Why measure your sweat rate
During exercise, your body cools itself by sweating. The amount varies enormously from one person to another, from about half a liter to more than 2 liters per hour. Knowing your own rate helps you drink enough during long sessions without drinking too much, which can be dangerous too. The test only needs a bathroom scale.
The formulas
- Sweat loss = weight before − weight after + fluid drunk − urine passed
- Sweat rate = sweat loss ÷ duration in hours
- 1 kg of body weight lost ≈ 1 liter of sweat; 1 lb ≈ 16 fl oz
- Dehydration = weight lost ÷ weight before × 100
- Rehydration ≈ 1.5 × weight lost, because you keep losing fluid in urine after exercise
How to run the test
- Empty your bladder, then weigh yourself with little or no clothing.
- Exercise for about an hour at your usual intensity, in the conditions you want to plan for.
- Note everything you drink. If you have to urinate, estimate or measure the volume.
- Towel off and weigh yourself again in the same clothing.
Worked example
A runner weighs 160.0 lb before and 158.2 lb after 1 hour 15 minutes, and drank 20 fl oz along the way.
- Weight lost: 1.8 lb, or about 0.82 kg.
- Sweat loss: 0.82 L + 0.59 L = 1.41 L, about 48 fl oz.
- Sweat rate: 1.41 ÷ 1.25 h = 1.13 L per hour, or about 38 fl oz per hour.
- Dehydration: 1.8 ÷ 160 = 1.1% of body weight, under the 2% mark.
- To rehydrate afterwards: about 41 fl oz over the next few hours.
Reading your result
| Weight lost | What it means |
|---|---|
| Weight gained | You drank more than you sweated; cut back |
| Under 2% | Good: hydration kept up with sweat |
| 2 to 3% | Endurance and heat tolerance start to drop |
| Over 3% | Clear loss of performance and higher risk of heat illness |
Planning your drinking
- Do not aim to replace everything. Drinking to limit losses to under 2% of body weight is enough for most sessions.
- Do not overdrink. Gaining weight during a long event can lead to hyponatremia, a dangerous drop in blood sodium.
- Sodium matters in sessions over 2 hours or in heat. Sports drinks or salt tablets help replace what sweat removes.
- Retest in different conditions: summer and winter rates can differ by a factor of two, and acclimatization to heat increases sweating.
- Thirst is a reasonable guide for sessions under an hour.