Introducing a precise open water swimming temperature scale

I get asked various questions related to cold because I like to write about it. Those questions often inspire further writing ideas. One thing I realised was there was no good agreement on cold water.

A poll of open water swimmers by DNOWS gave the following results.

  • Under 5ºC (41ºF) – 0%
  • Under 10ºC (50ºF) – 7%
  • Under 12.5°C (54.5°F) – 14%
  • Under 15°C (59°F) – 20%
  • Under 17.5°C (63.5°F) – 24%
  • Under 20°C (68°F) – 15%
  • Under 22.5°C (72.5°F) – 17%

I remember Finbarr Hedderman once saying to me that; “10ºC is the point at which you can start to do some proper distance”. But that’s when the temperature is going up in the late spring. What about when it is dropping in the autumn and winter?


I think it would be fair to say that many, if not most (but not all), of the (serious) Irish and British swimmers would fall into the 7% category, it’s getting cold under 10° C.

So here’s my purely personal swimmer’s temperature scale:

Over 18°C (65°F): This temperature is entirely theoretical and only happens on TV and in the movies. The only conclusion I can come to about the 32% who said this is cold are that they are someone’s imaginary friends. Or maybe foetuses.

16°C to 18°C (61 to 64°F): This is paradise. This is the temperature range at which Irish and British swimmers bring soap into the sea. The most common exclamation heard at this stage is “it’s a bath”!!! Sunburn is common. Swimmers float on their backs and laugh and play gaily like children. They wear shorts and t-shirts after finally emerging. They actually feel a bit guilty about swimming in such warm water. Possible exposures times are above 40 hours for us. It’s a pity we have to get out to sleep and eat.

14°C to 16°C (57° to 61°F): Aaahhh, summerAll is well with the world, the sea and the swimmers. Exposure times are at least 20 to 40 hours. Sandycove Swimmers will swim 6 hour to 16 hour qualification swims, some just for the hell of it and because others might be doing so. Lisa Cummins will see no need to get out of the water at all and will just sleep while floating, to get a head start on the next day’s training.

13°C (55° to 56°F): GrandYou can do a 6 hour swim, and have a bit of fun. Daily long distance training is fine. Barbecues in Sandycove. The first Irish teenagers start to appear.

12°C (53/54°F): Well manageable! You can still do a 6 hour swim, it’ll hurt but it’s possible. Otherwise it’s fine for regular 2 to 4 hour swims. This the temperature of the North Channel.

11°C (51/52°F): Ah well (with a shrug). Distance training is well underway. Ned, Rob, Ciarán, Craig, Danny C., Imelda, Eddie, Jen Lane, Jen Hurley & myself, at the very least, have all recorded 6 hour qualification swims at this temperature. Lisa did 9 hours at this temperature. Swimmers chuckle and murmur quietly amongst themselves when they hear tourists running screaming in agony from the water, throwing children out of the way… 

10°C (50°F): Usually known as It’s Still Ok”. A key temperature. This is the one hour point, where one hour swims become a regular event when the temperature is rising. We start wearing hats after swims.

9°C (48/49°F):A Bit Nippy”No point trying to do more than an hour, it can be done, but you won’t gain much from it unless you are contemplating the Mouth of Hell swim. Christmas Day swim range. Someone might remember to bring a flask of tea. No milk for me, thanks.

8°C (46/48°F): The precise technical term is “Chilly”. Sub one-hour swims. Weather plays a huge role. Gloves after swims. Sandycove Swimmers scoff at the notion they might be hypothermic.

7°C (44/45°F): “Cold”. Yes, it exists. It’s here. The front door to Cold-Town is 7.9°C.

6°C (42/43°F): “Damn, that hurts”. You baby. You went alll your lifebeing mostly unaware you had sinuses. This is the temperature at which you are diabused of that notion, as they tried to escape out the top of your head.  

