Sweat Rate Calculator
Estimate your sweat rate and fluid replacement needs from pre/post-workout weight, fluid drunk, and duration.
Your sweat rate
Fluid lost treats each kilogram of body-weight loss as roughly one liter of sweat, plus whatever you drank during the session. The replacement figure follows the common sports-science guidance of replacing roughly 125-150% of fluid lost over the hours after exercise.
Getting an accurate reading
Weigh in with as little clothing as practical, right before and right after exercising, on the same scale each time. Try not to urinate between the two weigh-ins if you can help it, since that adds fluid loss the formula doesn't separately account for. Track exactly how much you drank during the session — a marked bottle makes this easy — since that volume gets added back into the total.
Sweat rate tends to run higher in heat and humidity, during longer or harder efforts, and for people who are already fit and heat-acclimatized (acclimatization actually increases sweat rate as the body gets better at cooling itself). None of that is unusual — it's simply useful to know your own number.
Frequently Asked Questions
How is sweat rate calculated?
Fluid lost is estimated as your body-weight loss during the session (treating each kilogram lost as roughly one liter of sweat, since water is close to that density) plus whatever fluid you drank during the workout — that fluid still left your body as sweat even though you replaced some of it. Dividing total fluid lost by your workout's duration gives the rate in liters per hour.
Why weigh myself before and after instead of just estimating?
Sweat rate varies enormously between individuals — heat, humidity, clothing, fitness level, and genetics all change how much a given person sweats during the same workout. A same-day weigh-in captures your actual rate under actual conditions far better than a generic average would.
How much fluid should I actually replace?
This tool suggests replacing roughly 125-150% of the fluid you lost, which is common sports-science guidance — some fluid taken in during rehydration doesn't stay in circulation immediately, so slightly overshooting the raw loss figure helps restore balance faster. Spread replacement over the hours following exercise rather than drinking it all at once.
Does this account for electrolytes, not just fluid volume?
No — this estimates fluid volume only. Sweat also carries sodium and other electrolytes, and heavy or prolonged sweating (especially in heat, or for known 'salty sweaters') may call for an electrolyte drink or added sodium alongside plain water, not just more volume.
How accurate is a single measurement?
One session gives a useful snapshot, but sweat rate shifts with temperature, humidity, intensity, and acclimatization. Athletes who rely on this for serious training or racing typically repeat the measurement across a few different conditions to get a realistic range rather than a single number.
This tool provides general estimates only, not professional medical or sports-science advice. If you have a heart, kidney, or fluid-balance condition, talk to a doctor before changing how much you drink around exercise.