UK and Irish water temperatures rarely climb much past 18°C even in summer, and can sit in single figures over winter — which makes wetsuit thickness one of the first real buying decisions a new diver has to get right.
Rough thickness guide by water temperature
| Water temperature | Typical UK season | Suggested thickness |
|---|---|---|
| 16°C and below | Winter / early spring | 7mm semi-dry or drysuit |
| 16-18°C | Spring / autumn | 5-7mm semi-dry |
| 18-20°C | Summer | 5mm wetsuit |
| 20°C+ | Warm summer spells / abroad | 3mm wetsuit or shortie |
These are starting points, not rules — how cold you personally run, how long your dives are, and how many dives you’re doing in a day all shift the right answer. Divers who chill quickly often go a size thicker or move to a semi-dry earlier than this table suggests.
Wetsuit vs semi-dry vs drysuit
A standard wetsuit works by trapping a thin layer of water against your skin that your body heats up — it’s simple and affordable, but that layer flushes out with movement, so it loses effectiveness in colder water. A semi-dry uses tighter seals at the neck, wrists and ankles to reduce that flushing significantly. A drysuit keeps you completely dry with an air layer for insulation, and is the standard choice for UK diving through autumn and winter, or for regular divers who want one suit for the whole year.
Fit matters as much as thickness
A wetsuit that’s too loose lets water flush through constantly, undoing much of the insulation benefit regardless of thickness. A snug fit through the torso with full range of movement in the shoulders is worth prioritising over chasing a slightly thicker suit that doesn’t fit well.
Not sure which thickness suits your local dive sites? Browse our diving and snorkelling range or get in touch and tell us where and when you’re diving.