How to Calculate Your Sweat Rate and Fluid Needs During Exercise
October 21, 2026
Your sweat rate is how much fluid you lose per hour of exercise, and it varies a lot between people, from about 0.5 to over 2 liters per hour. You can measure it with a scale: weigh yourself nude before and after exercise, add any fluid you drank, and divide by the hours exercised. A loss of 1 kg of body weight equals about 1 liter of fluid lost.
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Sweat rate (L/hour) = (weight before − weight after + fluid consumed) ÷ hours of exercise. All weights in kilograms and fluid in liters.
Example: you weigh 70.0 kg before a one-hour run, 69.0 kg after, and drink 0.5 liters during it. Fluid lost is 1.0 + 0.5 = 1.5 liters, so your sweat rate is 1.5 L per hour.
Planning Fluid Intake
Most athletes cannot drink as fast as they sweat, and they do not need to replace everything during exercise. A common guideline is to limit weight loss to under 2% of body weight, which for a 70 kg person is 1.4 kg. Drinking enough to stay within that is a reasonable target.
In the example, drinking about 0.8 to 1.0 liter per hour would keep losses within 2%. Over a longer event, spread intake out in small amounts, roughly 150 to 250 ml every 15 to 20 minutes.
Heat, Intensity, and Electrolytes
Sweat rate rises with heat, humidity, intensity, and body size. Measure in conditions similar to your event, because a rate measured on a cool morning can understate losses on a hot afternoon.
Sweat also contains sodium. For sessions longer than an hour or two, or for heavy sweaters, a drink with sodium can help hydration and reduce the risk of low blood sodium from drinking plain water in large volumes.
- Weigh before and after, in minimal clothing.
- Include fluid you drank during exercise.
- Repeat in different conditions.
- Do not drink more than you sweat.
Frequently Asked Questions
How do I calculate my sweat rate?
Subtract your post-exercise weight from your pre-exercise weight, add the fluid you drank, and divide by the exercise time in hours.
What is a normal sweat rate?
Typical rates range from roughly 0.5 to 2 liters per hour, depending on size, intensity, heat, and fitness.
Can you drink too much during exercise?
Yes. Overdrinking plain water can dilute blood sodium and cause hyponatremia, so match intake to losses, not above them.
Do I need electrolytes for a short workout?
Usually not for sessions under about an hour. Water and normal meals are generally enough.