Free triathlon tool
Will you need a wetsuit?
Water temperature decides whether your wetsuit is compulsory, optional, or banned outright — and the thresholds move depending on the rulebook, your swim distance and your age. This tool puts sixteen years of temperature history for your venue against the rules that actually govern your race, so you know what to expect months before the official reading is taken on race morning.
62 venues are pre-calculated — every UK race Verus covers, plus the major international races — and any other venue on earth can be looked up by name.
The short answer
A wetsuit is mandatory below 14°C for triathlon swims up to 1500m under British Triathlon rules, and below 16°C for swims of 1501m and longer. It is forbidden above 22°C for most athletes, rising to 24.6°C for those aged 60 and over or racing beyond 1500m. Below 11°C the swim cannot legally take place at all. IRONMAN and World Triathlon set their own thresholds, which the calculator below applies separately.
Wetsuit Calculator
Whether you'll need a wetsuit at a given venue and date — under the rulebook that actually governs your race.
Big East Triathlon · 16 May
Wetsuit mandatory
Below 14°C a wetsuit is compulsory for a 1500m swim under rule 4.2(a).
Typical water temp
11.9°C
10.6–13.3°C in 8 of 10 years
Coldest on record
9.7°C
across 16 years
Warmest on record
14.2°C
calibrated to measurements
What would have happened in each of the last 16 years
- Swim cancelled3 of 16
- Wetsuit mandatory12 of 16
- Optional (recommended)1 of 16
Calibrated estimate. Fitted to measured sea-surface data and accurate to within about 1.2°C of it, then applied across 16 years of temperature records. Only the official reading taken an hour before the start decides the actual rule on the day.
Triathlon wetsuit rules by temperature
Three rulebooks cover almost every race an age-grouper will enter, and they disagree with each other. The same 23°C water bans your wetsuit under British Triathlon at Olympic distance, permits it under IRONMAN, and permits it under British Triathlon too if you happen to be 60. These are the published thresholds.
| Rulebook | Swim length | Age | Mandatory below | Forbidden above |
|---|---|---|---|---|
| British Triathlon | Up to 1500m | Up to 59 | 14°C | 22°C |
| British Triathlon | Up to 1500m | 60 and above | 14°C | 24.6°C |
| British Triathlon | 1501m and above | All ages | 16°C | 24.6°C |
| World Triathlon | Up to 1500m | Age-group, under 60 | 15.9°C | 22.0°C |
| World Triathlon | 1501m and above | Age-group, under 60 | 15.9°C | 24.6°C |
| World Triathlon | All distances | Age-group, 60+ | 15.9°C | 24.6°C |
| World Triathlon | Up to 1500m | Elite / U23 / junior | 15.9°C | 20.0°C |
| IRONMAN / 70.3 | All distances | All ages | 16.0°C (60.8°F) | 24.5°C (76.1°F)* |
* IRONMAN alone has a middle band. Between 24.5°C and 28.8°C (76.1-83.8°F) you may still wear a wetsuit, but doing so makes you ineligible for age-group awards and World Championship slots. Above 28.8°C it is banned outright.
British Triathlon
Races in Great Britain permitted by British Triathlon, Triathlon England, Scotland or Wales
British Triathlon Competition Rules, sections 4.2 and 4.4
World Triathlon
International races, and the basis most national federations adopt
World Triathlon Competition Rules (2026), wetsuit temperature table
IRONMAN / 70.3
All IRONMAN and IRONMAN 70.3 branded races worldwide
IRONMAN Competition Rules, swim course and wetsuit provisions
How cold is too cold to race?
Below 11°C, British Triathlon rule 4.4(a) does not permit open-water competition at any distance. Between 11°C and 13°C the swim must be shortened on a fixed ladder — this is the rule that turns a middle-distance race into a sprint on a cold morning.
