Sweat Rate Calculator
Your sweat rate from a weigh-in before and after exercise — and the drinking rate that keeps you within a safe fluid deficit.
Sweat rate is the weight you lost plus everything you drank, over the time you exercised.
How the sweat rate calculator works
Sweat rate is the weight you lost plus everything you drank, over the time you exercised. A kilogram of weight lost is a litre of fluid, which makes the arithmetic unusually clean.
The target is to keep the deficit under about 2% of body mass, since performance falls measurably beyond that. Replacing 100% is not the goal either — overdrinking causes hyponatraemia, which is more dangerous than mild dehydration.
Formula: sweat rate = (weight lost + fluid drunk) / hours
Worked examples
| Inputs | Sweat rate (L/hour) | Note |
|---|---|---|
| 1.4 kg lost over 75 minutes | 1.52 | 1.52 L/hour |
| Drinking more | 2.08 | higher sweat rate, smaller deficit |
| A cool day | 0.64 | a much lower rate |
FAQFrequently asked questions
How do I measure my sweat rate?
Weigh yourself before and after exercise, add whatever you drank, and divide by the hours. One kilogram lost is one litre of fluid.
What is a normal sweat rate?
Half a litre to two litres an hour. Heat, humidity, intensity and individual variation all move it, so measuring your own is the only way to know.
Should I replace all of it?
No. Staying within a 2% deficit is the target; drinking to replace every millilitre risks hyponatraemia, which is more dangerous than mild dehydration.
Do I need electrolytes?
For sessions over about 90 minutes, or in heat, yes. Sweat contains roughly 950 mg of sodium per litre, though this varies enormously between people.
Why does a 2% deficit matter?
Because measurable performance decline begins around there — endurance capacity and thermoregulation both suffer beyond it.
Where these figures come from
- Riegel PS, "Athletic records and human endurance", American Scientist 1981 — T2 = T1 × (D2 ÷ D1)^1.06
- Cooper KH, "A means of assessing maximal oxygen intake", JAMA 1968 — the 12-minute run test
- Ainsworth BE et al., Compendium of Physical Activities (2024 update) — MET values by activity
- OpenPowerlifting — DOTS coefficients (Tim Konertz, 2019) — the DOTS polynomial used for scoring
- Naismith's rule (1892) as used by UK and Australian walking bodies — 1 hour per 5 km plus 1 hour per 600 m of ascent
- UK Sport — national high-performance body
Last checked: September 2026. Formulas are fixed by their published sources; the MET values come from the Compendium of Physical Activities.