About the Sweat Rate Calculator
Your sweat rate is how much fluid you lose per hour of exercise, and it varies enormously — from under half a litre to more than two litres an hour depending on body size, fitness, heat, humidity and intensity. The simplest reliable way to measure it is to weigh yourself before and after a workout, then account for anything you drank or passed as urine. This calculator does the arithmetic.
Runners, cyclists, triathletes and team-sport athletes use it to build a personal hydration plan: how much to drink per hour or per 15 minutes in training and races, and how much to drink afterwards. The American College of Sports Medicine recommends drinking enough to keep body-weight loss under 2%, without drinking so much that you gain weight.
Weigh yourself nude or in the same dry kit, towel off sweat before the second weigh-in, and repeat the test in different conditions — a sweat rate measured on a cool day will underestimate what you need in summer heat.
With the default inputs, the sweat rate per hour (l/h or fl oz/h) is 1.3. Change any value above to recalculate instantly.
How to use the sweat rate calculator
- 1Weigh yourself nude or in dry kit just before exercise.
- 2Exercise as normal, noting how much you drink and any urine passed.
- 3Towel dry and weigh yourself again in the same clothing.
- 4Enter the weights, fluid, urine and duration to get your sweat rate.
- 5Use the per-15-minute and post-exercise figures to plan your hydration.
Formula and method
One kilogram of body-weight change during exercise is treated as one litre of fluid, because short-term weight loss is almost entirely water (fuel burned and respiratory losses are small for workouts of a few hours). Fluid you drank is added back, since it replaced sweat, and any urine passed is subtracted because it was not sweat.
Percentage weight loss = (pre − post) ÷ pre × 100; the ACSM position stand advises keeping it under 2% to limit performance loss. Recovery fluid is 1.5 × the weight lost, because some of what you drink afterwards is passed as urine. The "stay under 2%" rate is the fluid needed to cover sweat plus urine minus the 2% of body weight you can afford to lose, spread over the session. Imperial inputs use 1 lb = 0.4536 kg and 1 US fl oz = 29.57 mL.
- Pre / post
- Body weight before and after exercise
- Fluid
- Everything drunk during the session
- Urine
- Urine passed during the session
Worked examples
1-hour run: 70.0 → 69.2 kg, drank 500 mL
Weight loss is 0.8 kg ≈ 0.8 L, plus 0.5 L drunk = 1.3 L of sweat in one hour. That is 1.14% of body weight, under the 2% target. Rehydrate with 0.8 × 1.5 = 1.2 L.
Hot 90-minute ride: 80 → 77.9 kg, drank 750 mL, 150 mL urine
2.1 kg lost + 0.75 L drunk − 0.15 L urine = 2.7 L over 1.5 h, or 1.8 L/h. Losing 2.6% of body weight is past the 2% mark; to hold the loss to 1.6 kg (2%) the rider would need 2.7 + 0.15 − 1.6 = 1.25 L, about 833 mL per hour.
Imperial: 160 → 158 lb in 75 minutes, drank 16 fl oz
2 lb is 0.907 kg ≈ 30.7 fl oz of sweat, plus 16 fl oz drunk = 46.7 fl oz over 1.25 hours, about 37 fl oz per hour. Rehydrate with 1.5 × 30.7 ≈ 46 fl oz.
Frequently asked questions
What is a normal sweat rate?+
Most athletes sweat between about 0.5 and 2.0 litres per hour (17–68 fl oz). Heavy sweaters in hot, humid conditions can exceed 2.5 L/h. Larger, fitter and heat-acclimatised people generally sweat more.
How much should I drink during exercise?+
Enough to keep weight loss under about 2% of body weight, but not so much that you gain weight. For many people that is roughly 400–800 mL per hour, but your own sweat-rate test is the best guide.
Why multiply weight lost by 1.5 for recovery?+
The ACSM recommends about 1.5 L of fluid for each kilogram lost because you urinate some of what you drink after exercise. Including sodium from food or a sports drink helps you retain more of it.
Does 1 kg of weight loss really equal 1 litre of sweat?+
Close enough for practical use. Sweat is mostly water, which weighs 1 kg per litre. A small part of weight lost comes from burned fuel and breathing, so the method slightly overestimates sweat in very long events.
Can I drink too much during exercise?+
Yes. Drinking well beyond your sweat losses, especially in events over four hours, can dilute blood sodium and cause hyponatremia, which can be dangerous. Gaining weight during exercise is a warning sign.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.