Weigh in before and after a workout to find your sweat rate in fl oz or liters per hour, the share of body weight lost, and how much to drink to stay under the 2% limit.
Sweat Rate Calculator - Fluid Loss and What to Drink
Weigh in before and after a workout to find your sweat rate in fl oz or liters per hour, the share of body weight lost, and how much to drink to stay under the 2% limit.
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What a before-and-after weigh-in tells you
Step on the scale before a run, again after it, add what you drank and subtract any trip to the bathroom: the difference is your sweat. This sweat rate calculator does that arithmetic in pounds and fluid ounces or in kilograms and milliliters, gives the rate per hour and per 15 minutes, and shows how much to drink to keep the loss under 2% of body weight. A runner who weighs 154.0 lb before a 60-minute run, 151.8 lb after it and drank 8 fl oz sweated about 42 fl oz, a rate of 1.23 L/h.
The weigh-in test, done properly
- Units - pounds and fluid ounces, or kilograms and milliliters. Every result is shown in both.
- Weight before - after using the bathroom, nude or in dry underwear, on a scale that reads to 0.2 lb or 0.1 kg.
- Exercise time - the minutes between the two weigh-ins, from 10 to 600. For 1 h 30 min type 90.
- Fluid drunk and urine - weigh or measure the bottle before and after; add gels and bars eaten in between by their weight. Leave urine empty if you did not go.
- Weight after - towel off, take off wet clothes, same scale, before eating or drinking. Then read the rate, the percentage lost and the table of longer sessions.
Why a scale beats a rule of thumb
The American College of Sports Medicine sets one target for drinking during exercise: keep the body weight lost from a water deficit under 2%. Sweat rates differ between people several times over, so the position stand recommends a personal plan and names the method for building one, weighing before and after exercise.
The formula comes from Lindsay Baker's review of sweat-testing methods at the Gatorade Sports Science Institute: sweat equals weight before minus (weight after minus fluid in plus urine out), divided by the time. Across about 500 athletes the whole-body rate usually fell between 0.5 and 2.0 L per hour, roughly 17 to 68 fl oz.
Replacing the loss afterwards takes more than the loss itself. Shirreffs and colleagues gave cyclists 50, 100, 150 and 200% of the weight they had lost and found that only volumes above the loss restored fluid balance, provided the drink carried enough sodium; without it the extra simply left as urine. A follow-up study by Maughan, Leiper and Shirreffs used 150%, and that is the reference the calculator uses: 23 fl oz for every pound lost.
Four sessions run through the calculator
Each row is a separate test with the same formula as the form. The last column says when that person would reach a 2% loss with nothing to drink.
| Session | Before / after | Drunk / urine | Sweat | Rate | Weight change | To 2% |
|---|---|---|---|---|---|---|
| Run, 60 min | 154.0 / 151.8 lb | 8 / 0 fl oz | 42 fl oz | 1.23 L/h | 1.4% | 1 h 8 min |
| Bike ride, 120 min | 176.0 / 172.0 lb | 34 / 7 fl oz | 88 fl oz | 1.31 L/h | 2.3% | 1 h 13 min |
| Gym, 45 min | 132.0 / 131.4 lb | 10 / 0 fl oz | 19 fl oz | 0.76 L/h | 0.5% | 1 h 35 min |
| Soccer, 90 min | 143.0 / 143.4 lb | 50 / 0 fl oz | 44 fl oz | 0.86 L/h | +0.3% | 1 h 30 min |
The runner stays inside the limit but would cross it after just over an hour of a longer run without a drink, and should drink about 51 fl oz afterwards. The cyclist drank a little over a quart and still lost 2.3%, partly because some of the fluid left as urine. Matching the sweat would take about 11 fl oz every 15 minutes; the ride averaged a little over 4. The soccer player gained 0.4 lb and can drink less next time.
Minutes until a 2% loss without a drink
Rows are sweat rates, columns body weight before exercise. A heavier person has more margin: 2% of 210 lb is about 64 fl oz, of 120 lb only about 37 fl oz.
| Sweat rate | 120 lb | 150 lb | 180 lb | 210 lb |
|---|---|---|---|---|
| 0.5 L/h (17 fl oz/h) | 2 h 11 min | 2 h 43 min | 3 h 16 min | 3 h 49 min |
| 1.0 L/h (34 fl oz/h) | 1 h 5 min | 1 h 22 min | 1 h 38 min | 1 h 54 min |
| 1.5 L/h (51 fl oz/h) | 44 min | 54 min | 1 h 5 min | 1 h 16 min |
| 2.0 L/h (68 fl oz/h) | 33 min | 41 min | 49 min | 57 min |
Findings from sweat-testing research
Who weighs in before and after
Marathon and half marathon runners use the rate to decide how many aid stations to use and how much to take at each. Cyclists turn it into a number of bottles per hour for long rides in summer. Coaches of team sports weigh a whole squad before and after practice to spot the players who finish 3% down. Gym-goers and hikers learn whether a water bottle is even needed for a 45-minute session.
Sweat test questions
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