal guidelines' and notes that someone diving all year gets colder than a holiday diver. BARE's blog recommends thinner suits at the same temperatures. Treat the charts as a range of opinions with a laboratory standard underneath, not as one answer.

What wetsuit for 20°C (68°F) water?

A 5mm full suit, with a hood packed. That matches Waterproof's rating for its 5mm suits (16-24°C) and sits inside BARE's 17-21°C band, where it recommends a 5mm or 3mm. Twenty degrees is also 4°C below the point where DAN expects significant thermal stress, so we would not plan on a 3mm here unless you already know you run warm.

Move up to a 7mm or a semi-dry if you are making two or three dives a day; if most of the dive is at 20 m or deeper, where neoprene has lost around 40% of its insulation; if you are slim or chill easily; or if the dive is a slow drift where you hardly move. The Mares guest author would already be in an 8mm or a 6mm semi-dry at this temperature.

Off Malta, 20°C falls between late May and June: the 10-year averages are 18.9°C for May and 22.7°C for June, as the worked example at the end of this article shows.

How much warmth does a wetsuit lose at depth?

About a fifth of its insulation by 5 m and around 40% by 20 m, according to a 2025 laboratory study of 33 commercial neoprene sheets in 3, 5 and 7mm. Under a pressure equivalent to 20 m, the sheets lost 64.3% of their thickness on average (range 52.8-72.9%) and an estimated 40.9% of their thermal resistance (range 19.3-53.2%). At 5 m the average loss was 21.5%. Most of the compression happened in the first 5-10 m, which is why a suit feels thinner long before you reach your planned depth.

The study gives its own example: a 5mm suit with a surface thermal resistance of 0.12 m²K/W falls to about 0.07 at 20 m, which the authors say may be too little for extended dives below 15°C. They also argue that selection guides based on surface testing may underestimate the thickness needed for deeper recreational dives. The EN standard builds in the same expectation: a class A material needs at least 0.15 m²K/W at the surface but only 0.03 at 6 bar. BARE's manual warns that the drop 'may be substantial' and that the suit loses buoyancy as it compresses, which your BCD has to make up.

Neoprene also compresses for good. DAN notes that repeated exposure to pressure reduces a suit's thickness and warmth over time, and suggests treating a wetsuit as a depreciating item: when it stops keeping you as warm as it used to, replace it.

Cold tolerance, dive length and repetitive dives

These move the choice more than a degree or two of water temperature. DAN lists body composition as a driver of cooling: a higher lean-to-fat ratio and a lower body-mass-to-surface-area ratio both speed it up. Effort matters as well. BARE points out that a drift diver barely moves and feels heat loss sooner than someone swimming against a current.

Time adds up. DAN notes that immersion hypothermia can develop even in warm or tropical water as the body slowly cools, and BARE suggests that people who run cold wear a thin suit even above 22°C, especially on days with more than one dive. On a week of three dives a day, choose the suit for the last dive of the fifth day, not the first dive of the first.

Temperature also changes decompression stress. In a US Navy study summarised by DAN's Neal Pollock, divers kept warm on the bottom and cool during ascent and stops had a 22% DCS rate, while the reverse pattern had 1.3% despite more than twice the bottom time. Pollock's advice is to avoid overheating early in the dive, be mildly warm during ascent and skip the hot shower straight afterwards, because it can promote bubble formation. A suit that leaves you shivering on the safety stop is a decompression issue as well as a comfort one.

Hood, gloves and boots: when do you need them?

Add a hood before you add millimetres. BARE says hoods and vests add warmth to your core, and that thicker boots and gloves help fight off hypothermia even in warm water. The Mares guest post tells divers who get cold quickly to always carry a hood, and suggests a hooded vest to take an existing 5mm into colder water. O'Neill calls a hooded suit with gloves and boots a must in cold winter water.

Our rule of thumb: a hood or hooded vest from about 20°C down, gloves and thicker boots once you are in 7mm territory below about 19°C, and all three with a semi-dry.

Wetsuit, semi-dry or drysuit: where are the thresholds?

A semi-dry is a thick wetsuit built to stop water flushing through, with tight seals and a gas-tight zip. That matters because, as DAN puts it, a wetsuit's warmth depends mostly on limiting water circulation. Waterproof's manual notes that the EN laboratory rating does not account for those seals and zips, so a semi-dry's advantage is not captured in its class.

The published switch points:

• Wetsuit: Waterproof rates its 5mm suits for 16-24°C and its 7mm suits for 10-18°C.
• Semi-dry or warmest wetsuits: EN class A, 7-12°C, in both the Waterproof and BARE manuals.
• Drysuit: BARE treats water below 10°C (50°F) as a serious safety issue and a drysuit as mandatory below 6°C (43°F). The Mares guest post switches to a drysuit below 20°C.
• Cold shock: DAN warns that sudden immersion below 15°C (59°F) without thermal protection can trigger an inhalation gasp.

How should a dive wetsuit fit?

Skin tight, without restricting your breathing. DAN explains that new divers often choose suits that are too loose because they are not used to the pressure on the chest, and that poor fit undoes the warmth of even the best suit. Waterproof warns that a suit that does not fit can cause hypothermia or constriction. BARE's manual gives five checks for a properly fitted suit:

• Take a full, deep breath without resistance or extra effort.
• Join your hands above your head.
• Reach the centre of your back just below the neck.
• Touch your toes.
• Squat and kneel without restriction.

Worked example: four dive regions by season

The chart gives very different answers depending on where and when you dive. The table below takes one well-known spot in four regions and reads the 10-year average sea surface temperature for its coldest and warmest month against the chart above.

Cozumel barely changes: 26.8°C in February and 29.7°C in September, so a 3mm works all year, with a 5mm for divers who run cold. Sharm el-Sheikh in the northern Red Sea drops to 22.7°C in March, which moves you up to a 5mm in late winter and spring. Malta swings the most, from 15.5°C in February and March to 27.2°C in August. That makes three suits over a year: a 3mm from July to October, a 5mm in June and November, and a 7mm or a 5mm with hood, gloves and boots from December to May, with a semi-dry worth a look in February and March.

Plymouth, on England's south coast, peaks at 16.6°C in August, so even the summer pick is a 7mm or a 5mm with hood, gloves and boots. From November to June the water sits in the chart's two coldest bands, where the options run from a 7mm with hood and gloves to a drysuit.

Three caveats. These are surface averages for one spot, and a whole coastline can differ by a degree or two. Deeper water can be colder than the surface: PADI's guide to Kaş on the Mediterranean mentions thermoclines on deeper dives, so early-summer water at 25 m may read below these figures. And a 10-year average smooths over warm and cold years.

Log the temperature at depth, your suit and how cold you felt on every dive; after a season your own threshold beats any chart. Our dive logging guide shows what to record, and our destination guides cover where to dive in each region.