3mm vs 5mm vs 7mm wetsuits: which thickness for scuba diving
Summary
Three wetsuit thicknesses cover most recreational scuba diving. A 3mm suits warm water of 72–77°F (22–25°C), a 5mm is the temperate workhorse at 63–72°F (17–22°C) with a hood at the colder end, and a 7mm covers cold water of 50–63°F (10–17°C), usually with a hood and gloves. Each step up in thickness buys warmth and costs lead, mobility, and effort. Fit matters more than thickness: a well-fitted 5mm is warmer than a loose 7mm, because a loose suit flushes cold water continuously. For a single suit, most divers are best served by a 5mm plus a hood.
What each thickness costs
- 3mm: the least lead, easiest swimming, easiest to don and doff, and packs small for travel. Runs out of warmth below about 72°F (22°C).
- 5mm: a few pounds more lead than a 3mm and noticeably stiffer, but covers the widest range of real dive sites, and with a hood it holds down to about 63°F.
- 7mm: several pounds more lead again and meaningfully restrictive, and difficult to put on when wet. In return it opens up water most divers would otherwise skip.
Fit beats thickness
A wetsuit keeps a diver warm by trapping a thin layer of water against the skin, which the body then heats. If the suit is loose, that water flushes and refreshes with every kick, so the diver is continuously warming new cold water. A well-fitted 5mm comfortably outperforms a baggy 7mm. This is the most common problem with rental suits: a suit loose at the small of the back, the armpits, or the neck should be exchanged before the dive.
Which one to buy first
- Warm-water traveller: a 3mm. Tropical rentals are 3mm regardless, so owning one that fits properly is the real upgrade.
- Most divers, most places: a 5mm plus a hood. It spans more sites than either neighbouring thickness and absorbs a cold surprise.
- Cold-water local: a 7mm semi-dry with hood and gloves, and a drysuit if diving year-round.
Layering is an underrated alternative: a 3mm suit with a 3mm hooded vest beneath approaches a 5mm in warmth while remaining two packable, more versatile pieces.
Recheck weighting when the suit changes
Because neoprene is buoyant, each step up in thickness requires a few more pounds of lead. Divers who move from a 3mm to a 5mm and keep their previous lead commonly find they cannot descend, and attribute it to themselves rather than the suit. Suit thickness is an input to the weight calculation, and the new number should be confirmed with a surface buoyancy check.
Frequently asked questions
Should I buy a 3mm, 5mm, or 7mm wetsuit for diving?
Buy for the water you actually dive, weighted toward its colder end. A warm-water traveller wants a 3mm, since tropical rentals are 3mm anyway. Most divers in most places are best served by a 5mm plus a hood, which spans the widest range of dive sites. A cold-water local wants a 7mm semi-dry with hood and gloves.
Is a well-fitted thinner wetsuit warmer than a loose thicker one?
Yes. A wetsuit works by trapping a thin layer of water that the body then warms. A loose suit lets that water flush and refresh with every kick, so the diver continuously heats new cold water. A well-fitted 5mm outperforms a baggy 7mm.
How much more weight does a thicker wetsuit need?
Each step up in thickness adds a few pounds of lead, because neoprene is buoyant. Divers who move from a 3mm to a 5mm and keep the same lead commonly struggle to descend, so weighting should be rechecked whenever the suit changes.