Sweat rate calculator for runners

Find how much you sweat per hour from one run: weigh yourself before and after, note what you drank, and the calculator gives your sweat rate in litres or fluid ounces per hour.

How the calculator works

Almost all the weight you lose during a run is water. Sweat lost is the drop in body mass, plus anything you drank, minus any urine you passed. One kilogram of lost body mass counts as one litre of sweat. Divide by the hours you ran to get the rate.

Lose one kilogram on a one-hour run while drinking 500 millilitres, and you sweated 1.5 litres: 1.5 litres per hour. The American College of Sports Medicine's position stand on exercise and fluid replacement recommends estimating individual sweat rates this way, by weighing before and after exercise. Read the position stand.

How to run the test

Use a run of about an hour at a steady effort, in conditions like the race you are planning for. Weigh yourself just before, with an empty bladder, in minimal clothing. Weigh again straight after, in the same clothing, after towelling off. Use the same scale both times and note everything you drank.

Sweat rate changes with heat, humidity, pace and fitness, so one test in cool weather will not hold in summer. Repeat it a few times in different conditions.

Using the result

The same position stand describes the aim of drinking during exercise as preventing a loss of more than 2% of body weight from water. The calculator shows your percentage change so you can compare. Drinking more than you sweat also carries risk: if you gain weight during a run, you drank too much. In practice, a plan that replaces part of your sweat loss is the usual starting point.

What it does not measure

The calculator measures fluid only. It does not measure how much sodium you lose, which varies a lot between people, and it is not medical advice. To plan carbohydrate for the same race, use the marathon fuelling calculator.

From a number to race day

We make coach steve, an AI running coach you can text. Bring your sweat rate and your race, and talk through what to drink and when.

Common questions

Can I use pounds and fluid ounces?

Yes. Choose pounds and fluid ounces under units. The result shows fluid ounces per hour.

What if I went to the toilet during the run?

Enter an estimate of the urine passed. It is subtracted, because it left your body without being sweat.

Where are my numbers saved?

Nowhere. The calculator keeps them in the page while it is open.