Free hydration tool
Sweat Loss Calculator
Weigh yourself before and after a workout, note what you drank and any urine you passed, and the calculator estimates how much you sweated and your sweat rate. An estimate for one session, not advice about what to drink.
Worked example
Weighing 75 kg before and 74.2 kg after 60 minutes, and drinking 500 ml with no urine passed, gives a sweat loss of about 1.3 L, a rate of about 1,300 ml an hour. The body mass lost net of the drinking is 1.1%.
Estimate your sweat loss
The result below starts from the example's numbers. Change any field and it updates as you type.
Track this target every day
Use the number above as your daily goal in CalorTracker, then log meals by photo, voice or text and see where your day stands against it. Free to start, no card needed to explore.
Results are estimates from published formulas, not medical advice. Your own body can sit above or below them.
Where the formulas come from, and how we check them: how we calculate.
How the estimate works
Over a session your body weight falls mainly with the sweat you lose, and rises with everything you drink. So the sweat you lost is roughly the weight you lost, plus what you drank, minus any urine you passed. The calculator counts 1 kg of body weight as 1 litre of fluid, and divides by the length of the session to give a rate per hour.
This is the method described in the American College of Sports Medicine position stand on exercise and fluid replacement (Sawka and colleagues, 2007), which says that individual sweat rates can be estimated by weighing before and after exercise. The calculator counts only sweat, drinks and urine; it ignores other small changes in weight.
Weigh yourself so the subtraction means something
Use the same scale, wear the same clothes (or none) at both weighings, and dry off after the session, since a wet shirt weighs more. Count every drink, and any food, taken during the session, and subtract any urine you passed. The result is only as good as those four numbers.
What the percentage means
The body mass change tile is the net weight you lost as a share of your starting weight, after the drinking. The position stand says the aim of drinking during exercise is to prevent dehydration of more than 2% of body weight, and that a loss above that can impair performance. The calculator shows a note if your net loss is above it. That is a reference point, not a diagnosis, and it is not a target to drink to.
What this does not tell you
It says nothing about the salt in your sweat, which the same position stand describes as varying a great deal between people. It does not say how much to drink or what to add to it, and it should not be used in place of that advice. If you have a medical condition, take medication, or have had symptoms such as dizziness or confusion during exercise, talk to a doctor. For help with fluid and electrolyte plans for training or racing, a sports dietitian or your doctor can work from your own measurements.
This page is not for children, and it is not for pregnancy or for anyone advised to limit fluids. One session in one set of conditions is not your sweat rate in all conditions: heat, humidity, intensity and clothing all move it.
For the daily water target rather than what one session costs, use the water intake calculator or the chart by body weight.