How to measure sweat rate: a practical weigh-in method

Set up a repeatable test

Choose a normal session rather than an unusually hard effort. Use the same scale on a firm surface. Weigh before and promptly after training, drying off and removing wet clothing. Record the duration, conditions, drinks consumed and any urine passed. Keep drinking normally for the session; do not deliberately withhold water to simplify the calculation.

Calculate litres per hour

Estimated sweat loss in litres = starting weight in kg − finishing weight in kg + drinks in litres − urine in litres. Divide by exercise duration in hours. For example, 70.0 kg before and 69.4 kg after, with 0.5 L drunk and 0.1 L urine over one hour, gives approximately 1.0 L of sweat, or 1.0 L/h.

Know the limits of the estimate

The method approximates one kilogram as one litre of water. Food intake, breathing losses, wet kit and fuel use can affect body mass, particularly over long sessions. If the number is negative or implausible, review the measurements. Repeat comparable tests before deciding that a single result describes your usual sweating.

Use the number to improve logistics

A result can help with bottle capacity and refill opportunities. It is not a requirement to replace every millilitre during exercise. Comfortable drinking and avoiding excess intake matter too. Sweat sodium concentration needs a separate measurement; salt marks on clothing cannot turn a weigh-in into a sodium dose.

Common questions