Measure your sweat rate with a bathroom scale

The short answer: weigh yourself before and after a ride of about an hour, add what you drank, and divide by the time. Do it on a few rides at different temperatures and you know your sweat rate better than any table can tell you.

Why bother?

Sweat rates among athletes typically range from about 0.5 to 2 litres an hour, and they rise with heat and effort ¹. A plan built on an average can be off by a litre an hour for you. Changes in body weight during exercise are the simplest accurate way to measure your own rate ^1,2.

What you need

A bathroom scale that shows tenths of a kilogram, a towel, and a bottle you know the size of.

Step by step

1. Go to the toilet, then weigh yourself without clothes. Write it down.
2. Ride for about an hour at a steady effort, the kind you would ride in your event. Note the temperature.
3. Drink from a bottle you know the volume of and note how much you drank. Try not to eat; if you do, note what.
4. Afterwards, dry off with the towel and weigh yourself again without clothes, before you drink or go to the toilet.

The sum

Your sweat loss in litres is your weight before minus your weight after, in kilograms, plus the litres you drank. A kilogram of sweat is about a litre. Divide by the hours you rode to get your rate.

Example: 72.0 kg before, 71.2 kg after, 0.5 litres drunk, one hour. That is 0.8 + 0.5 = 1.3 litres of sweat an hour.

Getting it right

The review behind this method lists the main things that throw it off ¹:

• Weight that changes for other reasons than sweat: food, drink and going to the toilet. Weigh before eating or drinking afterwards, and leave out rides where you ate a lot.
• Sweat trapped in your kit. Weigh without clothes and dry off first.

Sweat rate changes with temperature and effort, so measure on a few rides: a cool one, a warm one, a hot one.

Using it in the FondoLab planner

The planner does not take a measured rate yet. Compare your numbers with its sweat table on the method page and pick the sweat profile whose row is closest. A sweat rate calculator that stores your measurements is planned.