Sweat Rate Calculator
Sweat rate varies with heat, intensity, and individual physiology, so this is an estimate, not medical advice. The suggested intake replaces about 125% of fluid lost; drink to thirst and avoid overdrinking. Consult a professional for personalized hydration plans.
Your sweat rate is how much fluid you lose per hour of activity, and knowing it helps you drink enough to stay hydrated without overdrinking. This calculator works it out from the change in your body weight across a session, adjusted for any fluid you drank and urine you passed, then expresses the loss as litres per hour. It also suggests an hourly intake to replace what you lost.
Formula
sweat rate (L/hr) = (preWeight - postWeight + fluidIntake - urine) / hours
- preWeight
- Body weight before exercise (kg)
- postWeight
- Body weight after exercise (kg)
- fluidIntake
- Fluid consumed during exercise (L)
- urine
- Urine passed during exercise (L)
- hours
- Duration of the session in hours
How it works
- Weigh yourself (ideally nude and towel-dried) before and after a timed workout, and note any fluid you drank and urine you passed during it.
- The calculator treats each kilogram of weight lost as one litre of fluid, then adds back the fluid you drank and subtracts urine to find total sweat loss.
- It divides that total by the session length in hours for your sweat rate, and multiplies by 1.25 to suggest a fuller fluid-replacement target.
Worked example
A runner weighs 70 kg before and 69 kg after a 1-hour run, having drunk 0.5 L and passed no urine.
- Total sweat loss = 70 - 69 + 0.5 - 0 = 1.5 L.
- Sweat rate = 1.5 L / 1 hr = 1.5 L/hr (1500 ml/hr).
- Suggested replacement = 1.5 x 1.25 = 1.875 L/hr, about 1875 ml/hr.
A sweat rate of 1.5 L/hr, with a suggested fluid replacement of roughly 1875 ml per hour.
Frequently asked questions
- Why is one kilogram treated as one litre of fluid?
- Sweat is mostly water, and water has a density very close to 1 kg per litre. Over a single session, almost all short-term body-weight change comes from fluid loss, so each kilogram dropped is taken as roughly one litre of sweat.
- Why does the tool suggest drinking more than I lost?
- Not all fluid you drink is retained; some is lost as urine. Sports-science guidance often suggests replacing around 125 to 150% of the deficit after exercise to fully rehydrate, which is why the suggested intake exceeds the measured loss.
- How can I make the measurement accurate?
- Weigh yourself on the same scale just before and after, with minimal dry clothing, towel off sweat first, and record every drink and bathroom break during the session. Repeat in different conditions, because heat and intensity change your sweat rate a lot.
- Is this medical or clinical advice?
- No. It is an educational estimate to guide training hydration, not medical advice. If you have a heart, kidney, or electrolyte condition, or train in extreme heat, get a personalized plan from a qualified professional.