Maximum swim distance in cold water
- 13°C max 2000m
- 12°C max 1000m
- 11°C max 500m
- Below 11°C no swim permitted
These are water temperatures alone and assume wind chill is negligible. Where wind chill is significant, organisers may shorten the swim at higher temperatures than these — and may adapt the limits for current, heavy rain or changing conditions. The final decision is made one hour before the start.
Water temperature at every UK race we cover
36 open-water venues, each estimated for the week its race actually runs, and ruled under British Triathlon at that race's own swim distance for a 35-year-old. Coldest first — Ironman 70.3 Bolton at 11.3°C, through to Cairns Airport IRONMAN Cairns at 25°C. Pool swims are excluded, since the question doesn't arise.
| Race | Date | Water | Swim | Typical | Likely ruling |
|---|---|---|---|---|---|
| Ironman 70.3 Bolton | 6 Jun | Inland lake | 1900m | 11.3°C (9.6–12.4) | mandatory |
| Big East Triathlon | 16 May | Tidal estuary | 1500m | 11.9°C (10.6–13.3) | mandatory |
| Cotswold113 | 6 Jun | Inland lake | 1900m | 12.4°C (10.8–13.3) | mandatory |
| Grafman Middle Distance | 6 Jun | Reservoir | 1900m | 12.4°C (11.1–13.5) | mandatory |
| Chester Triathlon | 6 Jun | River | 1500m | 12.4°C (10.5–13.4) | mandatory |
| Blenheim Palace Triathlon | 5 Jun | Inland lake | 750m | 12.5°C (10.8–13.3) | mandatory |
| IRONMAN 70.3 Staffordshire | 11 Jun | Reservoir | 1900m | 12.5°C (11.1–13.6) | mandatory |
| Shropshire Triathlon | 19 Jun | Inland lake | 1500m | 13.3°C (11.3–14.4) | mandatory |
| Royal Windsor Triathlon | 13 Jun | River | 1500m | 13.4°C (12.6–14.8) | mandatory |
| Cholmondeley Castle Triathlon & Multisport Festival | 26 Jun | Inland lake | 1500m | 13.7°C (12.4–15.6) | mandatory |
| Triathlon y Bala — Legend | 6 Sept | Inland lake | 1900m | 15°C (13.8–16) | mandatory |
| Hever Castle Triathlon & Multisport Festival | 26 Sept | Inland lake | 1500m | 15°C (14.2–16.8) | optional — and recommended |
| Grafham Triathlon | 26 Sept | Reservoir | 1500m | 15.2°C (13.7–16.7) | optional — and recommended |
| Cotswold Classic | 11 Jul | Inland lake | 1900m | 15.3°C (14.5–17.5) | mandatory |
| Challenge Canberra 100 | 22 Nov | Inland lake | 2000m | 15.3°C (14.1–16.8) | mandatory |
| Cowman Triathlon | 20 Sept | Inland lake | 1900m | 15.5°C (14.4–16.9) | mandatory |
| Wimbleball Middle Distance | 30 Aug | Reservoir | 1900m | 15.6°C (14.4–16.9) | mandatory |
| St Neots Triathlon | 20 Sept | River | 1500m | 15.9°C (14.7–17.3) | optional — and recommended |
| Ironman Leeds | 15 Aug | Inland lake | 3800m | 16.6°C (14.6–17.9) | optional — and recommended |
| New Forest Middle Distance | 6 Sept | Inland lake | 1900m | 16.7°C (15.8–18) | optional — and recommended |
| IRONMAN 70.3 Melbourne | 8 Nov | Open sea | 1900m | 16.7°C (15.6–18) | optional — and recommended |
| Outlaw Original | 25 Jul | Inland lake | 3800m | 16.8°C (15.8–18.5) | optional — and recommended |
| Ironman Wales | 13 Sept | Open sea | 3800m | 17.1°C (16.5–18.4) | optional — and recommended |
| Ironman 70.3 Weymouth | 20 Sept | Open sea | 1900m | 17.7°C (17.1–18.9) | optional — and recommended |
| IRONMAN 70.3 Western Sydney | 2 May | Inland lake | 1900m | 17.7°C (16.9–19) | optional — and recommended |
| Thorpe Park Triathlon | 23 Aug | Inland lake | 750m | 17.8°C (16.6–19.8) | optional — and recommended |
