Sweat rate
calculator
Weigh in before and after a session and this returns your sweat rate — the number ACSM recommends measuring over any formula — plus the fluid plan it sets: enough to keep body-mass loss under 2%, but no more than you actually sweat.
What your sweat rate is
Your sweat rate is how much fluid you lose per hour of exercise, and the honest way to find it is a scale, not an app. Weigh yourself before and after a session: one litre of sweat is one kilogram of body mass, so the drop in weight — plus anything you drank, minus anything you passed as urine — is the sweat you lost. Divide by the hours and you have the rate. For most people it lands between 0.5 and 2.0 litres per hour (ACSM position stand, Sawka et al. 2007), and it climbs with heat, humidity, and intensity.
Most sweat-rate tools stop at that number and tell you to drink it all back. This one goes one step further, to the plan the same position stand actually implies — which is bounded on both ends, not a single "replace 100%" figure.
How the math works
The rate comes straight from the body-mass balance ACSM recommends over any predictive formula. The fluid plan then sits between two limits from the same source: keep body-mass loss under about 2%, and don't drink more than you sweat.
Sweat rate (L/h) = sweat loss ÷ hours
Body-mass loss (%) = (weight before − weight after) ÷ weight before × 100
Fluid plan for a similar session [ACSM 2007, bounded both ways]
floor = max(0, sweat loss − 2% of bodyweight) ← keep loss under ~2%
ceiling = sweat loss ← don't exceed sweat rate
1 L sweat = 1 kg body mass · 1 L = 33.814 fl oz · 1 kg = 2.2046 lb [NIST SP 811, exact]
The two limits are the point. The floor stops dehydration: ACSM's guidance is to keep exercise body-mass loss under roughly 2% of bodyweight, so on a session where you sweat more than that 2%, you have to drink the difference. The ceiling stops the opposite mistake: drinking past your sweat rate on a long event can dilute blood sodium to a dangerous level (exercise-associated hyponatremia), so 100% replacement is the top of the range, not a target to beat.
Worked example
A lifter goes from 154.3 lb (70.0 kg) before to 152.1 lb (69.0 kg) after a 60-minute session, having drunk 16.9 fl oz (0.5 L) and not urinated:
- Sweat loss: (70.0 − 69.0) + 0.5 = 1.5 L (50.7 fl oz)
- Sweat rate: 1.5 ÷ 1 h = 1.5 L/h — the heavy end, but inside the 0.5–2.0 L/h band
- Body-mass loss: 1.0 kg (2.2 lb) ÷ 70.0 = 1.4% — under the ~2% limit, so his 0.5 L kept him in range
- Plan for next time: 2% of 70 kg is 1.4 L, so drink at least 1.5 − 1.4 = 0.1 L and no more than the 1.5 L he sweats
Contrast a bigger athlete on a hot day: 176.4 lb (80.0 kg) down to 171.1 lb (77.6 kg) over 90 minutes on 20.3 fl oz (0.6 L). Sweat loss 3.0 L (101 fl oz), a 2.0 L/h rate — but a 3.0% body-mass loss, over the 2% line. He under-drank: the plan says at least 1.4 L and up to 3.0 L (about 0.9–2.0 L/h), against the 0.4 L/h he managed.
When this calculator is wrong
A sweat test is the right measurement, but the result is one session's number under one set of conditions, not a fixed personal constant — and the number to act on is a plan, not a "replace 100%" figure. ACSM (Sawka et al. 2007) puts sweat rate anywhere from 0.5 to 2.0 L/h, and Baker et al. (2019) show it moves with sport and heat, so a cool-weather test underreads a hot race. The plan is bounded on both ends by the same source: keep body-mass loss under about 2% (the floor on how much to drink) and don't drink in excess of your sweat rate (the ceiling, or you risk exercise-associated hyponatremia). The counter-case: when you test in the conditions you'll actually race in, the number is trustworthy and worth planning around — the fault is treating one arbitrary test as universal, not the measuring itself.
- Not all the weight lost is sweat. Over long efforts, glycogen and its bound water, respiratory losses, and fuel eaten during the session all move the scale too. Keeping the test to about 60–90 minutes and accounting for food and drink keeps those from distorting the rate.
- Conditions change the number. Heat, humidity, altitude, clothing, and intensity all push sweat rate up. Re-measure on a hot day or a hard session rather than reusing a mild-weather figure — that is the honest way to fold conditions in.
- The 2% floor and the sweat ceiling can both bite. On a high-sweat session the floor demands a lot of fluid; on a long, slow one the ceiling matters more. Under-drinking and over-drinking are both real risks, which is why the plan is a band and not a single number.
What to do with the result
Use the rate to build a drinking plan for sessions like the one you tested, and keep it inside the band this page prints. Aim past the floor so body-mass loss stays under about 2%, and stop short of the ceiling — matching your sweat rate is the most you should drink, not the least. On efforts over about an hour, pair the fluid with sodium: sweat carries salt as well as water, and replacing water alone is what drives hyponatremia.
Re-test when the weather or the effort changes, not every session — one number in race-like heat is worth more than ten in mild conditions. To turn the sweat volume into a sodium target, the electrolyte calculator multiplies it by sweat sodium concentration; for the daily baseline your training sits on top of, the water intake calculator handles the non-exercise side.
Common questions
- How do I calculate my sweat rate?
- Weigh yourself before and after a session. Sweat loss in litres equals the weight drop in kilograms, plus any fluid you drank, minus any urine passed; divide by the hours for the rate in litres per hour. One litre of sweat is one kilogram of body mass, so a 1 kg drop over an hour with 0.5 L drunk is a 1.5 L/h sweat rate.
- What is a normal sweat rate?
- Most people fall between 0.5 and 2.0 litres per hour during exercise (ACSM, Sawka et al. 2007). It rises with heat, humidity, and intensity, and varies a lot between individuals, so a number at either end of that range is not by itself a problem — it is a figure to plan fluids around.
- How much should I drink per hour when exercising?
- Enough to keep your body-mass loss under about 2%, but not more than your sweat rate. That gives a band: at minimum, replace the sweat above the 2% tolerance; at maximum, replace all of it. Drinking past your sweat rate on long events risks exercise-associated hyponatremia, so full replacement is the ceiling, not a goal to exceed.
- How long should a sweat-rate test be?
- About 60 to 90 minutes at a steady, representative effort. Shorter tests magnify small weighing errors; much longer ones let glycogen water and respiratory losses creep into the weight change, which inflates the apparent sweat rate.
- Can you drink too much water during exercise?
- Yes. Drinking in excess of your sweat rate over a long event can cause exercise-associated hyponatremia — dangerously diluted blood sodium (ACSM, Sawka et al. 2007). The guidance is to replace what you sweat, with sodium on longer sessions, rather than forcing fluid past thirst on a fixed schedule.
- Is a high sweat rate a sign of fitness?
- Partly, and it cuts both ways. Trained, heat-acclimated people often sweat earlier and more, which cools them better — but a high rate also means higher fluid and sodium losses to replace. It is a number to plan around, not a fitness grade.