How to measure your sweat rate
Your sweat rate is one of the most useful — and most personal — numbers in endurance running. Two runners side by side in the same race can lose wildly different amounts of fluid. The only way to know yours is to measure it.
- Weigh yourself just before your run, ideally undressed and after towelling dry.
- Note any fluid you drink during the run.
- Weigh again straight after, having towelled off sweat and removed wet clothing.
- Enter the numbers above. Sweat loss = weight lost + fluid drunk; sweat rate = that divided by the run's duration.
Repeat the test in different conditions — a cool morning easy run and a hot tempo will give very different rates.
What your dehydration percentage means
The most important output is body-mass loss as a percentage. Once you lose more than about 2% of your body weight to sweat, endurance performance measurably declines, and in the heat the risk of heat illness climbs.
| Body-mass loss | What it means |
|---|---|
| Under 2% | Well hydrated — the target for most runs. |
| 2–3% | Noticeable performance drop; drink more next time. |
| 3–5% | Significant impairment and heat-illness risk in warm conditions. |
| Weight gained | You overdrank — a rare but real risk (hyponatraemia). Drink less. |
How much should you actually drink?
Ideally you replace enough fluid to keep body-mass loss under 2%. In practice, though, most runners can only absorb 0.4–0.8 litres per hour while running. If your sweat rate is higher than that — common in heat — chasing full replacement causes stomach sloshing and does more harm than good.
The practical approach: drink to thirst, aim for the intake shown above, and accept a small deficit on hot, hard runs. Sports drinks with sodium help you retain what you take in. And never drink so much that you finish heavier than you started.
Train smarter through the heat
The Running Genie factors conditions into your plan and helps you adjust effort on hot days — so hydration and pacing work together, not against each other.
Frequently asked questions
How do I calculate my sweat rate?
Weigh yourself right before a run and again right after. Your sweat loss in litres is the weight you lost in kilograms plus any fluid you drank during the run. Divide that by the run's duration in hours to get your sweat rate in litres per hour. This calculator does the maths for you.
What is a normal sweat rate for running?
Most runners sweat between 0.5 and 2.0 litres per hour, but it varies widely with heat, humidity, intensity, and physiology. Some athletes exceed 3 litres per hour in hot conditions. There is no single normal — the point is to measure your own rate in the conditions you train and race in.
How much should I drink while running?
Aim to keep body-mass loss under about 2%. In practice most runners can only absorb 0.4 to 0.8 litres per hour, so if your sweat rate is higher, drink to thirst and accept some deficit rather than overdrinking. Never drink so much that you gain weight during a run.
What does dehydration percentage mean?
It is the percentage of body mass you lost during the run. Losing more than 2% of your body weight to sweat is associated with a measurable drop in endurance performance and, in the heat, a higher risk of heat illness. Under 2% is the usual target.
Should I weigh in kilograms or pounds?
Either — this calculator supports both and converts internally. The key is to use the same units and the same scale for both weigh-ins, and to towel off sweat and remove wet clothing before the post-run weight so you measure fluid loss, not damp kit.
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