| IRONMAN Australia | 18 Oct | River | 3800m | 17.8°C (17.1–19.3) | optional — and recommended |
| IRONMAN 70.3 Port Macquarie | 18 Oct | River | 1900m | 17.8°C (17.1–19.3) | optional — and recommended |
| The Bedford Triathlon | 15 Aug | River | 1500m | 18°C (16.4–20.2) | optional — and recommended |
| T100 London | 21 Aug | Enclosed dock | 2000m | 18.6°C (16.8–20) | optional — and recommended |
| Nepean Triathlon | 22 Nov | Inland lake | 1000m | 19.3°C (18.1–20.4) | optional — and recommended |
| IRONMAN 70.3 Sunshine Coast | 13 Sept | Open sea | 1900m | 19.7°C (19.3–20.8) | optional — and recommended |
| IRONMAN Western Australia | 6 Dec | Open sea | 3800m | 20.7°C (19.5–22) | optional — and recommended |
| IRONMAN 70.3 Geelong | 21 Mar | Open sea | 1900m | 21.3°C (19.1–22.9) | optional — and recommended |
| Garmin Noosa Triathlon | 1 Nov | Open sea | 1500m | 22.7°C (22–23.5) | not permitted |
| Cairns Airport IRONMAN Cairns | 13 Jun | Open sea | 3800m | 25°C (23.8–25.7) | not permitted |
Figures in brackets are the range covering 8 of every 10 years. 9 of these venues are calibrated against measured sea-surface data; the rest are modelled from air temperature. See the method below.
Water temperature at the major international races
The same treatment for the races people travel for, each judged under the rulebook it actually runs under — which is why Kona shows a different outcome from a British race at a similar temperature. Coldest first.
| Race | Swim venue | Swim | Typical | Likely ruling |
|---|---|---|---|---|
| IRONMAN Hamburg | Binnenalster, Germany | 3800m | 13.4°C (11.3–15.8) | mandatory |
| Escape from Alcatraz Triathlon | San Francisco Bay, California, USA | 2400m | 14.5°C (13.7–15.8) | mandatory |
| IRONMAN 70.3 Oceanside | Oceanside Harbor, California, USA | 1900m | 16°C (14.9–17.5) | optional — and recommended |
| IRONMAN New Zealand | Lake Taupō, New Zealand | 3800m | 16.6°C (16–17.4) | optional — and recommended |
| Norseman Xtreme Triathlon | Hardangerfjord, Eidfjord, Norway | 3800m | 16.9°C (14.6–20.2) | optional — and recommended |
| IRONMAN Vitoria-Gasteiz | Landa reservoir, Spain | 3800m | 17.1°C (15.9–18.4) | optional — and recommended |
| IRONMAN 70.3 St. George | Sand Hollow Reservoir, Utah, USA | 1900m | 17.4°C (15.6–18.6) | optional — and recommended |
| IRONMAN European Championship (Frankfurt) | Langener Waldsee, Germany | 3800m | 18°C (16.8–20.9) | optional — and recommended |
| Challenge Roth | Main-Donau Canal, Hilpoltstein, Germany | 3800m | 18.3°C (17.1–19.8) | optional — and recommended |
| IRONMAN Copenhagen | Amager Strandpark, Denmark | 3800m | 18.7°C (17.5–20) | optional — and recommended |
| IRONMAN Switzerland (Thun) | Lake Thun, Switzerland | 3800m | 19°C (17–19.9) | optional — and recommended |
| IRONMAN Mont-Tremblant | Lac Tremblant, Quebec, Canada | 3800m | 19.1°C (18.1–20.4) | optional — and recommended |
| IRONMAN Lake Placid | Mirror Lake, New York, USA | 3800m | 19.2°C (17.7–20.2) | optional — and recommended |
| IRONMAN Austria (Klagenfurt) | Wörthersee, Austria | 3800m | 19.3°C (18.5–21.4) | optional — and recommended |
| IRONMAN Portugal (Cascais) | Baía de Cascais, Portugal | 3800m | 19.7°C (18.8–20.6) | optional — and recommended |
| IRONMAN 70.3 Dubai | Jumeirah Beach, UAE | 1900m | 19.7°C (18.9–20.6) | optional — and recommended |
