Step on the scale before. Step on the scale after.
Weigh yourself before and after a workout. We compute how much you sweat per hour, how dehydrated you got, and how much you should drink in similar conditions to keep performance from dropping.
One litre of sweat weighs almost exactly one kilogram, so body-weight loss during a workout is a near-direct readout of fluid loss. We add what you drank back in (since drinking masks the true sweat), then normalize per hour. The method is so reliable that sports-science labs use it as the reference test against which sweat patches and predictive equations are validated. The only caveats: very long sessions where glycogen depletion adds ~500 g of weight loss, and bathroom breaks, which we capture in the optional "urine voided" input.
Aim to drink 80–100% of your sweat rate during similar future sessions. Above that, you risk hyponatremia (dangerous over-dilution); below that, you accept performance loss. Sweat rate scales with effort, climate and clothing — re-test on hot days, hard days and race-pace days separately. Pair with our fueling planner to translate the litres-per-hour into bottle counts and sodium grams for race day.
The formulas and ranges above are grounded in the following peer-reviewed literature.
Source for the body-weight-loss method this calculator uses, the 80-100% replacement target, and the dehydration impact thresholds (2% / 4%).
Read paperVery — for sessions under 90 minutes the only meaningful loss is sweat, and 1 kg of body weight ≈ 1 L of sweat. For sessions over 2-3 hours, glycogen depletion adds ~500 g of "weight loss" that isn’t actually fluid; this calculator’s output skews 5-10% high on ultra-distance sessions. Standard practice is to subtract 0.5 kg for every 2 hours of effort.
Target 80-100% of your measured sweat rate as a starting point. So if you sweat 1 200 ml/h, drink 960-1 200 ml/h. Going much above that risks hyponatremia (low blood sodium) on long sessions; going much below means accepting some dehydration. Combine with our fueling planner to convert this into bottle counts.
Salty sweaters lose 800-1 200 mg of sodium per litre of sweat — well above the 500 mg/L average. Signs: white residue on dark race kit, salty taste after a session, cramps despite drinking. Sweat rate (volume) is independent of saltiness, but if you’re a salty sweater you also need a high-sodium drink, not just water — see our fueling planner.
Sports-science consensus: under 2% body-weight loss has no measurable performance effect. 2-4% loss costs 1-3% of pace in the heat (more in long endurance events). Above 4% you risk significant performance loss, cramps and GI distress. For most amateurs, finishing a marathon at 2-3% is fine; over 4% suggests under-drinking that’s costing you 5-10 minutes.
Three big drivers: heat (a 10°C jump can double sweat rate), intensity (each 1% of effort adds roughly 1% to sweat rate), and clothing (heavy or non-breathable kit traps heat, raising sweat rate). Test in the conditions you race in — measuring on a cool morning won’t tell you what you’ll sweat in race-day heat.
Yes. The method is sport-agnostic. For cycling, sweat rate is typically 10-20% lower than running at the same effort because airflow cools you better. For triathlon, the bike leg is the lowest sweat rate (airflow), the run leg the highest (no airflow + accumulated body-core heat). Test each separately.