How it works
sweat (L) = weight before − weight after + fluid drunk; rate = sweat ÷ hours
Almost all the weight you lose during a run is sweat, since one litre of sweat weighs about one kilogram. But drinking during the run replaces some of that loss and hides it on the scale, so total sweat is your body-mass loss plus whatever you drank. Divide by the time in hours to get your sweat rate. For example, dropping from 70 kg to 69 kg while drinking 0.5 L over an hour means you sweated 1 + 0.5 = 1.5 litres, a rate of 1.5 L/h. Your body-mass loss as a percentage matters too: losing more than about 2% of your weight starts to impair performance, which is the threshold this tool flags. To fully replace what you lose you would drink at your sweat rate, though the gut absorbs only so much per hour, so spread intake out and include electrolytes on long, hot efforts.
Sources
- Sweat-rate measurement Sweat loss = (pre-exercise body mass − post-exercise body mass) + fluid ingested; 1 L sweat ≈ 1 kg body mass. Standard field method (e.g. ACSM/NATA hydration guidance).
- 2% dehydration threshold Body-mass losses beyond ~2% are commonly associated with measurable declines in endurance performance and thermoregulation.
- Fluid absorption limits Gastric emptying and intestinal absorption cap practical fluid intake, so very high sweat rates cannot always be fully replaced during exercise.
FAQ
How do I calculate my sweat rate?
Weigh yourself (dry, ideally unclothed) right before and right after a run of known duration, and record how much you drank. Sweat loss is the weight you lost plus the fluid you drank; divide by the hours to get litres per hour. This calculator does the arithmetic for you.
Why does fluid I drank count as sweat?
Because drinking masks part of your weight loss. If you sweated 1.5 L but drank 0.5 L, the scale only shows a 1 L drop — yet you actually lost 1.5 L of sweat. Adding the fluid back in gives your true sweat loss.
What is a normal sweat rate?
It varies widely — commonly 0.5 to 2.0 litres per hour, and more in hot, humid conditions or for larger, fitter runners. There is no single right number; what matters is knowing yours so you can plan fluid intake for long runs and races.
How much should I drink?
Aim to limit body-mass loss to under about 2%. Fully replacing sweat means drinking close to your sweat rate, but the gut can only absorb so much, so drink to thirst plus a plan, spread it out, and add electrolytes when sweating heavily for a long time.
Does heat change my sweat rate?
A lot. Hot and humid conditions can double your sweat rate compared with cool weather, so test yourself in conditions similar to your goal race. Use the heat-adjusted pace tool alongside this to plan effort and hydration together.
Should I weigh in with clothes on?
For the most accurate result, weigh dry and with minimal clothing both times, because sweat-soaked kit adds weight and understates your loss. Towel off before the post-run weigh-in. If you must keep clothes on, use the same dry clothing each time.
Sweat-rate figures are estimates from your weigh-in and ignore urine and respiratory losses. Hydration needs are individual; over-drinking can be dangerous (hyponatremia). General information for training, not medical advice.