How it works
sweat (L) = weight before − weight after + fluid drunk; rate = sweat ÷ hours
A liter of sweat weighs a kilogram, which turns a bathroom scale into a measuring instrument. Whatever mass disappeared during the run left mostly as sweat, but anything you drank replaced part of the loss invisibly, so it gets added back: finish 1 kg lighter after drinking half a liter and you actually sweated 1.5 L. Dividing by hours produces the rate — with the default 60-minute session, that example reads 1.5 L/h. The percentage line compares the loss against your starting weight and the ~2% performance threshold. One caution the arithmetic hides: a high sweat rate cannot always be matched glass for glass mid-run, because the gut absorbs fluid more slowly than skin loses it.
Sources
- Sweat-rate measurement Sweat loss = (pre-exercise body mass − post-exercise body mass) + fluid ingested; 1 L sweat ≈ 1 kg body mass. Standard field method (e.g. ACSM/NATA hydration guidance).
- 2% dehydration threshold Body-mass losses beyond ~2% are commonly associated with measurable declines in endurance performance and thermoregulation.
- Fluid absorption limits Gastric emptying and intestinal absorption cap practical fluid intake, so very high sweat rates cannot always be fully replaced during exercise.
FAQ
How do I run the sweat test properly?
Weigh in dry and lightly dressed, immediately before and right after the run — sweat-soaked kit on the scale hides real loss. Log every sip in between, and time the session accurately, since the rate divides by hours and a sloppy duration skews everything.
What counts as fluid drunk?
Anything swallowed during the run itself — water, sports drink, the cup grabbed at an aid station. Pre-run coffee and the post-run recovery shake stay out of it; the field only corrects for fluid that masked weight loss between the two weigh-ins.
What is a typical sweat rate for runners?
Most people in most conditions fall between 0.5 and 2.0 liters per hour, with heat, humidity, body size, and fitness pushing the figure around. Heavy sweaters in summer can sit above that range — exactly the runners who benefit most from knowing their number.
How much of the loss should I drink back during a run?
Enough to stay inside roughly 2% body-mass loss, not necessarily all of it. An easy hour may need nothing; a hot long run calls for a plan of small, regular amounts with electrolytes as the session stretches. Chasing full replacement can exceed what your gut will absorb.
Do pounds and ounces work as well as metric?
Yes — the weight fields toggle between kg and lb, and the fluid field between ml and oz. Everything converts to metric internally, where the formula is at home: one liter of sweat, one kilogram on the scale.
Why repeat the test in different conditions?
Because a single result is a snapshot. Sweat rate can roughly double between a cool morning and a humid afternoon, and it shifts with pace and heat acclimation, so a small collection of tests — easy against hard, cool against hot — is what actually lets you plan race hydration.
The method reads all weight change as sweat, so bathroom stops, breathing losses, and damp clothing bias the result. Hydration needs are individual, and drinking far beyond thirst can cause dangerous hyponatremia — use the rate for planning, and raise persistent heavy losses or symptoms with a clinician.