How to use the sweat rate calculator
- Pick US (lb and fl oz) or metric (kg and mL) units.
- Enter your weight before and after the session, measured with little clothing on the same scale, after towelling off.
- Enter the fluid you drank during the session and any urine passed, then the session length in minutes.
- Read your sweat loss in total and per hour, and the share of body weight you lost.
Formula
Sweat loss = (weight before - weight after) + fluid drunk - urine passed
Sweat rate per hour = sweat loss / hours of exercise
Body weight lost (%) = (weight before - weight after) / weight before x 100
A kilogram of body weight lost is treated as one liter of fluid, because a milliliter of water weighs about one gram. One pound is 0.45359237 kilogram and one US fluid ounce is 29.5735295625 milliliters.
A one hour run
A runner weighs 160 lb before and 158.4 lb after, and drinks 16 fl oz during a 60 minute run. The weight lost is 1.6 lb, which is 0.7257 kg or about 726 mL of fluid. Adding the 16 fl oz drunk (473 mL) gives a sweat loss of about 1,199 mL, or 40.5 fl oz.
Over one hour that is a sweat rate of 1.20 liters per hour, or 40.5 fl oz per hour. The body weight lost is 1.6 / 160 = 1.00%.
Why this method works
During exercise your body loses water mostly through sweat. If you weigh yourself before and after, the difference in weight is mostly water, so it tells you how much more you sweated than you drank. Adding back what you drank gives the total you sweated, and dividing by the time gives a rate. The same session in a hot room and a cool one gives very different numbers, so a rate belongs to the conditions, effort and clothing of that session.
To keep the numbers meaningful, weigh yourself with the same scale and clothing, towel off first, and note any bathroom stops. Passing urine during the session reduces your weight without sweating, so subtract it. A bathroom stop before the final weigh-in matters for the same reason.
Using the result
People use sweat rates to plan how much to drink in a long run, ride or match. The tool does not tell you how much you should drink and does not judge your number. It is arithmetic on your own measurements, not medical advice. Needs depend on heat, humidity, effort, health and medicines, and drinking too much can also be harmful. For advice about fluids, salt or a health condition, talk to a doctor or sports dietitian.
It also ignores the small weight changes from fuel burned, from breathing and from food eaten. If the weight after plus your drinking suggests you gained weight, the tool shows a message so you can check the numbers. For heart rate zones during exercise, see the heart rate zone calculator, for your daily water see the water intake calculator, and for pace use the pace calculator.
Unit notes
Weight in pounds and fluid in US fluid ounces are converted exactly to kilograms and milliliters before the calculation. Results are shown in milliliters, liters and fluid ounces.
Assumptions and limits
- A milliliter of fluid is treated as one gram, so weight lost in grams equals fluid lost in milliliters.
- Food, breathing and fuel burned are ignored, and a single session does not describe other days or conditions.
- Weigh with the same scale and clothing, and subtract any urine passed, as the tool expects.
- This is arithmetic on your numbers and not medical advice. It does not say how much you should drink.
Frequently asked questions
How do I calculate my sweat rate?
Weigh yourself before and after exercise, add the fluid you drank, subtract any urine, and divide the total by the hours you exercised. The result is your sweat rate per hour for that session.
Is it normal to lose weight during a workout?
The weight lost over a session is mostly water. The calculator shows it as a share of your body weight, but it does not judge whether your number is healthy. Ask a doctor or sports dietitian about fluids and your situation.
Why does the calculator say I gained weight?
If your weight after plus the fluids you entered works out to more than before, there is likely a typing mistake or a changed scale or clothing. Check the numbers and try again.
Does it tell me how much to drink?
No. It only reports how much you sweated in one session. Fluid plans depend on many other factors and are best made with a professional.