| IRONMAN Texas | Lake Woodlands, The Woodlands, Texas, USA | 3800m | 20.3°C (18.3–21.8) | optional — and recommended |
| IRONMAN African Championship | Hobie Beach, Gqeberha, South Africa | 3800m | 20.9°C (20.4–21.7) | optional — and recommended |
| IRONMAN Wisconsin | Lake Monona, Madison, Wisconsin, USA | 3800m | 21.6°C (20.3–22.3) | optional — and recommended |
| IRONMAN Chattanooga | Tennessee River, Tennessee, USA | 3800m | 22.9°C (21.1–25.6) | optional — and recommended |
| IRONMAN France (Nice) | Baie des Anges, Nice, France | 3800m | 23.4°C (22.3–25.5) | optional — and recommended |
| IRONMAN Barcelona | Calella beach, Spain | 3800m | 23.4°C (22.1–24.1) | optional — and recommended |
| IRONMAN Maryland | Choptank River, Cambridge, Maryland, USA | 3800m | 24.3°C (22.9–25.4) | optional — and recommended |
| IRONMAN Italy (Emilia-Romagna) | Cervia beach, Italy | 3800m | 24.3°C (22.3–25.7) | optional — and recommended |
| IRONMAN World Championship (Kona) | Kailua Bay, Kailua-Kona, Hawaii, USA | 3800m | 27.2°C (24.5–28.1) | Allowed, no awards |
| IRONMAN Cozumel | Chankanaab, Cozumel, Mexico | 3800m | 28.8°C (28.5–29.5) | Allowed, no awards |
12 of these 26 venues are calibrated against measured sea-surface data. Dates are the month each race typically runs; organisers move them by a week or two, and the calculator above lets you set your own.
What each temperature actually feels like
A number on a page is not the same as getting in. This is roughly what to expect, and what most people find hard about each band.
Below 11°C
Racing not permitted
British Triathlon does not allow open-water competition here. Cold shock is a genuine drowning risk for unacclimatised swimmers.
11-14°C
Seriously cold
Wetsuit compulsory, and swims are shortened. Expect gasping for the first two minutes, numb hands, and a real struggle to sight. Neoprene cap and booties are usually permitted — check with the organiser.
14-16°C
Cold
The typical British race-morning temperature. Manageable in a wetsuit, still a shock on entry. Get in before the start if the organiser allows an acclimatisation swim.
16-19°C
Comfortable in neoprene
Where most UK summer races land. Wetsuit optional under some rulebooks but almost everyone wears one for the buoyancy and speed.
19-22°C
Warm
A wetsuit becomes a heat-management decision rather than a warmth one. Overheating on a long swim is a real possibility.
Above 22°C
Wetsuit likely banned
Above the British Triathlon threshold for most athletes. Expect a swimskin-or-nothing swim.
Cold water affects people very differently, and acclimatisation matters more than almost anything else. This is general guidance for healthy athletes, not medical advice — if you have a heart condition or any concern about cold-water immersion, speak to a doctor before entering an open-water race.
How these estimates are made
Nobody publishes a sixteen-year water-temperature record for a gravel pit in Gloucestershire. Here is exactly what this tool does instead, including where it is weakest.
Sea and estuary — calibrated
Marine reanalysis gives a real measured sea-surface temperature, but only back to late 2022 — three summers, far too short to know what a cold year looks like. So the air-temperature model is fitted to those measurements over the overlap, and the fit applied across the full 16-year record. The residual error is typically around 1°C, and it is shown on the result.
