Why sweat rate matters
Sweating is how your body sheds the heat produced by running. Every litre of sweat that evaporates removes a large amount of heat — but it also removes water and salt. Lose too much and your blood volume drops, heart rate climbs and pace suffers; drink far too much and you risk the opposite problem, low blood sodium (hyponatraemia).
Sweat rates vary enormously between runners — commonly anywhere from about 0.5 to 2 litres per hour, and more for heavy sweaters in the heat. Generic advice like “drink a cup every 20 minutes” can be far too little for one runner and far too much for another. Measuring your own sweat rate takes the guesswork out.
How to measure your sweat rate
- Weigh yourself before the run, naked or in minimal dry clothing, after going to the toilet.
- Run for about an hour at a pace and in weather similar to your target race.
- Record everything you drink and any urine passed during the run.
- Towel off and weigh yourself again in the same clothing.
- Enter the numbers above. 1 kg of body mass lost ≈ 1 litre of sweat.
body-mass loss (%) = (weight before − weight after) ÷ weight before × 100
Worked example
A 70.0 kg runner weighs 68.9 kg after a 60-minute run and drank 300 ml. Sweat loss = 70.0 − 68.9 + 0.3 = 1.4 litres in one hour — a sweat rate of 1.4 L/h. Their body-mass loss was 1.1 kg, or 1.6 %.
How much should you drink?
The American College of Sports Medicine’s position is to drink enough during exercise to prevent excessive dehydration — generally a loss of more than about 2 % of body weight — while avoiding drinking more than you sweat. Most runners find a sustainable approach is to replace somewhere between half and three-quarters of their sweat losses, guided by thirst.
| Sweat rate | Loss over 4 h (no drinking) | Drink ~50 % | Drink ~75 % |
|---|---|---|---|
| 0.5 L/h | 2.0 L | 250 ml/h | 375 ml/h |
| 0.75 L/h | 3.0 L | 375 ml/h | 562 ml/h |
| 1.0 L/h | 4.0 L | 500 ml/h | 750 ml/h |
| 1.25 L/h | 5.0 L | 625 ml/h | 938 ml/h |
| 1.5 L/h | 6.0 L | 750 ml/h | 1125 ml/h |
| 2.0 L/h | 8.0 L | 1000 ml/h | 1500 ml/h |
Do not over-drink. Gaining weight during a race means you drank more than you sweated. That is the main risk factor for exercise-associated hyponatraemia, which can be dangerous. Never force fluids beyond thirst and your measured losses.
Factors that change your sweat rate
Temperature & humidity
Sweat rate can double between a cool morning and a hot, humid afternoon. Test in race-like weather.
Pace
Faster running produces more heat and more sweat. Test at goal race effort, not an easy jog.
Body size
Larger runners generally sweat more in absolute terms.
Acclimatisation
After 1–2 weeks of training in the heat you start sweating earlier and more, which improves cooling.
Sodium and electrolytes
Sweat also contains sodium, and the concentration varies a lot from person to person — some runners finish covered in white salt marks. For runs longer than about 2 hours, especially in the heat, many runners use sports drinks or electrolyte tablets. A sweat-sodium test at a sports lab can personalise this further.
Plan carbohydrate alongside fluids with the race fueling calculator, and adjust pace for hot days with the heat and humidity pace calculator.
- American College of Sports Medicine; Sawka MN et al. (2007). Exercise and fluid replacement. Med Sci Sports Exerc 39(2):377–390.
- Hew-Butler T et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med 25(4):303–320.
Last reviewed: September 2026 · Health disclaimer