5°C (40/41°F): Holy F*ck!That’s a technical term. Swimmers like to remind people this is the same temperature as the inside of a quite cold domestic fridge. Don’t worry if you can’t remember actually swimming, getting out of the water or trying to talk. Memory loss is a fun game for all the family. This occurs usually around the middle to end of February.

Under 5°C (Under 40 °F). This is only for bragging rights.There are no adequate words for this. In fact speech is impossible.  It’s completely acceptable to measure exposure times in multiples of half minutes and temperatures in one-tenths of a degree. This is hard-core.  When you’ve done this, you can tell others to “Bite me, (’cause I won’t feel it)”. (4.8°C 1.4°C is mine, Feb. 2013). Carl Reynolds starts to get a bit nervous. Lisa make sure her suntan lotion is packed.

Ned Denison during the winter

2.5°C  to 5°C. South London Swimming Club and British Cold Water Swimming Championships live here. If you are enjoying this, please seek immediate psychological help. Lisa might zip up her hoodie.

1.5°C to 2.5°C: Lynn Coxian temperatures. You are officially a loon.

0°C to 1.5°C: Aka “Lewis Pughiantemperatures. Long duration nerve damage, probably death for the rest of us. Lisa considers putting on shoes instead of sandals. But probably she won’t.

*Grand is a purely Irish use that ranges from; “don’t mind me, I’ll be over here slowly bleeding to death, don’t put yourself out … Son“, to “ok” and “the best“, indicated entirely by context and tone.

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60 thoughts on “Introducing a precise open water swimming temperature scale

  1. Hello there, I’ve recently begun cold water swimming, came across your blog, and really loved your articles (and as a professional writer and editor by trade, I can say, well done!). Besides just to say thanks for being informative and funny–all at the same time–I’m writing as I’m having trouble locating a good thermometer for ocean swimmers. I haven’t come across too many sites that talk about good/reliable ones, so I was wondering if you could recommend one. I’m hoping for something I can tuck inside a skin suit or clip onto a watch, but anything helps….thanks for any guidance you can give.


    • Hi,

      Yes, good water thermos are difficult. Watch thermos are usually inaccurate, typically reading 1 degree higher than reality. I don’t know of any accurate wearable ones, and the only ones that I’d rate for accuracy are lab grade or thermos that can be professionally calibrated. Most thermos, and all cheap thermos, (infrared or sensor) usually have a plus or minus 1 to 2 degree variance, which is utterly insufficient for safety. I’d suggest a long waterproof probe thermo that you can check before a swim and leave to settle for at least a minute. Unfortunately, reliable and accurate thermometers are not cheap.


  2. There is some good research ( Prof Mike Tipton ) on cold water swimming and the very real problem of self regulation. Even very experienced swimmers can swim to unconsciousness and the exposure limits and physiological effects are extremely varied and relate to body size, composition, habituation etc etc. The primary danger is a belief that every swimmer can self monitor and get out before hypothermia and or swim failure occurs. It is a conundrum for every swimmer regardless of experience. The solid advice of not swimming alone, having a get out plan and knowing what to do with someone who is showing signs of hypothermia is critical. Once we get water temperatures under 15C (most of the year in UK and Ireland) we should be more aware of ourselves and fellow swimmers.


    • Great comment Rick.

      There’s a name from the past. Zirganos appears in Sam Rockett’s marvelous ‘Cold in the Channel’ book about the Butlin’s Cross Channel Races and of course his death in the North Channel should be remembered.
      As a lone swimmer, my only choice is swim alone, or do not swim. I believe in using time as a good measure when in really cold water (<10 degrees). Tipton's (who is hugely important in cold water studies) paper is focused on marathon distances or equivalent times. I am not dismissing it at all, nor your message, but as you point out, there are things we can do to better educate people, and to get better at cold water safety.

      My criticism for example of the ISSA was in part because for all their guff about safety, they had not done a single thing to educate people or the hazards, nor done any data collection from all the Ice Mile swims. They contributed NOTHING to helping people understand cold water swimming.