Inland — modelled
Lakes, reservoirs and rivers are estimated from the 30-day trailing mean air temperature at the venue — the standard first-order approximation for a shallow water body, capturing the two-to-three week lag behind air temperature. No correction factor is applied on top, because any coefficient invented here would imply precision we don't have.
- Estimates cover 2010 to 2025 and are stored per week of the year, not per day — water temperature does not meaningfully differ between a Tuesday and a Wednesday, and a daily figure would imply a resolution the method cannot support.
- Results are reported as counts across real years ("mandatory in 15 of the last 16") rather than as a probability, because a count is a fact about the record and a probability would be an inference dressed up as one.
- Spot-checked against published figures where they exist: the model puts Grafham Water at 15.2°C in late June against a reported figure of about 15°C, and Weymouth at 17.0°C in late September against a reported 16-18°C.
- Small, shallow, sheltered venues will tend to run warmer than the inland model shows. A large deep reservoir will track it closely.
- Weather data comes from Open-Meteo, which draws on ERA5 reanalysis. No live API call is made when you load a race page — the figures are computed when the site is built.
None of this decides your race. Under British Triathlon rules the official temperature is measured one hour before the start, at three points on the swim course and at 60cm depth, and where the average is 27°C or below the lowest of those readings is the one that counts. Pack for the range, not the median.
How to use this calculator
- 1 Pick a race we cover, or search any venue in the world by name.
- 2 Set your race date — the estimate is specific to that week of the year, not an annual average.
- 3 Choose the rulebook your race runs under: British Triathlon, World Triathlon or IRONMAN.
- 4 Set your swim distance and age, since both move the temperature thresholds.
- 5 Read the year-by-year outcome: how often each ruling would have applied over the last sixteen years.
Wetsuit rules: common questions
At what temperature is a wetsuit mandatory in a triathlon?
Under British Triathlon rules a wetsuit is compulsory below 14°C for swims up to 1500m, and below 16°C for swims of 1501m and longer. World Triathlon sets a single mandatory threshold of 15.9°C, and IRONMAN requires a wetsuit below 16°C (60.8°F).
At what temperature are wetsuits banned?
Under British Triathlon, wetsuits are forbidden above 22°C for swims up to 1500m (24.6°C if you are 60 or over, or if the swim is 1501m or longer). IRONMAN permits a wetsuit up to 24.5°C (76.1°F) with full award eligibility, and between 24.5°C and 28.8°C you may still wear one but forfeit age-group awards and World Championship slots. Above 28.8°C (83.8°F) wetsuits are prohibited entirely.
How cold is too cold to swim in a triathlon?
British Triathlon rule 4.4 prohibits open-water competition below 11°C. Between 11°C and 13°C the swim must be shortened: a maximum of 500m at 11°C, 1000m at 12°C and 2000m at 13°C. Organisers can also shorten or cancel a swim at higher temperatures if wind chill is significant.
When is the official water temperature decided?
Under British Triathlon rules the temperature is measured one hour before the start, at the middle and two other points on the swim course, at 60cm depth. Where the average is 27°C or below, the lowest of those readings counts as the official temperature. The final call on wetsuits is announced to athletes by the organiser after that reading.
Can I wear a wetsuit if it is optional?
Yes. British Triathlon rule 4.2(b) explicitly recommends a wetsuit for all novices and age-groupers up to 22°C (24.6°C if you are 60 or over). Beyond warmth, a wetsuit adds buoyancy and is usually worth a meaningful amount of time — most age-groupers wear one whenever it is permitted.
How thick can a triathlon wetsuit be?
Wetsuit thickness is capped at 5mm at any point under both British Triathlon and IRONMAN rules, and the suit may not cover the face, hands or feet. Wetsuits may cover the arms but not the hands.
Racing one of these?
Verus is where athletes keep the record of what they're building toward — the races on the calendar, the training behind them, and the story of how it went. Add your name to a race and it becomes part of your profile, alongside everyone else chasing the same start line.