Sweat Rate & Fluid Replacement Calculator
Work out your hourly sweat rate from before-and-after body weight, and how much fluid to drink to replace it.
How it works
Sweat loss(L) = (weight before − weight after) + fluid drunk · sweat rate = loss ÷ duration · body-mass loss% = (before − after) ÷ before × 100
Because sweat is mostly water, a kilogram of body-mass lost during exercise represents about a litre of fluid, once you add back anything you drank along the way. Dividing that by the session length gives your personal hourly sweat rate, which varies enormously with heat, intensity and individual physiology. Endurance guidance suggests keeping body-mass loss under about 2% to protect performance, and replacing roughly 125–150% of losses afterward to cover ongoing urine output.
Worked example
You start at 70.0 kg and finish at 69.0 kg after 1 hour, having drunk 0.5 L. Sweat loss = (70 − 69) + 0.5 = 1.5 L, a rate of 1.5 L/h. Net mass loss is 1.0 ÷ 70 = 1.4%, and replacing 150% suggests about 2.25 L/h in similar conditions.
Frequently asked questions
How do I weigh myself correctly for this?
Weigh nude or in minimal dry clothing immediately before and after exercise, towel off sweat first, and note exactly how much you drank in between. Empty your bladder before the pre-weight if possible.
Why replace more than 100% of what I lost?
You keep losing fluid through urine and continued sweating after you stop, so drinking about 125–150% of the deficit over the following hours restores balance. Include some sodium to help retain the fluid.
Is a 2% loss really a problem?
Losses beyond roughly 2% of body mass are associated with measurable drops in endurance performance and, in heat, higher risk of heat illness. Use your sweat rate to plan drinking that keeps you under that threshold.
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Educational estimate only — not medical advice. Individual needs vary; consult a qualified professional before making health decisions.