Hong Life CalculatorsUnderstanding the sweat rate calculator — concept and principle
Sweat losses differ enormously between people and conditions, so the best drinking plan is one based on your own numbers. This calculator uses your body weight before and after a training session, plus what you drank, to estimate how much you sweat per hour, how much body weight you lost and how much to drink afterwards.
How it is calculated
- Sweat rate (L/h) = (weight before − weight after + fluid drunk) ÷ (duration in minutes ÷ 60)
- Body-weight loss (%) = (weight before − weight after) ÷ weight before × 100
- Drink to recover (L) = (weight before − weight after) × 1.5
Weights are in kilograms, fluid in litres. One kilogram of weight lost is treated as one litre of sweat.
The principle
Over a single session, almost all short-term weight change comes from water. Sweat is mostly water, and a litre of it weighs about a kilogram, so the weight drop plus the fluid you drank equals the sweat you produced. Dividing by time gives a rate you can use to plan drinking in future sessions under similar conditions. The 2% threshold comes from research showing that losing more than about 2% of body weight as water can impair endurance performance and thinking, especially in heat. The 150% rehydration figure exists because you keep producing urine while you drink, so replacing only 100% leaves you short.
Worked example
- Default inputs: before 72.0 kg, after 71.2 kg, drank 0.5 L, 60 minutes.
- Weight change = 72.0 − 71.2 = 0.8 kg.
- Sweat rate = (0.8 + 0.5) ÷ (60 ÷ 60) = 1.3 L/h.
- Body-weight loss = 0.8 ÷ 72.0 × 100 = 1.11%.
- Drink to recover = 0.8 × 1.5 = 1.2 L.
The calculator shows 1.3 L/h, 1.11% loss and 1.2 L to drink.
| Weight before (kg) | Sweat rate (L/hour) |
|---|---|
| 18 | -52.7 |
| 36 | -34.7 |
| 54 | -16.7 |
| 72 | 1.3 |
| 90 | 19.3 |
| 108 | 37.3 |
| 144 | 73.3 |
How to read the result
Sweat rates commonly range from about 0.5 to 2 L per hour depending on fitness, body size, intensity, clothing and weather. A loss below 2% is generally acceptable; above that, drink more during sessions. Gaining weight during exercise means you drank more than you lost, which raises the risk of low blood sodium in long events. Heavy, salty sweaters may also need sodium in their drinks or food.
Common mistakes
- Weighing in sweaty clothes, or before and after eating, drinking or using the toilet without adjusting.
- Applying one test to all conditions; repeat it in hot and cool weather.
- Trying to replace all losses during exercise; the aim is to limit loss to under 2%.
Frequently asked questions
Should I weigh myself naked?
Ideally yes, or in the same dry minimal clothing, after towelling off.
Does urine during exercise affect the result?
Yes. Subtract the volume or weight of any urine passed, otherwise sweat is overestimated.
What should I drink to recover?
Water with a meal or snack is usually enough; adding sodium helps retain fluid after big losses.
Sources: Sawka MN et al. American College of Sports Medicine position stand: Exercise and fluid replacement. Med Sci Sports Exerc. 2007.