Exercise & Sweat Rate Calculator

How to Use the Exercise & Sweat Rate Calculator

To use the sweat rate calculator, weigh yourself right before and right after a workout, note how much you drank and how long you exercised, and enter those numbers. It returns your sweat rate in liters per hour along with how much body mass you lost.

1. Weigh in before you start. Use the toilet first, then weigh yourself nude or in minimal dry clothing. Enter it as pre-exercise weight.
2. Train as normal and track every drink. Measure bottles before and after so you know exactly how much went in. Enter the total as fluid drunk.
3. Note any toilet stops. If you urinated mid-session, estimate the volume (a typical void is 200–400 ml). Otherwise leave it at zero.
4. Weigh in again immediately after. Towel off sweat and weigh in the same state as before, ideally within 10 minutes of stopping. Enter it as post-exercise weight.
5. Enter the duration in minutes, from the first weigh-in to the end of exercise.

The big number is your sweat rate: how much fluid your body lost per hour. Body mass change shows how dehydrated you finished; losses under 2% are generally well tolerated, while losses above 2% start to affect endurance performance and heat tolerance. Sweat replaced tells you what share of your losses you covered by drinking, and the rehydration target is how much to drink over the next few hours.

How the Exercise & Sweat Rate Calculator Works

The calculator uses the body-mass balance method: any weight you lose during a single session is almost entirely water, so the change on the scale, plus what you drank, minus what you urinated, equals what you sweated.

• Pre and post weight: body weight in kg (pounds are converted at 1 kg = 2.205 lb). Each kilogram lost counts as 1 liter of fluid.
• Fluid intake and urine output: in liters (fluid ounces are converted at 1 fl oz = 29.57 ml).
• Duration: exercise time in minutes, converted to hours.

Worked example: A runner weighs 70.0 kg before a 60-minute run and 69.2 kg after, and drinks 500 ml on the way. Sweat loss = 0.8 + 0.5 − 0 = 1.3 L. Sweat rate = 1.3 ÷ 1 = 1.30 L/hr (about 44 fl oz/hr). Body mass change = 0.8 ÷ 70 × 100 = 1.1%. The rehydration target is 125–150% of the 0.8 kg lost, or 1.0–1.2 L.

A few things nudge the result. The method treats 1 kg of mass lost as exactly 1 L of sweat and ignores small losses from breathing and fuel burned, which slightly overstates sweat rate in very long sessions. Food eaten during exercise adds weight and should be weighed if it's significant. Results are rounded to two decimals for L/hr and to the nearest 10 ml elsewhere.

Exercise & Sweat Rate Calculator FAQs

What is a normal sweat rate during exercise?

Most active adults sweat between 0.5 and 2.0 liters per hour, and some heavy sweaters exceed that in hot weather. Where you land depends on body size, exercise intensity, temperature, humidity, clothing and heat acclimatization, which is why measuring beats guessing.

How accurate is the weigh-in method for sweat rate?

It is the field method sports scientists and the American College of Sports Medicine recommend, and it is accurate to within about 0.1 L when you use a scale that reads to 0.1 kg (0.2 lb). Most errors come from weighing in wet clothes, forgetting to log a drink, or skipping a toilet stop in the calculation.

Do I need to include urine in the sweat rate formula?

Only if you urinated between the two weigh-ins. Urine is body mass that left without being sweat, so the calculator subtracts it; leave the field at zero if you did not go.

How much should I drink after a workout?

Aim for 1.25 to 1.5 liters of fluid for every kilogram of body weight you lost (roughly 20 to 24 fl oz per pound), spread over the next 2 to 4 hours. The extra 25–50% covers the urine your kidneys keep producing while you rehydrate, and including some sodium helps you hold on to the fluid.

Why should I test my sweat rate more than once?

A single session only describes that day's conditions. Testing in cool, warm and hot weather, and at easy and hard efforts, gives you a range you can actually plan races and long sessions around.

Can I drink too much during exercise?

Yes. If you finish heavier than you started, you drank more than you sweated, which raises the risk of exercise-associated hyponatremia (low blood sodium). Use your measured sweat rate as a ceiling for hourly intake, not a target to exceed.