A field estimate, not a fixed prescription

Sweat-rate calculator

Use measurements from one representative training session to estimate hourly fluid loss. Repeat under different conditions to build a useful range.

Before measuring: Use the same scale, minimal dry clothing, and a session long enough to observe meaningful change. Towel off and remove wet clothing before the final weigh-in.

The equation

(mass before − mass after + fluid consumed − urine produced) ÷ hours

When mass is in kilograms and drink/urine volumes are in litres, the result is litres per hour. Pounds are converted to kilograms. The estimate assumes one litre of water-like fluid weighs approximately one kilogram.

How to interpret the result

This is your observed sweat rate for this session—not a mandate to replace every millilitre while moving. Temperature, humidity, clothing, intensity, fitness, acclimatization, and duration all change losses. Repeat the measurement across conditions and do not aim to finish prolonged exercise heavier than you started.

Medical boundary: This tool does not assess dehydration, sodium needs, kidney or heart conditions, heat illness, or hyponatremia. Confusion, seizure, collapse, severe weakness, or very high body temperature requires urgent help—not another calculator result.

Read the complete measurement guide · Understand overhydration · Hydration collection