Sweat Rate Calculator — Measure Your Own Fluid Loss

Measured, Not Predicted

Sweat rate is one of the few physiological variables you can measure accurately at home with a bathroom scale. Prediction models exist, but individual sweat rates vary several-fold between people in identical conditions, so a measured figure beats any estimate.

Sweat loss (L) = (weight before − weight after) + fluid drunk − urine passed Rate (L/h) = sweat loss ÷ hours

One kilogram of body-mass change is taken as one litre of fluid. The true figure is slightly under — some mass leaves as oxidised substrate and respiratory water — but over a single session the error is small.

The Protocol

1. Weigh yourself undressed and towelled dry. 2. Train at the intensity, and in the heat and humidity, that you want the number for. 3. Record everything you drink. Note any urine passed. 4. Towel dry and weigh again undressed.

Typical Ranges

Most people sit between 0.5 and 1.5 L/h**. Above roughly **2.5 L/h it stops being practical to replace losses during prolonged exercise in the heat — gastric emptying becomes the limit before willingness does.

Why the Drink Range Is Lower Than the Loss

This calculator gives a replacement band of 60–80% of measured loss, not 100%, and that is deliberate.

Drinking to match every millilitre of sweat means drinking past thirst for hours. That is the mechanism behind exercise-associated hyponatraemia: plasma sodium diluted by fluid intake that exceeds loss. It is rare, it happens mostly in slower marathon and ultra finishers, and unlike dehydration it kills people. Few sports scientists now advocate like-for-like replacement.

The practical guidance: drink to thirst, use your measured rate as an upper bound rather than a target, and when sessions run long add sodium rather than more water.

A 2% Body-Mass Threshold, With a Caveat

Performance decrements are conventionally anchored at 2% body-mass loss — the figure behind the dehydration and strength evidence. It is a useful flag, but it comes largely from laboratory protocols where participants were dehydrated before exercising, rather than losing fluid during it, and field studies of self-paced racing find weaker effects. Treat 2% as a reason to check your plan, not as a cliff.

This Is Not the Electrolyte Tool

Sweat rate tells you how much** fluid you lose. The electrolyte loss calculator estimates **what is in it — sodium, potassium and magnesium. Both matter, and for long hot sessions the electrolyte side matters more.