How is sweat rate calculated?

Sweat loss equals body weight lost plus fluid consumed during exercise. Dividing that total by the session duration in hours gives sweat rate in litres per hour, assuming 1 kg of weight loss equals roughly 1 litre of fluid.

Why does the 2% threshold matter?

Research shows endurance performance starts to decline once you lose more than about 2% of body weight in fluid. The calculator flags whether your net loss crossed that line so you can adjust your drinking plan.

Should I drink to replace 100% of sweat losses?

Not always. ACSM advises drinking to limit losses to under 2% of body weight rather than fully matching sweat, since overdrinking can cause hyponatraemia. Use the recommended rate as a ceiling, adjusted to thirst.

Does this account for urine or food weight?

It assumes you did not urinate or eat during the session. If you did, the estimate will be slightly off. For best accuracy, weigh under identical conditions and avoid bathroom breaks during the test.

How often should I retest?

Sweat rate varies with heat, humidity, fitness, and intensity. Retest across different conditions, especially before racing in a new climate, and treat each result as specific to that environment.

Athlete Sweat Rate & Hydration Calculator

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Know exactly how much you sweat

Two athletes running the same race can lose wildly different amounts of fluid. Your personal sweat rate is the only reliable basis for a hydration plan, and the only way to measure it is the weigh-in / weigh-out test. This tool turns those scale readings into a litres-per-hour figure and a practical drinking target.

How it works

Sweat loss during exercise is calculated as the weight you lost plus any fluid you drank along the way:

sweat loss (kg) = (pre weight − post weight) + fluid consumed (kg)
sweat rate (L/hr) = sweat loss / duration (hours)

Because 1 litre of sweat weighs almost exactly 1 kilogram, the kilograms-lost figure converts directly to litres. We also compute your net body-weight change as a percentage of starting weight. Per ACSM guidance, the goal is to keep that net loss under 2% — beyond that, endurance performance and thermoregulation start to suffer.

The recommended drinking rate aims to replace enough sweat to stay inside the 2% limit over the session, capped so you are never advised to overdrink.

Tips and example

Suppose you weigh 70 kg, drink 0.5 L during a 1-hour run, and finish at 69.0 kg. Sweat loss is (70 − 69.0) + 0.5 = 1.5 kg, so your sweat rate is 1.5 L/hr. Your net loss was 1.0 kg, or 1.4% of body weight — under the threshold, but with little margin in hotter conditions.

For the most accurate test, run at race intensity in race-day weather, weigh nude on the same scale, and avoid the bathroom. Repeat in heat versus cold to build a small personal table you can reference before any event.