Sweat Rate & Hydration Calculator
Stop guessing at 500ml an hour. Measure what you actually lose.
Sweat rates between athletes vary by a factor of four, which is why generic hydration advice fails so many people. Weigh yourself before and after one session, note what you drank, and you have a number that is yours. This tool turns that into a drinking target and a sodium range — and flags the two ways a hydration plan becomes dangerous.
Very high sweat rate
You sweat 1.9 litres an hour — drink 1140 to 1200 ml per hour to stay ahead of it
Measured from 1.4 kg of mass lost plus 500 ml drunk over 1:00:00 in Mild (15–22°C) conditions. The target is deliberately less than your full sweat rate: your gut can only absorb around 0.6 to 1.2 litres an hour, and drinking to full replacement over a long race is how hyponatraemia happens.
Where your sweat rate sits
View as a table
| Band | Score |
|---|---|
| Low | Under 0.8 L/h |
| Moderate | 0.8–1.2 L/h |
| High | 1.2–1.8 L/h |
| Very high | 1.8–2.5 L/h |
| Extreme | Over 2.5 L/h |
| You | You — 1.9 L/h |
A high sweat rate is not a fitness problem and not something to fix — fitter, more acclimatised athletes often sweat more, and earlier. It just means your plan has to carry more fluid.
Your numbers
| What | Figure | Why |
|---|---|---|
| Sweat rate | 1.9 L/h | Mass lost plus fluid drunk, per hour. |
| Body mass lost | 1.4 kg (1.9%) | Performance declines measurably past about 2%. |
| Drink during exercise | 1140–1200 ml/h | Capped at the gut's absorption ceiling — you cannot absorb more than this however much you drink. |
| Sodium | 360–720 mg/h | For sessions past 90 minutes. Salty sweaters sit at the top of this range or above. |
| Rehydrate afterwards | 2.1 L | Roughly 1.5 litres per kilogram still missing, taken over the following hours. |
What happens when the weather changes
Sweat rate is not one number. These project your tested figure into other conditions — treat them as a planning starting point and retest when the season turns.
| Conditions | Projected sweat rate | Drink about |
|---|---|---|
| Cool (under 15°C) | 1.5 L/h | 1140 ml/h |
| Mild (15–22°C) — you tested here | 1.9 L/h | 1200 ml/h |
| Warm (22–28°C) | 2.4 L/h | 1200 ml/h |
| Hot or humid (28°C+) | 2.9 L/h | 1200 ml/h |
The two ways this goes wrong
- Drinking too littlePast roughly 2% of body mass, endurance performance declines measurably, perceived effort rises and your heart rate drifts up at the same pace. It is uncomfortable and it costs you time, but it is rarely dangerous on its own.
- Drinking too muchExceeding your sweat rate dilutes blood sodium. Nausea, headache, confusion, puffy hands and feeling worse the more you drink are the warning signs of exercise-associated hyponatraemia. This one is genuinely dangerous, and it is the reason the target above is not simply your full sweat rate.
Tri Alliance sessions that close these gaps
- Coach or accredited sports dietitian review A sweat rate at this end of the range needs a plan built around it rather than a rule of thumb, and that is a conversation with a coach or an accredited sports dietitian.
- Brick session (bike into run) Hydration plans fail on the run, not on the bike. Rehearse this intake in a brick session before you trust it on race day.
- One-to-one coach consult Heat acclimatisation is trainable and takes about ten to fourteen days. Worth planning deliberately if your race is somewhere hot.
This is general guidance for healthy athletes, not individual medical or dietetic advice. Anyone with a heart, kidney or blood-pressure condition, anyone on medication affecting fluid balance, and anyone who has previously experienced hyponatraemia should have a hydration plan set by a doctor or accredited sports dietitian before racing.
The short answer
Sweat rate is calculated as body mass lost during exercise plus fluid consumed, divided by the duration in hours, where one kilogram of mass lost is treated as one litre of sweat. Typical endurance athlete sweat rates range from 0.5 to 2.5 litres per hour, varying with heat, humidity, intensity and individual physiology. During exercise, athletes should aim to replace roughly 60 to 80% of sweat losses rather than 100%, because the gut can only absorb about 0.6 to 1.2 litres per hour and drinking to full replacement risks exercise-associated hyponatraemia. Body mass loss above 2% of starting weight is associated with measurable performance decline, and losses above 4% require intervention. Gaining weight during a session means fluid intake exceeded sweat loss and is a warning sign for hyponatraemia, which is more dangerous than equivalent dehydration. Sweat sodium concentration ranges from about 400 to 1500 mg per litre, so sessions beyond two hours generally warrant 300 to 600 mg of sodium per litre consumed.
How to use this tool
- Weigh yourself immediately before the session, after emptying your bladder, nude or in dry kit.
- Train as normal, and keep track of exactly how much fluid you drink — measure your bottles rather than estimating.
- Weigh yourself immediately afterwards, towel dried and out of wet kit.
- Enter both weights, the session duration, the fluid drunk and the conditions.
- Repeat the test in different conditions — your hot-weather sweat rate can be double your winter one.
Common questions
How do I calculate my sweat rate?
Weigh yourself before and after a session, then add the fluid you drank to the mass you lost and divide by the hours elapsed. One kilogram of body mass lost is treated as one litre of sweat. So an athlete who drops 1.4 kg in an hour while drinking 500ml has a sweat rate of 1.9 litres per hour. Do the test on a session that resembles what you are training for, because sweat rate rises sharply with both intensity and heat.
How much should I drink per hour?
Aim to replace roughly 60 to 80% of your sweat rate during exercise, not 100%. Two reasons: the gut can only absorb around 0.6 to 1.2 litres per hour regardless of how much you pour in, and drinking to full replacement over a long race carries a real risk of exercise-associated hyponatraemia. A small deficit during a session is normal and well tolerated. Make it up afterwards, at roughly 1.5 litres for every kilogram still missing.
Is it dangerous to drink too much during a race?
Yes, and it is more dangerous than the equivalent degree of dehydration. Exercise-associated hyponatraemia happens when fluid intake exceeds sweat loss and dilutes blood sodium. Symptoms include nausea, headache, confusion, puffy hands and feeling worse the more you drink, and severe cases can be fatal. If you finish a session heavier than you started, you drank too much. Drink to a plan built on your own sweat rate rather than drinking at every aid station because it is there.
How much sodium do I need?
Sweat sodium concentration varies from about 400 to 1500 mg per litre between individuals, and unlike sweat rate it does not change much with training or acclimatisation. For sessions under about 90 minutes, plain water is usually fine. Beyond that, 300 to 600 mg of sodium per litre of fluid is a sensible starting range, and salty sweaters — visible white marks on your kit and skin after training — sit at the top of it or above. A sweat sodium test is the only way to know your own figure.
Does losing weight during a session always mean dehydration?
Not entirely. Some of the mass lost is fuel burned and water released from glycogen stores, and a small amount is lost through breathing. Over a typical one to three hour session this accounts for only a few hundred grams, so treating mass lost as sweat is close enough for practical planning. It becomes less accurate over very long sessions, where the substrate contribution grows and this method will slightly overstate your true sweat rate.