How much you really sweat

Generic advice on how much to drink during exercise is written for an average person who does not exist: sweating varies enormously from one person to another, with heat, intensity and training status. The simplest way to find your own rate is a weigh-in test: you weigh yourself before and after a session, account for what you drank and get how many litres of sweat you lose each hour.

This calculator turns the weigh-ins into a sweat rate, the percentage of body weight lost, the fluid per hour needed to stay within the 2% threshold and the fluid to replace afterwards. It works for running, cycling, football, tennis, the gym or any activity that makes you sweat.

How to do the weigh-in test

• Before: go to the toilet, then weigh yourself in minimal dry clothing or none, on the same scale you will use afterwards.
• During: train as usual and note how much you drink; the easiest way is to weigh or measure your bottle before and after.
• After: towel off, remove wet clothing and weigh yourself again before eating or drinking.
• Urine: if you urinated during the session, estimate the amount; otherwise leave zero.

Repeat the test in different conditions, for example a cool day and a hot one, or an easy session and a hard one: your sweat rate can change a lot, and it is worth knowing your extremes.

How the calculation works

A kilogram of weight lost during a session corresponds, in practice, to a litre of water. Total sweat is therefore the weight lost plus what you drank, minus any urine passed.

• Total sweat: (weight before − weight after) + fluid drunk − urine.
• Sweat rate: total sweat divided by the session length in hours.
• Body weight loss: weight lost divided by starting weight, as a percentage.
• Fluid per hour to stay within 2%: how much you would have needed to drink each hour for the loss not to exceed 2% of body weight in a similar session.
• Fluid afterwards: one and a half times the weight lost.

The verdict is based on the percentage of body weight lost, which does not depend on body size and compares directly with the thresholds in sports guidelines.

What the 2% threshold means in kilograms

Many sports guidelines, starting with those of the American College of Sports Medicine, advise avoiding losses above 2% of body weight in endurance sessions, because beyond that threshold performance tends to drop, especially in the heat. In kilograms the threshold depends on your weight:

The threshold is a reference for endurance sessions, not a hard limit: in short or cool sessions a slightly larger loss often has little effect, while in long races in the heat it matters more.

How much to drink during: it depends on duration

The 2% threshold is a fixed reserve, so how much you need to drink depends on both your sweat rate and the session length. The table uses a 70 kg person and a 90-minute session.

The result surprises many people: in a session of an hour and a half, a light sweater can stay within 2% while drinking almost nothing, while a heavy sweater would need more than a litre an hour, an amount many people struggle to tolerate. That is why knowing your own rate matters more than any fixed rule.

Three example tests

The five bands of the verdict

• Weight gained: you drank more than you sweated. It does not improve performance and, in long sessions, raises the risk of hyponatraemia.
• Below 1%: a negligible loss for almost any activity.
• 1% to 2%: a moderate loss, within the threshold most guidelines set.
• 2% to 3%: above the threshold; the calculator shows how much to drink per hour to stay within it.
• 3% and above: marked dehydration, with lower endurance and concentration and more risk in the heat.

Rehydrating afterwards: why one and a half times

After the session you keep losing fluid through urine, so drinking only the weight lost is not enough to fully recover it. Guidelines suggest drinking about one and a half times the weight lost over the following hours, spread out rather than all at once. A salty meal or a drink with sodium helps the body hold on to the fluid.

What changes your sweat rate

Sweat rates during exercise usually range from about half a litre to more than two litres an hour. They rise with temperature and humidity, with intensity, with body size and with heat acclimatisation, which makes you sweat more and sooner. Clothing and wind matter too. That is why a test done in winter underestimates what you will need in summer.

Limits of the calculator

The calculation assumes that a kilogram equals a litre and ignores the small weight losses from breathing and energy use, which matter little in sessions of a few hours. Accuracy depends on the scale: a 0.1 kg difference weighs heavily in short sessions. The calculator does not measure salt lost in sweat and is not a substitute for medical advice, especially with heart or kidney conditions or medicines that affect fluid balance.

Frequently asked questions

How do you calculate sweat rate?

Weigh yourself before and after the session, add the fluid you drank and subtract any urine passed: that gives the litres of sweat, which divided by the hours gives the rate. For example, going from 70.0 to 69.2 kg with 0.5 L drunk in an hour gives 1.3 L/h.

How much do people sweat during exercise on average?

Usually between half a litre and two litres an hour, but the variation between people is large and grows with heat, humidity and intensity. That is why it pays to measure your own rate rather than rely on an average.

Why the 2% threshold?

Many sports guidelines indicate that beyond 2% of body weight lost, endurance performance tends to drop, especially in the heat. It is a reference for long sessions, not a hard limit.

How much should I drink during training?

Enough not to exceed 2% of body weight lost, but without drinking more than you sweat. The calculator shows the fluid per hour needed to stay within the threshold in a session like the one you tested.

Is it a problem if I weigh more after training?

It means you drank more than you sweated. In short sessions it matters little, but in long events drinking too much can lower blood sodium, a condition called hyponatraemia.

How much should I drink after training?

Guidelines suggest about one and a half times the weight lost, spread over the following hours, because you keep losing fluid through urine in the meantime.

Is a kilogram lost really a litre of sweat?

In practice, yes: sweat is almost all water and a litre of water weighs about a kilogram. The small losses from breathing and energy use matter little in normal sessions.

Do I need to weigh myself without clothes?

It is not required, but it is best to weigh yourself in minimal dry clothing, the same before and after, or none: clothes soaked with sweat distort the reading.

How accurate does the scale need to be?

As accurate as possible: with a scale that rounds to 0.1 kg the error matters little in an hour of running in the heat, but a lot in a short session with little sweating.

Does sweat rate change with the seasons?

Yes, a lot: you sweat more in the heat and once acclimatised. Repeat the test in summer and winter, and at different intensities, to know your extremes.

Does the calculator account for salt loss?

No, it only measures fluid. The amount of sodium in sweat varies from person to person and is measured with specific tests.

Does it work for any sport?

Yes: running, cycling, team sports, the gym or hiking. You just need to weigh yourself before and after and note how much you drink.