  3. I started doing cold water swimming last summer. Well.. it was not “cold” water as per say, but the water of the river was 16°C while the air was around 30 to 35°. Let’s say it was refreshing. So I didn’t stopped…Then in late october I started swimming in the lake, because water temperature was finally colder than the river. The lake temperature dropped to 4-point-something degrees last february, at which point my swims were no longer than 10 minutes. And then I found out about your website, read some articles, which confirmed some of my theories about how the body react in cold water, and how you get used to it, the 7 degrees limit to “real cold”, etc. I mean, I was even doing the joke about the fridge temperature when the lake was around 5°.

    Anyway, thanks for all the great articles, I learned a lot and it helped me to reassure my wife that I was not going to die of a heart attack while swimming in 10° water 😀


    • Thank you very much. Messages like yours are so important to remind that what I write can help people and have a real tangible result.

      Btw, I also make a joke about fridge temperature in the winter, usually to the effect that I wish I felt that warm. Best of luck & well done !


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  5. Hilarious article thank you, although it’s left me a bit worried about being imaginary, given that the water temp here in SE Australia is well over 18C between December and April, maybe even May.
    It’s the depths of “winter” now and we’re down to about 13. Bracing!!


  6. This rating for non-wetsuit swims?

    I’ll stick with the wetsuit in the sea through winter. Through spring, I can handle quick dips without. Once the jellyfish arrive back, that’s the sign the sea’s too warm to enjoy – time to get into the cooler freshwater lochs and out of the wetsuit.

    I’ll take cold water over jellyfish anyday!!!


  7. I’m trying to figure out how true the weather channel water temperature recordings are.
    I’m in Italy. Usually the summer water temperature is 29 degree.
    So I kept swimming into the fall, when in middle semptember sea temperature got to 21 degree, there were still people swimming but not many. Then October came, water (still according to the weather) was 18 degree and I was one of the few who was stil swimming. Middle of October it was 17 degree and this is where I started to need some acclimatation, splashing my face before entering and would shiver after 20 minutes in.

    Today 20 November it was 15 degree according to the weather channel, but to me it felt as cold as ice.
    As soon as I put my feet into the water they begin to hurt, you know like when you keep your bare hand into the snow, hurting like “needle stings”. It was very hard to swim for even 3 minutes and even after getting out of the water my feet and legs stayed cold whiel my torso was “burning” and getting red, the same as when you put your bare hands in the snow, after some feeling of cold they become hot and red.

    So I really don’t know whether you’re underestimating how cold 15 degree is for a body which is 37 or whether I’m not hardened enough that even what is warm to you is freezing cold to me or if the temperature the weather claims is just wrong and it was actually 6 degree.

    Do you think 15 degree water can have that effect on feet and feel that cold, make your skin red and such?



    • It’s really about tolderance I think. I live in BC and 11-15 degrees is a normal swimming temp for me. I consider it to start getting cold sub 11 degrees


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  9. I just started going with my colleagues in the winter in a lake in Switzerland. They have been doing it for years, me since last October (2016). I am not able to stay in very long for the moment (we are talking minutes). I am used to swimming in a pool for about 45-50 mins (so not even hour long swims there).
    Back in October, when the water was 15 C I thought it was cold. Last week, I went twice (Monday 4.7 C, and Thursday 3.5 C) actually, I try to do twice each week. I am hoping to work up to 20 minutes or so but am taking it quite slowly. I swam for 5 minutes in the 4.7 C and got nervous in the 3.5 C and left after 2 mins. On both occasions I suffered less after exiting than I did on earlier swims but really felt the cold while in the water.
    I usually check the temperature after exiting. But I have done many of the temperatures that you list (on the way “down”). Each time I thought that they were cold, only to feel worse when it was colder.
    I can say I am looking forward to the water getting above 10 C, I did not know how good I had it when it was that warm before.
    I really like your blog and have read a lot (to learn a bit more). I just thought I should at least comment and how useful I have found it.


