Sweat Rate Calculator
Estimate sweat rate from pre/post-exercise body weight and fluids consumed.
Overview
A sweat-rate calculator quantifies how much fluid you lose per hour during exercise by comparing your weight before and after a session, accounting for anything you drank in between. The result is a personal hydration baseline: instead of guessing at "drink to thirst" or copying generic 500 ml-per-hour advice, you find out exactly how much water your body loses under your typical training conditions.
This matters most for endurance athletes, hot-weather runners, and anyone with a history of cramping or hyponatraemia. Sweat rates vary widely — from under half a litre per hour for a small person on a cool day to over two litres for a large athlete in heat — so generic advice misses the mark for both ends of the distribution.
How it works
The basic formula is sweat_rate_l_per_hour = ((pre_weight − post_weight) + fluid_consumed_l) / duration_h. Pre- and post-weights are in kilograms (where 1 kg of weight loss equals 1 L of sweat), and any fluid drunk during the session is added back so it is not mistaken for fluid retained.
Urine losses are usually ignored unless they happen mid-session, in which case they should be added to the numerator. The result represents net sweat output and is the basis for replacement planning: a runner sweating 1.2 L/hour and able to drink 0.6 L/hour will accumulate a 0.6 L deficit per hour, which is sustainable for short events but problematic over many hours.
Examples
- Pre 70.0 kg, post 68.5 kg, drank 0.5 L, 60 minutes:
((70.0 − 68.5) + 0.5) / 1= 2.0 L/hour. - Pre 60.0 kg, post 59.7 kg, drank 0.4 L, 60 minutes:
((60.0 − 59.7) + 0.4) / 1= 0.7 L/hour. - Pre 85.0 kg, post 83.4 kg, drank 1.0 L, 90 minutes:
((85.0 − 83.4) + 1.0) / 1.5= 1.73 L/hour. - Pre 75.0 kg, post 75.0 kg, drank 0.6 L, 30 minutes:
(0 + 0.6) / 0.5= 1.2 L/hour.
FAQ
Why weigh after exercise?
Weight loss during exercise is dominated by fluid loss. Other contributors (glycogen, fat oxidation) account for under 5% over a typical session.
Should I weigh clothed or unclothed?
Unclothed and dry-toweled, both before and after. Sweat-soaked kit can add a kilogram or more of error.
Does it have to be water I drink back?
The calculation accepts any fluid by volume. Sports drinks count, though they affect electrolyte rather than fluid balance.
How often should I test?
At least once per season and again whenever temperature, humidity, or fitness changes noticeably. Personal rates drift across the year.