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Sweat Rate Calculator

Know exactly how much you sweat per hour

How it is calculated

The tool works out total fluid lost as sweat: loss = (pre − post) + fluid − urine, then divides by the session length: rate = loss / hours. Body weights are in kilograms and are treated as litres of fluid, one kilogram of weight change being counted as one litre. fluid is what you drank during the session and urine is what you passed, both in litres. The results are the total loss in litres and the rate in L/hr.

All inputs must be zero or more, and the duration must be greater than zero, given in hours (use 1.5 for 90 minutes). The tool assumes you weighed yourself in the same clothes, dry, and does not subtract food eaten, weight lost through breathing or fuel burned, which are small but not zero. A negative rate can appear if you drank more than you lost.

Worked example

An athlete weighs 70.5 kg before and 69.3 kg after a 1.5 hour run, drinks 0.6 L and does not urinate. Weight change: 70.5 − 69.3 = 1.2 kg. Add the fluid drunk: 1.2 + 0.6 = 1.80 L total loss. Divide by the duration: 1.80 / 1.5 = 1.20 L/hr sweat rate.

Try this example →

Common mistakes

  • Entering the duration in minutes; the field takes hours, so 90 minutes is 1.5, not 90.
  • Forgetting to add back the fluid you drank, which makes the sweat loss look much lower than it really was.
  • Weighing pre-exercise in dry clothes and post-exercise in a soaked shirt; wet clothing adds weight and hides loss, so weigh in the same dry state both times.
  • Entering weights in pounds; the tool treats the difference as kilograms and litres, so convert first.
  • Using one session as your permanent number; the rate changes with heat, humidity, intensity and fitness, so repeat the test in the conditions you plan for.

Sources

About this calculator

Sweat rate is calculated from the change in body weight before and after exercise, adjusted for what you drank and lost as urine: sweat rate (L/hr) = [(pre-exercise weight − post-exercise weight) + fluid intake − urine loss] ÷ exercise duration. Body weight is used as a proxy for fluid loss because one liter of sweat weighs almost exactly one kilogram.

Athletic trainers, endurance coaches, and sports dietitians use this number to build individualized hydration plans — a marathoner who loses 1.5 L/hr needs a very different fluid strategy than one who loses 0.5 L/hr, even at the same pace. It's also used in military and occupational heat-stress settings to set safe work-rest cycles in hot environments.

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Comments (1)

  • Editorial team

    Weights go in as kilograms and the tool treats one kilogram as one litre. For a 1 hour run with 70.0 kg before, 68.9 kg after, and 0.5 L drunk, the rate is 1.60 L per hour. Weigh yourself in the same clothes, towelled dry, and repeat in hot and cool conditions, since the figure only describes the session you measured. What sport do you test it for?

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