Nutrition calculator
Sweat rate calculator
Weigh in before and after a session to find how much fluid you actually lose per hour — and how much to drink during and after. Free, instant, no sign-up.
Sweat rate changes with heat and intensity — the coach adapts your plan as conditions do.
The athlete sweat test
This is the protocol sports scientists use, not a general daily-water guideline. Weigh yourself nude or in dry kit right before training and again straight after, towelled off. Track every millilitre you drink in between. The weight you are missing is fluid, and adding back what you drank gives the total you sweated out.
How this calculator works
Sweat loss = (weight before − weight after) + fluid drunk, taking one litre of sweat as one kilogram. Divide by session length in hours and you have a sweat rate in litres per hour. We also show body-mass loss as a percentage, because the standard guidance is to keep it under 2 % — past that, performance and thermoregulation start to suffer.
Replacing what you lost
During a comparable session, drink at roughly your sweat rate, spread across the hour rather than in one go. Afterwards, the usual guidance is about 1.5 litres for every kilogram you are down, over the following few hours, because some of it leaves as urine. Sweat sodium varies widely — around 500 to 2,000 mg per litre — so the sodium figure here is an order-of-magnitude estimate, not a prescription. Retest in different weather: sweat rate is not one number for the year.