CalcVitalsSweat Rate & Hydration Calculator
Sweat rate is measured, not guessed: weigh before and after, add what you drank. Losing 1 kg in a one-hour session while drinking 0.5 L means a sweat rate of 1.5 L/h — and a rehydration target of about 1.25–1.5 L afterwards. Do the test once in conditions like your events and you can plan drinking properly.
How it works
Generic drinking advice fails because sweat rates are wildly individual — measured values run from 0.5 to over 2.5 litres per hour depending on the athlete, the sport and the weather. The weigh-in method on this page is what sports-science labs and professional teams actually use: nude dry weight before, towelled weight after, everything consumed counted. One test in event-like conditions turns hydration from guesswork into a plan.
The numbers have two distinct uses. During exercise, aim to limit losses to under about 2% of body mass — beyond that, endurance performance and thermoregulation measurably degrade — while never drinking so much that you gain weight, which is the path to exercise-associated hyponatraemia, the one hydration outcome more dangerous than dehydration. After exercise, replace 125–150% of the net loss over the next few hours, with sodium, because the kidneys keep producing urine and plain replacement of exactly 100% under-restores you.
Repeat the test across seasons: the same runner can sweat 0.8 L/h on a cool morning and 1.8 L/h in summer heat, and acclimatisation over 10–14 hot days raises the rate further (while making the sweat more dilute). Your rate is a property of you-plus-conditions, not a fixed trait.
The method
Sweat rate = (pre-weight − post-weight + fluid consumed) ÷ hours, treating 1 kg of weight change as 1 L of sweat, per ACSM position-stand methodology. (Urine losses during the session, if any, should be added to fluid consumed.) Post-exercise replacement 125–150% of net loss per the same guidance.
Frequently asked questions
Why add the fluid I drank to the weight I lost?
Because the scale only shows net loss. If you sweated 1.5 L but drank 0.5 L, the scale shows 1 kg — your glands still produced 1.5 L, and your sweat rate (which plans future drinking) is the gross figure.
How much should I drink during a marathon?
Enough to finish no more than ~2% down and never heavier than you started — for most runners 0.4–0.8 L/h, guided by thirst. Your measured sweat rate from this test turns that range into your number.
Do I need electrolytes or is water enough?
For sessions under about an hour, water suffices. Beyond that — especially in heat — sodium matters: it retains the fluid you drink and replaces what sweat removes (roughly 0.5–1 g of sodium per litre of sweat).