How to use it
- Under Units, pick kg and liters or lb and fl oz. Numbers already typed are converted when you switch.
- Enter Weight before and Weight after, both in minimal clothing. Towel dry before the second weighing.
- Enter the Session length in minutes and the Fluid drunk during it. If you passed urine during the session, add it under Urine passed during it.
- Read your sweat rate per hour and how much your weight changed. Keep it with Copy link, Save image or Share.
How the calculation works
The method is the one set out in a 2017 review by Lindsay Baker (Sports Medicine): whole-body sweat rate = [weight before − (weight after − fluid drunk + urine)] ÷ exercise time. One kilogram of weight lost counts as about one liter of sweat.
Example: 70.0 kg before, 69.0 kg after, 0.5 L drunk, no urine, 60 minutes. Sweat = 70.0 − (69.0 − 0.5 + 0) = 1.5 kg, about 1.5 L. Divided by 1 hour, that is 1.50 L per hour. The body weight change is 1.0 kg, or 1.4%.
| Input | Metric | US |
|---|---|---|
| Weight before | 70.0 kg | 154.3 lb |
| Weight after | 69.0 kg | 152.1 lb |
| Fluid drunk | 0.5 L | 16.9 fl oz |
| Sweat rate | 1.50 L/h | 51 fl oz/h |
How accurate is this?
Weighing before and after is the simplest and most accurate field method, according to Baker's review, as long as the result is corrected for drinks and urine. Uncorrected, urine loss alone made sweat loss look 16–37% bigger in one study the review cites.
Some of the weight you lose is not sweat: breathing out water and burning fuel account for about 5–15% combined, more in sessions over 2–3 hours. So the calculator slightly overstates sweat in long sessions. Sweat trapped in clothing works the other way.
Scales matter too. A home scale that reads to 0.1 kg can be off by 0.1 kg each time, which is 0.2 L on a one-hour session. Same scale, same clothing, before and after.
What the number tells you
Sweat rates vary a lot between people and between days. Baker's review reports that most athletes tested sweat about 0.5 to 2.0 liters per hour, and more than 3 liters is rare. Heat, humidity, intensity and body size all change it. The American College of Sports Medicine recommends customized fluid plans for that reason and notes that your own rate can be estimated by weighing before and after exercise; Baker's review suggests testing in conditions like the ones you train or compete in.
The ACSM's 2007 position stand describes drinking during exercise as a way to prevent excessive dehydration, meaning a loss of more than 2% of body weight. The National Athletic Trainers' Association adds that people should not drink so much that they gain weight during exercise. The calculator shows your weight change so you can see where a session landed. It doesn't tell you what to drink.
Getting a clean measurement
Use the toilet before the first weighing. Weigh in the same minimal clothing both times, and towel dry before the second. Measure what you drink in a bottle with marks. If you eat during the session, its weight counts like fluid.
Daily water needs are a different question from sweat during one session. The water intake calculator shows the official daily reference values.
Questions
- What is a normal sweat rate?
- Most athletes tested by researchers fall between about 0.5 and 2.0 liters per hour (Baker 2017). Above 3 liters per hour is rare and usually tied to heat, high intensity or a large body.
- Why does 1 kg of weight count as 1 liter of sweat?
- Sweat is mostly water, and a liter of water weighs about a kilogram. That is close enough for this method.
- Do I have to measure urine?
- Only if you went during the session. If you did and leave it out, the calculator counts it as sweat and the rate comes out too high.
- Can I use pounds and ounces?
- Yes. Choose lb and fl oz. The calculator converts to kilograms and liters for the math and shows the result in fl oz per hour.
- Is anything I type sent to a server?
- No. The math runs in your browser. Copy link puts the numbers after a # in the link, a part that browsers don't send to servers.