A wetsuit's thickness is written in millimetres, and those two or three numbers on the label tell you more about where a suit will work than anything else on the packaging. Choose badly and you will either shiver through a twenty-minute session or drag a suit so stiff around your shoulders that paddling becomes a chore. Reading the ratings properly takes about five minutes, and it saves both money and misery.

Here is how the numbers work, what UK water actually does through the year, and how to match the two.

What 3/2, 4/3 and 5/4/3 actually mean

The first number is the thickest part of the suit, normally the torso and thighs, because your core is what needs protecting. Any numbers after it describe thinner panels, usually in the shoulders, arms and sometimes the lower legs, where you need to move.

So a 3/2 has a 3 mm body and 2 mm arms. A 5/4/3 has a 5 mm core, 4 mm legs and 3 mm arms. A 6/5/4 hooded is exactly what it sounds like: a winter suit with a built-in hood.

Some makers also split the torso itself, which is why you occasionally see a four-number label. It is not a marketing trick so much as a map of where the warmth sits, with the chest and back often a fraction thicker than the sides.

Two suits with the same numbers can feel completely different. Stretchier neoprene, glued and blindstitched seams, taped or liquid-sealed seams, and a well-shaped neck all add warmth without adding millimetres. A good 4/3 will often outperform a cheap 5/4 in real conditions, simply because less cold water flushes through it. It is worth paying attention to seam construction and to how a suit fits across the chest and shoulders, because those details do more for your comfort than an extra millimetre of rubber.

UK water temperature through the year

Sea temperatures lag behind the air, which is why the water is at its coldest in late winter and at its warmest in late August or September. If you are buying in June, buy for the water you will be in come October. As a rough rule, UK coastal water moves between about 6°C and 18°C across the year, and the difference between a good day and a grim one is often the wind rather than the number on a thermometer.

Where you paddle matters too. The south-west and the west coast of Scotland catch the tail of the Gulf Stream, so Cornwall and parts of the Hebrides can be two or three degrees warmer than the east coast at the same time of year. The North Sea is shallow and cools quickly, so East Anglia and the north-east often feel a grade colder than the map suggests.

Summer

Around the south-west and the south coast, a good August can reach 16-18°C in sheltered bays. The North Sea and most Scottish coasts are usually a couple of degrees cooler. A 3/2 works well for shorter sessions and stronger swimmers; if you feel the cold, or plan to be in for an hour or more, a 4/3 is the safer pick. A rash vest underneath adds a little warmth for very little bulk, and it makes the suit easier to get on and off.

Spring and autumn

This is where most UK paddling actually happens, and the water typically sits somewhere between 10°C and 14°C. A 4/3 is the workhorse here, with a 5/4 for the colder end and for anyone who knows they chill quickly. Boots and gloves start to earn their place, and a hood or neoprene cap becomes genuinely useful on windy days.

Winter

Expect roughly 6-9°C around most of the coast, colder in the north. A 5/4/3 is a sensible minimum for regular surfing or swimming, with a 6/5/4 and a hood for long sessions. Even in a thick suit, time in the water matters: forty minutes in a good 5/4 beats ninety minutes in a 6/5 you have soaked through. Cold water also drains energy faster than most people expect, so plan shorter sessions, eat before you go and warm up properly afterwards.

Match the suit to the activity

Thickness is only half the decision. What you are doing changes how much you move, how often you get submerged and how much wind chill you face.

• Surfing: constant immersion and heavy paddling. A 4/3 covers spring to autumn, a 5/4/3 or 6/5/4 hooded handles winter. Thin arms keep your paddling efficient, which matters more than most people realise when a set is bearing down on them.
• Open water swimming: swim-specific suits are usually 1.5-3 mm, with smooth panels to reduce drag. A 3/2 is plenty for a summer sea swim, a 4/3 for the shoulder seasons. Check the rules of any organised event before you enter, as some ban buoyancy panels or cap thickness.
• Paddleboarding and kayaking: you are often wet rather than submerged, so wind chill is the real problem. A 2-3 mm suit for summer and a 4/3 with a windproof shell for cooler months works well.
• Diving: different kit altogether. UK diving wetsuits generally start around 5 mm and climb from there, and many divers use a semi-dry or drysuit instead.
• Family and casual use: if you are in and out of the water with children, a 3/2 for summer and a 5/4 for spring and autumn keeps everyone comfortable without a wardrobe of options.

Fit matters more than millimetres

A snug 4/3 with good seals will keep you warmer than a loose 5/4 every time. Water flushing through the neck, wrists and ankles is what makes people cold, not the label.

• The suit should feel firm but not restrict your breathing or cut into your neck.
• Check for gaps at the neck, wrists and ankles, and for bunching behind the knees or under the arms.
• Reach overhead, rotate your shoulders and squat deeply. If either movement is fighting the suit, size up or try a different cut.
• Remember that neoprene relaxes slightly in the water, so a suit that is merely comfortable in the shop can feel loose once it is wet.

Warmth you can add later

If you are between two sizes or two thicknesses, spend the difference on accessories. They are cheaper than a new suit, and they extend the range of what you already own.

• Boots: 3 mm for summer, 5 mm or 7 mm for winter. Cold feet end sessions early.
• Gloves or mitts: mitts are warmer because your fingers share heat. 2-3 mm is enough for most UK shoulder-season sessions.
• Hood or cap: the single biggest warmth gain per pound. A hooded vest under a suit can transform a 4/3 into a near-winter setup.
• Rash vest and thermal layers: a thin thermal top under a suit adds insulation without changing the fit.

If you only remember one thing, make it this: buy for the water temperature you will actually be in, not the air temperature on the day you order.

Building a two-suit quiver

Most UK paddlers do not need a full wardrobe. A 3/2 or 4/3 for late spring through early autumn, plus a 5/4/3 or hooded 6/5/4 for winter, covers almost every session on the coast. If you can only buy one, a good 4/3 with sealed seams is the closest thing to a year-round suit, and you can add boots, gloves and a hood as the water cools.

Try before you buy where you can, read the seams as carefully as the numbers, and ignore the temptation to over-suit. The right thickness is the one that lets you stay in long enough to enjoy it.

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