Sweat Rate & Hydration Calculator
Find your sweat rate and how much to drink per hour.
How sweat rate is measured
The most reliable field method is body weight. Any weight you lose during exercise is essentially water, so your sweat loss is the weight dropped plus any fluid you drank along the way. Divide that by the hours you exercised and you get your sweat rate in litres per hour — the number that tells you how much you need to drink to stay even.
Using the numbers
A practical fluid target is roughly your sweat rate per hour, adjusted for what your gut can tolerate. The sodium figure estimates how much salt you are losing each hour; pairing fluid with electrolytes matters more as sweat rate and session length climb. Sweat sodium varies a lot between people — 800 mg/L is a middle-of-the-road default.
Caution: don’t out-drink your sweat rate
More is not better. Drinking well beyond your sweat rate, especially plain water over long events, can dilute blood sodium and cause hyponatremia — which is dangerous and, at the extreme, fatal. Use your sweat rate as a ceiling guide, drink to thirst around it, and include sodium on long or hot efforts rather than chasing ever-higher fluid volumes.
Questions
How do I measure sweat rate?
Weigh yourself (ideally nude) before and after a timed workout, noting any fluid you drank. Sweat loss = (pre-weight − post-weight) + fluid consumed, divided by the hours exercised. This tool does that math.
How much should I drink?
Aim to replace most — not all — of what you lose, generally 0.4–0.8 L per hour, adjusted to your measured rate. Over-drinking past your sweat rate risks hyponatremia, which is more dangerous than mild dehydration.
What about sodium?
Sweat sodium varies widely (roughly 200–1500 mg per litre). For long or hot sessions, replacing sodium helps you absorb fluid and avoid cramping; the estimate here uses a moderate default you can adjust.