How to Calculate Your Sweat Rate (and How Much to Drink)
Measure your personal sweat rate with a simple before-and-after weigh-in, learn how much fluid to replace per hour, and when electrolytes actually matter.
Hydration advice is usually generic, but sweat rate is intensely personal — two runners side by side can lose anywhere from half a litre to over two litres an hour. The only way to know yours is to measure it, and the test takes one session.
The weigh-in test
The method is simple arithmetic based on the fact that one kilogram of body-mass loss during exercise is roughly one litre of sweat:
- Weigh yourself before exercise, with little clothing and towelled dry. Note it.
- Exercise at a representative intensity for one hour, tracking any fluid you drink.
- Towel off and weigh yourself again, in the same state as before.
- Sweat loss (litres) = (pre-weight − post-weight in kg) + litres you drank.
The sweat-rate & hydration calculator does this for you and converts the result into an hourly drinking target. Repeat it on a hot day and a cool day — your rate can easily double in the heat.
How much to actually drink
The goal is not to replace every drop — drinking litre-for-litre is unnecessary and can be counterproductive. The practical target is to keep total fluid loss under about 2% of body weight, the point where endurance performance starts to fall off. For most people that means sipping 400–800 ml per hour, more when it is hot. For efforts under about 90 minutes, drinking to thirst is a perfectly good guide.
When sodium matters
Sweat carries sodium, and on long or hot days the losses add up. Replacing some sodium helps your body hold on to the fluid you drink and keeps you from feeling flat late in a session. Pair your fluid plan with your carbohydrate intake so you are taking on energy and fluid together, the way race nutrition is actually consumed.
Questions
How do I calculate my sweat rate?
Weigh yourself before and after exactly one hour of exercise, with minimal clothing and towelled dry. Your sweat loss in litres is your weight drop in kilograms plus any fluid you drank during the hour (since 1 kg of body-mass loss is about 1 litre of sweat).
How much should I drink per hour?
Aim to replace enough to keep total loss under about 2% of your body weight, rather than to match sweat litre-for-litre. For most people that is 400–800 ml per hour, higher in heat. Drinking to thirst is a reasonable default for efforts under about 90 minutes.
Do I need electrolytes?
For long or hot sessions where you lose a lot of sweat, adding sodium helps you retain fluid and maintain performance. For short, cool sessions, water is usually enough.