Sweat rate test results are the most useful number most endurance athletes never measure. All you need is a set of bathroom scales, a drink bottle and one normal training session. At our Samford gym we ask every runner and triathlete preparing for a long event to do it at least twice a year.
Why bother measuring?
Two people can run the same route at the same pace and lose very different amounts of sweat. Generic advice like “drink 500 mL an hour” is too much for some and nowhere near enough for others.
Knowing your own rate lets you plan drinks for long runs, rides and race day instead of guessing. It also shows you how much summer changes the picture.
How to do a sweat rate test
- Before: go to the toilet, then weigh yourself in minimal clothing. Write it down.
- During: do a normal 60-minute session at a steady effort. Note exactly how much you drink. Try not to go to the toilet.
- After: towel off fully, then weigh yourself again in the same clothing.
The sweat rate test sum
Sweat loss (litres) = weight before minus weight after (kg), plus fluid drunk (litres).
| Example | Number |
|---|---|
| Weight before | 78.0 kg |
| Weight after | 77.1 kg |
| Weight lost | 0.9 kg |
| Drunk during the hour | 0.4 L |
| Sweat rate | 1.3 L per hour |
One kilogram of body weight lost in a session is roughly one litre of sweat.
Reading your result
- Under 0.5 L an hour: a light sweater, or a cool day. Sipping to thirst is usually enough.
- 0.5 to 1.2 L an hour: the common range. Plan drinks for anything over an hour.
- Over 1.2 L an hour: a heavy sweater. Long sessions need a clear drinking plan and some practice.
You do not need to replace every drop during a session. Most people aim to keep losses under about 2 percent of body weight on long efforts, then catch up afterwards with around 1.25 to 1.5 L for each kilogram lost.
Test in different conditions
A sweat rate test on a cool June morning tells you little about a humid February long run. Repeat it in each season and at different efforts. Many of our runners find their summer rate is close to double their winter one.
Also look at your kit and cap after hot sessions. White, crusty marks mean you lose a fair bit of salt in your sweat. Our electrolyte drinks guide explains when to add salt and carbohydrate for long, hot sessions.
Everyday drinking
Outside training, a glass of water with each meal and a bottle at your desk covers most people. Some clients also add a tablespoon of a liquid trace mineral to one glass a day. If you have seen these on the shelf, this page explains what colloidal minerals are and what colloidal means.
Use it in your race plan
Once you know your number, practise your drinking plan in long training runs, not on race day. Our marathon running plan and half marathon plan both have long runs where you can rehearse it. For fuel alongside fluid, see our energy gels for running guide.
Common questions
How accurate is a sweat rate test?
Accurate enough to plan with. Small errors from the toilet or a damp shirt matter less than the big difference between a light and a heavy sweater.
Should I drink my full sweat rate every hour?
No. Most people drink less than they lose during long efforts and catch up afterwards. Drinking well beyond thirst is a real risk in long events.
How often should I repeat the test?
At least once in summer and once in winter, and again before a big event in different conditions.
Does a sweat rate test work for gym sessions?
Yes, though strength sessions usually produce much lower numbers than running or cycling.
When to get it checked
If you feel dizzy, sick or confused during or after long sessions, stop and see your GP. An exercise physiologist can then help you build a training and drinking plan that suits your numbers.