Sweat Rate Calculator

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.

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.