Sweat rate calculator
Weigh yourself before and after, add what you drank, and see how much you sweat per hour.
Weigh nude or in dry minimal clothing, after towelling off, on the same scale.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health, especially if you have a heart or kidney condition or take diuretics. How we check our calculators.
What the sweat rate tells you
People differ enormously in how much they sweat, so a rule such as ‘drink eight ounces every fifteen minutes’ is a poor fit for many. The only way to know your own number is to measure it: the weight you lose in a session, corrected for what you drank and passed, is the fluid your body gave up as sweat. Measured in a few different conditions, it lets you plan drinking for training and for races.
The formula
Sweat lost (L) = weight before (kg) − weight after (kg) + fluid drunk (L) − urine (L), taking 1 kg of weight as 1 liter. Sweat rate = sweat lost ÷ exercise time in hours. Percent of body weight lost = (weight before − weight after) ÷ weight before × 100, before adding back what you drank. The drinking rate to finish within 2% loss is (sweat lost − 2% of your starting weight) ÷ hours, never below zero.
How do I calculate my sweat rate?
Weigh yourself nude or in dry minimal clothing before exercise and again after, after towelling off. Sweat lost = weight before − weight after + the fluid you drank − any urine. Divide by the hours you exercised. One kilogram of weight loss counts as about one liter of fluid. The ACSM position stand says individual sweat rates can be estimated by measuring body weight before and after exercise.
Worked example: 75 minutes of running
A runner weighs 70.0 kg before and 68.8 kg after 75 minutes, drinks 500 mL and does not urinate. Weight change is 1.2 kg, plus 0.5 L of drink, so 1.7 L of sweat (57 fl oz) over 1.25 hours: 1.36 L per hour, or 46 fl oz per hour. The runner lost 1.7% of body weight. To finish with no more than a 2% loss the runner would need to drink about 0.24 L per hour (8 fl oz) over this session. The result holds for that day's heat, pace and clothing only.
How much should I drink during exercise?
The ACSM goal is to drink enough to prevent excessive dehydration, defined as more than 2% loss of body weight from water deficit, and excessive changes in electrolyte balance. Because sweat rates and sweat salt content vary a great deal between people, the stand recommends customized plans rather than one number for everyone. NATA agrees: plans should account for sweat rate, environment, acclimatization, body size, duration, intensity and what the person tolerates. The ‘drink per hour to finish within 2%’ line is the minimum that holds the loss at that threshold; it is not a target to exceed.
Can I drink too much?
Yes. NATA states that drinking water or low-sodium drinks, sports drinks included, beyond what you sweat out can cause exercise-associated hyponatremia, a low blood sodium (below 135 mmol/L) that is a potential medical emergency and typically causes symptoms below 130 mmol/L. NATA notes it has been found in 10% to 20% of distance athletes after events, more often in women and after efforts longer than four hours. Gaining weight during exercise is a warning sign: for example, a person who gains 0.6 kg while drinking 1.8 L has sweated only 1.2 L. If you gain weight, drink less next time. See a doctor urgently for headache, nausea, confusion or swelling of the hands and feet during or after long exercise.
Why does body weight loss matter?
NATA reports that heat regulation is impaired once you are more than about 1% down, that core temperature climbs by roughly 0.15 to 0.20°C for each 1% of body weight lost, and that a deficit beyond 2% reduces sweating itself. Severe deficits, above 5%, threaten the ability to keep body temperature and exercise intensity under control. Those are the reasons for the 2% line in the ACSM statement.
Should I replace electrolytes?
The ACSM stand says beverages with electrolytes and carbohydrate can offer advantages over water alone under certain conditions, such as long sessions, heavy sweat losses or events where the drink is also your food. How much sodium you lose is individual and this calculator does not measure it. A salty white residue on your clothes after a hot workout suggests high losses, but it is only an observation. For long efforts in the heat, talk to a sports dietitian or physician.
Common mistakes
Weighing in wet or heavy clothing on one occasion and dry clothing on the other, forgetting to count a drink from a gel pack or a rinse, or skipping the bathroom visit. Weight change also includes small losses of water in breathing and of fuel burned, so the number is an estimate. One test says little, because sweat rate changes with temperature, humidity, sun, pace and fitness; repeat in a few conditions and keep the numbers. Finally, avoid using urine color or thirst alone: NATA says people left to thirst replace only about two-thirds of their sweat losses.
Sources and related calculators
Sources: M. N. Sawka and others, American College of Sports Medicine position stand, “Exercise and fluid replacement,” Medicine & Science in Sports & Exercise 39(2):377–390 (2007), abstract read; National Athletic Trainers’ Association, “Position statement: fluid replacement for the physically active,” Journal of Athletic Training 52(9):877–895 (2017), read in full, for the hyponatremia definition and figures, core temperature changes and the recommendations on sweat rate. Related: water intake calculator, heat index calculator (hot, humid days raise sweat rates), calories burned calculator, total body water calculator and rucking calories calculator.