Sweat loss hydration
The most reliable way to know how much you sweat is the scales. Weigh before and after a session, add what you drank, and get your sweat rate.
Your details
Weigh in minimal dry clothing, after going to the loo, and towel off first.
Sweat lost
1.4 litres
Under 2% of your weight
- Weight change0 st 2 lb (1.2%)
- Sweat each hour1.4 litres
- To replace afterwards1.1 to 1.4 litres over a few hours
- Plan the next oneadd the length of your next session or race to get a drinking plan
Losing more than 2% of body weight tends to slow you down. Use your sweat rate to plan drinks for a similar session.
The working
Weight lost plus what you drank
Sweat = (weight before − weight after) + drinks. 11 st 7 lb is 73.0 kg and 11 st 5 lb is 72.1 kg, a loss of 0.91 kg, so sweat is 0.91 + 0.5 = 1.4 litres in an hour. The 0.91 kg is 1.2% of body weight.
To rehydrate, ACSM guidance suggests about 1.25 to 1.5 litres for every kg lost, over a few hours.
Plan the next one
A drinking plan from your rate
Add the length of your next session and we scale your sweat rate up, take off the 2% of body weight you can lose, and spread the rest. At 1.4 litres an hour and 73.0 kg, a 3 hour race is about 4.2 litres of sweat; 2% is 1.5 kg, so drink at least 2.8 litres, about 920 ml an hour or 230 ml every 15 minutes. That is above the 0.4 to 0.8 litres an hour most people manage (ACSM), so an easier pace or a cooler start helps. Never drink so much that you gain weight.
Why 2%?
When it starts to matter
Losing more than about 2% of body weight in fluid during exercise tends to reduce performance, especially in the heat. Use your sweat rate to plan drinking in similar sessions so you stay under that.
Do not overdo it
Gaining weight is a warning sign
If you weigh more after than before, you drank more than you sweated. In long events that can dilute blood sodium (hyponatraemia), which is dangerous. Drinking to thirst is a good rule for most people.
Read with care
What this cannot tell you
- Different conditionsSweat rate changes with heat, humidity and effort.
- Food and the looEating or weeing during the session throws it off.
- ElectrolytesSee the electrolyte calculator for salt losses.
Questions
Common questions
What is a normal sweat rate?
Anywhere from 0.3 to 2.4 litres an hour, depending on the person, effort and heat.
How much should I drink after exercise?
About 1.25 to 1.5 litres for every kilo lost, spread over a few hours.
Should I drink during a run?
For runs over an hour, or in heat, small regular sips to thirst help.
Sources
- Sawka MN et al. Exercise and fluid replacement (ACSM position stand). Med Sci Sports Exerc 2007
- Thomas DT et al. Nutrition and athletic performance (ACSM joint position). Med Sci Sports Exerc 2016
- NHS: water, drinks and your health
Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.
If food feels hard
If eating, weight or food is on your mind a lot, you are not alone. Your GP can help, and Beat has a free helpline on 0808 801 0677 (England, weekdays 3pm to 8pm).
Related
Related calculators
- Electrolyte needs estimatorSalt lost in sweat, and when to replace it.Open
- Water intake calculatorA realistic daily drinks guide, with extras.Open
- Long run fuel calculatorCarbs an hour and gels for long sessions.Open
- Post workout refuelCarbs and protein after training, for your weight.Open
- Marathon carb loadYour carb load in grams a day before a race.Open