How to measure your sweat rate (and why it matters)

2026-10-30

Your sweat rate is how much fluid you lose per hour of exercise, and you can measure it with a bathroom scale: weigh yourself before and after a workout, add what you drank, subtract any urine, and divide by the time. A kilogram of weight lost is about a liter of sweat (a pound is about 15 fl oz). It matters because sweat rates differ by a factor of five or more between people, from about 0.3 to 2.5 liters per hour, so generic advice like "drink 500 ml per hour" can be far too much for one person and far too little for another.

Why bother measuring?

The American College of Sports Medicine recommends customized fluid plans precisely because sweat rates and sweat salt content vary so much between individuals, and it names weighing before and after exercise as the way to estimate them. A 2017 review of sweat-testing methods reached the same conclusion: body mass change is the simplest and most accurate way to measure whole-body sweat rate.

Knowing your number helps you:

stay under the commonly used limit of losing 2% of your body weight during exercise, where performance starts to suffer;
avoid the opposite mistake, drinking more than you sweat, which over long events can lead to low blood sodium (see can you drink too much water?);
plan how much to carry on a long run or ride, and how much to drink afterwards.

What you need

A digital scale that shows at least 0.1 kg (or 0.2 lb). The same scale before and after.
A bottle you know the volume of, or a kitchen scale to weigh it.
A watch or phone to time the session.
A towel.

Step by step

Before: go to the toilet, then weigh yourself with no clothes or minimal dry clothes. Write it down.
Exercise as you normally would for 45–90 minutes. One hour makes the maths easiest.
Track what goes in: note how much you drink (weigh the bottle before and after, or count the bottles). Gels and food also count by their weight.
Avoid the toilet if you can. If you can't, estimate the volume and note it.
After: towel off thoroughly, take off sweaty clothes (they can hold a surprising amount), and weigh yourself again right away, before eating or drinking.
Calculate:

1 kg of weight equals about 1 liter of sweat; 1 lb equals about 15 fl oz.

A worked example

Alex weighs 75.0 kg (165.3 lb) before a 60-minute run on a mild day. During the run Alex drinks 400 ml (about 14 fl oz) and doesn't stop for the toilet. Afterwards, dried off and undressed, the scale shows 74.1 kg (163.4 lb).

Weight lost: 75.0 − 74.1 = 0.9 kg (about 1.9 lb)
Plus fluid drunk: 0.9 + 0.4 = 1.3 liters of sweat
Over one hour: 1.3 liters per hour (about 44 fl oz)
Body mass lost: 0.9 ÷ 75 = 1.2%, comfortably under the 2% threshold

What Alex learns: on a mild day, drinking 400 ml covered about a third of the loss. That was fine for one hour. But on a two-hour run at the same pace without drinking more, Alex would lose about 2.6 kg, roughly 3.5% of body weight. For long efforts, Alex should plan to drink more, though still less than the full 1.3 liters per hour.

How to use your number

During exercise

The goal is not to replace 100% of your sweat. The German Nutrition Society's position on fluid replacement suggests covering no more than about 80% of the loss over longer sessions, and drinking to thirst within that frame. Two practical checks:

You should finish a little lighter than you started, never heavier. Gaining weight during exercise means you drank more than you sweated.
You should finish less than about 2% lighter. If you regularly lose more, drink a bit more next time.

For a full before/during/after plan, see how much to drink when you exercise.

After exercise

If you need to recover quickly, drink about 1.5 liters per kilogram lost (about 23 fl oz per pound), spread over a few hours with some food. In Alex's case: 0.9 kg lost means about 1.35 liters. If there's no hard session the next day, regular meals and drinks will usually do it.

Salt

A high sweat rate also means more sodium lost. Sweat contains on average about 900 mg of sodium per liter, but individual values range from roughly 175 to 1,500 mg per liter. If you sweat more than about 1.2 liters per hour and exercise for over two hours, you'll likely benefit from sodium during the session. White, gritty salt marks on dark clothing after a workout are a hint that you are a "salty sweater". More in electrolytes: when do you actually need them?

Things that change your sweat rate

One measurement is a snapshot. Your sweat rate rises with:

Heat and humidity: high humidity makes sweat evaporate less, so you sweat more for the same cooling.
Intensity: a tempo run produces more heat than an easy jog.
Body size: heavier people generally sweat more in absolute terms.
Fitness and heat acclimation: trained and heat-adapted people start sweating sooner and sweat more. That's a sign of a good cooling system, not a problem.
Clothing and equipment: layers, helmets and protective gear trap heat.

So measure a few times: an easy session and a hard one, a cool day and a hot one. After three or four tests you'll have a realistic range rather than a single number.

Common mistakes

Weighing in sweaty clothes. Wet clothing can add several hundred grams and hide part of the loss.
Eating or drinking before the second weigh-in. Weigh first, then refuel.
Using different scales. Home scales vary; use the same one both times.
Testing once and assuming it applies all year. Summer and winter numbers can be very different.

Keeping track

Write your results somewhere with the date, duration, temperature and how you felt. Over time you'll see a pattern. For the drinking itself, the running guide covers how to carry and pace fluids in races. Day to day, Water Tracker can log what you drink before and after training in one tap on your iPhone or Apple Watch (the Watch app works without your phone nearby), so you can see whether you actually caught up later in the day.

The bottom line

Sweat rate = (weight before − weight after + fluid drunk − urine) ÷ hours.
1 kg lost ≈ 1 liter of sweat; 1 lb ≈ 15 fl oz.
Most people sweat somewhere between 0.3 and 2.5 liters per hour; only a test tells you where you are.
Use it to drink enough to stay under 2% weight loss, but never so much that you gain weight.
Retest in different weather and intensities; one number doesn't fit every session.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.