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What 3/2, 4/3, and 5/4 mean
60 °F (15.3 °C). That’s the average water temperature in Monterey Bay, off Santa Cruz, California, in September, the warmest month of the year, based on our calculation from ten years of NOAA satellite readings [1]. To a wetsuit maker like Rip Curl, that’s fullsuit territory, a 4/3, not a spring suit [2]. Before you pick one, though, you need to know how to read those two numbers.
Wetsuit thickness is written as two numbers separated by a slash. The first is the thickness in millimeters of the torso panels, the second that of the arms and legs: a 4/3 is 4 mm on the torso and 3 mm on the limbs [2]. The logic is simple: thick neoprene insulates better, but it gets in the way when you paddle, pop up on the board, or swim, so the thickness goes where your body loses the most heat, the core, and gets trimmed where you need to flex, at the shoulders, elbows, and knees [2].
Which thickness for which water temperature
Rip Curl’s rule fits in a sentence: the warmer the water, the less neoprene you need, and every drop of about 4 °F (2 °C) usually moves you up a thickness [2]. The brand publishes the chart it uses as a baseline.
| Water temperature | Thickness | Type of wetsuit | Accessories |
|---|---|---|---|
| Above 75 °F (24 °C) | 0 to 1 mm | Boardshorts or rash guard | None |
| 64 to 75 °F (18 to 24 °C) | 2 mm | Spring suit or top | None |
| 61 to 64 °F (16 to 18 °C) | 3/2 mm | Fullsuit | Booties optional |
| 55 to 61 °F (13 to 16 °C) | 4/3 mm | Fullsuit | Booties recommended |
| 48 to 55 °F (9 to 13 °C) | 4/3 or 5/4 mm | Fullsuit | Booties and gloves |
| Below 48 °F (9 °C) | 5/4 or 6/5 mm | Hooded fullsuit | Booties, gloves, and hood |
Rip Curl’s chart, designed for surfing; °F values converted from the brand’s Celsius figures. Rip Curl adds that a 3/2 suits water of about 57 to 68 °F (14 to 20 °C) and a 4/3 water of 52 to 63 °F (11 to 17 °C): between 57 and 63 °F (14 and 17 °C), it’s your call.
The chart is only a starting point. Rip Curl names the factors that bump you up a thickness: a cold wind at dawn, a session that runs past 90 minutes, since heat loss adds up, and your own tolerance for cold. In 55 °F (13 °C) water, some surfers get by in a 3/2, while others reach for a 4/3 well above 61 °F (16 °C) [2].
Booties, gloves, and hood: at what temperature
Below 59 °F (15 °C), accessories do the real work: your hands, feet, and head lose heat the fastest, and covering them well can add an hour or more to a session without switching wetsuits [2].
- Booties: below 59 °F (15 °C), or sooner if you have a long walk over cold sand or sharp rocks [2].
- Gloves: below 55 °F (13 °C), they become essential for sessions longer than an hour [2].
- Hood: below 50 °F (10 °C), your head becomes the biggest source of heat loss; below 45 °F (7 °C), a hood built into the suit seals the neck better [2].
California, the East Coast, Florida: US waters, season by season
Here’s the detail that changes everything: which coast you’re on. To know what to expect, we calculated the average sea surface temperature, month by month, from 2016 to 2025, at six points off the US coast, from California to Maine and Florida, using NOAA’s daily satellite data (the OISST dataset, version 2.1) [1].
| Location | Coldest month | Warmest month | Thickness in the coldest month | Thickness in the warmest month |
|---|---|---|---|---|
| Southern California, off San Diego | 60 °F (15.3 °C), February | 71 °F (21.7 °C), August | 4/3, booties | 2 mm |
| Central California, Monterey Bay, off Santa Cruz | 54 °F (12.4 °C), April | 60 °F (15.3 °C), September | 4/3 or 5/4, booties and gloves | 4/3 |
| Northern California, off San Francisco | 53 °F (11.9 °C), April | 60 °F (15.4 °C), September | 4/3 or 5/4, booties and gloves | 4/3 |
| Maine, off Portland | 40 °F (4.3 °C), March | 65 °F (18.4 °C), August | 5/4 or 6/5, booties, gloves, and hood | 2 mm |
| North Carolina, Outer Banks | 57 °F (13.7 °C), February | 82 °F (27.6 °C), July and August | 4/3, booties | 0 to 1 mm |
| Florida, off Miami | 78 °F (25.5 °C), February | 86 °F (30.1 °C), August | 0 to 1 mm | 0 to 1 mm |
Calculation: average of NOAA OISST v2.1 satellite readings (one reading every five days, on a grid of about 16 miles, or 25 km), at one point offshore; °F converted from the Celsius averages and rounded to whole numbers. Thicknesses taken from Rip Curl’s chart. Close to shore, the water can stray from these averages, and no two years are alike.
