Work out your sweat rate and how much to drink per hour from your pre- and post-exercise weight, fluid intake and time.
General fitness information, not medical advice.
About this tool
The Sweat Rate Calculator estimates how fast you lose fluid during exercise and how much to drink to keep up. Weigh yourself before and after a session, note how much you drank and how long you trained, and it does the rest — locally in your browser, with nothing uploaded.
Net sweat loss is your weight change converted to volume plus the fluid you drank, minus any urine passed: sweat loss = (pre − post) + fluid − urine. A weight change of 1 kg is treated as about 1 litre (1 lb ≈ 0.45 L), and fluid volumes are handled in millilitres or ounces. Dividing by your session length in hours gives your sweat rate per hour, and the recommended intake is 125–150% of that rate, since not everything you drink is retained.
Hydration needs vary hugely with heat, humidity, intensity and your own physiology, so use this as a personalised starting point. This is general fitness information, not medical advice — never force fluids, and seek professional guidance for endurance events or any health concerns.
Frequently asked questions
How do I measure sweat rate accurately?
Weigh yourself (ideally nude and towel-dried) right before and after exercise, using the same scale. Record everything you drank during the session and how long it lasted for the best estimate.
Why convert weight loss to volume?
Most acute weight lost during exercise is water. Since about 1 kg of body-weight loss equals roughly 1 litre of fluid (1 lb ≈ 0.45 L), your weight change converts directly into the volume of sweat lost.
Why drink more than I lost — 125–150%?
You keep sweating and passing some fluid as urine after exercise, so replacing only what you lost leaves you short. Drinking about 1.25–1.5× your sweat rate restores balance over the following hours.
Can I over-hydrate?
Yes — drinking far more than you lose can dangerously dilute blood sodium (hyponatraemia). Use these numbers as a guide, not a quota. This is general fitness information, not medical advice.