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  11. Doing the loch lomond ice swim in february next year so looking for info.and this is very informative just at the acclimatisation stages but this is what I’m looking for


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  14. So true, I had a lovely 58 min swim in 8 degrees yesterday so yes, sub one hour! It’s still a joy at the moment, my real trouble will be after I get back from a diving/ swimming trip to Hurghada for New Year where the water will be (chilly for Egypt) around 23/24 degrees. Man it’ll feel sooooo cold after that. A useful post to explain to others how our minds work 🙂


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  21. hi Donal, thank you for such an excellent blog and thank you for confirming/admitting that for some swimmers, sometimes there is memory loss in ‘ice’ waters. i had this happen to me over last saturday at coney island while trying for an ice mile. 300yds to the end and, unbeknownst to me, i stood up and got out (into the hands of my safety crew). dont remember anything until prob 45min later while warming up in the car….it was like i had been out on the lash and had no recollection of getting home. trying to find out what stage of hypothermia memory loss occurs…at the ok stage or not-ok stage. thank you again for sharing all your experiences in this most excellent blog. – Melissa O’Reilly


      • I am trying to gague how serious the memory loss is….obviously it’s pretty much game over in the water if you are swimming. i would not attempt a swim pushing my limits without safety support – but can i ask you – does it happen to you often or just more so when you started cold water swimming or just when you are really on the edge? i am fairly new to this.


        • I haven’t had memory loss in the water. The one time it happened me was immediately after a 3 hour race in 12C , when I had much less experience than now, whereas during Channel training 3 years later, I swam for 6 hours in under 11C with no recurrence. That one time, I was very hypothermic and fighting wind waves. I know of it happening to a couple of other friend in a similar training situation, really trying to push the boundaries for the Channel. However in a Channel solo,where the person becomes very hypothermic, swimmers will sometimes not recall the last portion of the swim before their crew pulls them out, or after being pulled, this period can be two or three hours long.


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  24. What’s the generally accepted freshwater conversion for temperature perception, or is there one? I’ve heard variables of 3-5 degrees F and 1-3 degrees C between salt and fresh. We’re landlocked – rivers, lakes, ponds and bathtubs.
    Many thanks!


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  26. I’ve noticed a couple of times on reports from the crews of marathon swimmers that, when they mention the water is a balmy (whatever temp), it is soon followed by the swimmer aborting the swim due to hypothermia.


  27. having been asked to comment on such a quality blog I hope I don’t dissapoint so here goes…. nice humourous article. I think we can all agree that above 10c you can do some distance. Below that its trickier. I swim whatever the temperature all year but that doesn’t mean that I don’t think 12c water isn’t cold. Its just warmer. The body’s internal temp is cca 37c although well protected. Definately under 4c is tough but I would say it is under about 2.5c or 2c where its becomes really tough and you lose feeling in the extremities very quickly. I did 1km in 1c water 2 years ago bbrrrr, but last month we made a swim in the Norton sound (Bering sea) in alaska. Just 5 mins but highly saline and below zero, -1/-2c. I think I lost feeling in arms in about 10 seconds and I was sure I could feel my insides freezing when I got out. The iceswim is an interesting concept, late winter early spring in Europe probably best time to try it if you are not sure as after a good season of winter swimming 5c doesn’t feel too bad.


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  29. Love this post. The rivers where I swim in summer vary from 10-16*c, and I struggle at the lower end – it feels bloody freezing! I swim in a wetsuit in winter, the river was 6.5*c on Saturday and that physically hurt my exposed parts. One of my buddies is training for a channel relay and has replaced the C-word with ‘cosy’. She claims this makes her feel un-C…


  30. Good stuff. This is my first season doing weekly OW swims throughout winter. Just a newbie. And what I used to think was cold last year, is tropical compared to now.


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    • Thanks Kate, I had a lot of fun writing this post. All SLSC members are hardcore. I was on my way over to the BCWSC there with the other Sandycove guys last Jan. I was already checked in and sitting in my seat, when I was told that my ticket was for the next weekend… Ryanair: seat empty, flight half empty, still kicked off flight. Didn’t get over the embarassment for months.


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