Three takeaways stand out. Northern and Central California stay cool year-round: off Santa Cruz and San Francisco, the warmest month is September, and its average stops at 60 °F (15.3 and 15.4 °C), so a 4/3 is the summer standard. Southern California is milder: off San Diego, the water sits in 2 mm territory from June to October, at 71 °F (21.7 °C) in August. The East Coast swings the most: off Portland, Maine, the water climbs from 40 °F (4.3 °C) in March to 65 °F (18.4 °C) in August, which calls for two wetsuits, or a well-chosen season, while off Miami it never drops below 78 °F (25.5 °C) [1].
The same reasoning works on any coast: find the monthly average for your own stretch of ocean, then read it against the chart. These averages don’t replace the temperature on the day, which shifts with the weather. And before you head to a tidal beach, checking the tide times is part of the same habit.
Cold water: dangerous from the very first minutes
The cold isn’t only a risk after two hours in the water. The National Weather Service warns that cold shock can trigger involuntary gasping and very rapid breathing during the first 2 to 3 minutes in cold water, putting even strong swimmers at risk of drowning; water at 50 to 60 °F (10 to 15 °C) can cause a shock as severe as water at 35 °F (2 °C), and gasping can be triggered by water as warm as 77 °F (25 °C) [3].
Once you’ve found the right thickness, cold water is on your side: nearly empty beaches, low winter light, and that soft warmth seeping back into your wetsuit after the first few waves.
FAQ
What does 4/3 mean on a wetsuit?
It’s the neoprene thickness in millimeters: 4 mm on the torso, 3 mm on the arms and legs. The thickness is concentrated on the torso, where your body loses the most heat, and reduced on the limbs so you can move freely. According to Rip Curl’s chart, a 4/3 suits water of about 52 to 63 °F (11 to 17 °C).
What wetsuit do I need for 59 °F (15 °C) water?
According to Rip Curl’s chart, water at 55 to 61 °F (13 to 16 °C) calls for a 4/3 fullsuit, with booties recommended. Between 57 and 63 °F (14 and 17 °C), a 3/2 can do the job if you handle cold well, your session is short, and there’s no wind. The brand recommends going up a thickness for long sessions or in a cold wind.
How warm is the ocean in California in summer?
It depends on the stretch of coast. Our calculation for 2016 to 2025, based on NOAA satellite data, gives an average of 60 °F (15.3 °C) in Monterey Bay, off Santa Cruz, in September, the warmest month, and 71 °F (21.7 °C) off San Diego in August. Close to shore, the water can run warmer or colder. Going by Rip Curl’s chart, that’s still 4/3 territory in Northern and Central California at the height of summer, and 2 mm in Southern California.
How cold does water have to be to be dangerous?
Not as cold as you’d think. The National Weather Service says cold shock can be just as severe in water at 50 to 60 °F (10 to 15 °C) as at 35 °F (2 °C), and that involuntary gasping can be triggered by water as warm as 77 °F (25 °C). Ease in, don’t go out alone, and get out as soon as you start shivering.
Sources
- NOAA, NCEI, “Daily Optimum Interpolation Sea Surface Temperature (OISST), version 2.1,” data for 2016 to 2025 queried on October 22 and 23, 2026, from NOAA’s ERDDAP server, averages calculated by the editorial team. coastwatch.pfeg.noaa.gov https://coastwatch.pfeg.noaa.gov/erddap/info/ncdcOisst21Agg_LonPM180/index.html ↩
- Rip Curl, “Wetsuit Temperature & Thickness Guide,” accessed October 22, 2026. ripcurl.com.au https://www.ripcurl.com.au/blogs/products/wetsuit-thickness-guide ↩
- National Weather Service (NOAA), “Cold Water Hazards and Safety,” accessed October 22, 2026. weather.gov https://www.weather.gov/safety/coldwater ↩
Trail running, snow, water, nature and destinations
Nolwenn covers trail running, snow, water, nature and destinations for Outfarer. Curious about everything, from meteor showers to night-train schedules, she has a knack for digging up the one detail that changes everything, and nailing it in a single sentence.
Arnaud Couderc