Sweat rate calculator
Work out your sweat rate in litres per hour from a simple before-and-after weigh-in, and how much to drink on long runs and races in Bangkok's heat.
Weigh yourself without clothes right before and after a run (towel off first), and note how much you drank on the way.
You don't need to replace every drop: aim to lose no more than about 2% of body weight, and never drink so much that you gain weight during a run. Add electrolytes on long runs — sweat in this climate carries a lot of salt.
How it works
Weigh yourself without clothes before and after a run of about an hour, at race-like effort. Every kilogram lost is roughly one litre of sweat.
Sweat rate = (weight before − weight after + fluid drunk − urine) ÷ hours.
Most runners lose 0.5–2 litres an hour, and more in Bangkok's heat. Aim to replace enough that you lose no more than about 2% of body weight — not every drop. Repeat the test in different conditions, because your sweat rate changes with temperature and pace.
FAQ
How much should I drink during a run in Bangkok?
On runs over an hour, many runners drink 400–800 ml per hour, adjusted to their own sweat rate. Under an hour, drinking before and after is usually enough.
Can I drink too much?
Yes. Drinking more than you sweat can dilute blood sodium (hyponatraemia), which is dangerous. If you weigh more after a run than before, you drank too much.
Do I need electrolytes?
On long runs and in heat, yes — sweat contains a lot of sodium. Use electrolyte drinks or tablets on runs over about an hour.
Results are estimates for planning training, not medical advice.