Most hydration advice for runners is generic — "drink 16-20 oz per hour," "stay ahead of thirst" — and most of it is wrong for any specific individual, because sweat rate varies enormously between runners even at identical pace, temperature, and effort. A generic fluid plan built for an average sweater can badly under- or over-hydrate someone whose actual rate is well outside that average.
Why this number is worth measuring yourself
Sweat rate depends on body size, fitness level, heat acclimation, clothing, and genetics — factors no calculator can estimate from the outside. The only reliable way to know your own number is to measure it directly, and the test takes one run.
The test
Weigh yourself, nude or in minimal clothing, immediately before a run. Track exactly how much fluid you drink during the run in ounces. Weigh yourself again immediately after, before eating or drinking anything further. The weight lost, adjusted for what you drank, is your sweat loss for that run's duration and conditions.
The Sweat Rate Calculator does this math for you — pre-run weight, post-run weight, fluid consumed, and duration in, personal sweat rate in ounces per hour out.
Worked example: A runner who weighs 160.0 lb before a 60-minute run, drinks 12 oz during it, and weighs 158.5 lb after has lost 1.5 lb of body weight plus replaced 12 oz along the way — total sweat loss works out to roughly 36 oz for the hour, well above the commonly cited average range, and a real number that runner should plan around rather than a generic guideline.
Using the number
Once you know your rate, the Fluid Planner can build a per-hour target from it directly, adjusted for race duration and heat. Test your rate more than once — in different temperatures and at different paces, since it shifts with both — and use the result closest to your actual race-day conditions, not a single cool-weather training run, to plan a hot-weather race.