How to Calculate Your Sweat Rate at Home
Field note #781 · 2026-05-30 · 4 min read
Rapid answer
Weigh yourself naked before a 1-hour effort with no drinking. Weigh again after. Each 1 kg of mass lost equals 1,000 mL of sweat per hour. A 70 kg athlete who weighs 69.2 kg after an hour with no drinking has a sweat rate of 800 mL per hour.
Your sweat rate is the most important number in your hydration strategy, and most athletes have never measured it. The test takes about 75 minutes and requires only a scale.
The naked weight sweat test protocol
Weigh yourself naked (or in minimal, consistent clothing) before exercise. After a 1-hour session with no fluid intake, weigh again in the same state. Each 1 kg of mass lost represents 1,000 mL of sweat lost. Add back any fluid consumed during the test to get gross sweat rate.
Step-by-step:
- Use the bathroom before the test. Bladder contents affect weight.
- Weigh yourself naked on a consistent scale. Record the number.
- Exercise for exactly 60 minutes at your target race intensity. No drinking during the test.
- Immediately after exercise, dry off any sweat from your skin before weighing.
- Weigh yourself again naked.
- Subtract the post-exercise weight from the pre-exercise weight.
- The difference in kilograms is your sweat rate in liters per hour.
If you drank any fluid during the test (not recommended for the first test), add the volume consumed in mL to the mass loss in grams to get gross sweat rate.
Example calculation
| Pre-exercise weight | 70.0 kg |
| Post-exercise weight | 69.1 kg |
| Mass lost | 0.9 kg |
| Fluid consumed | 0 mL |
| Sweat rate | 900 mL/h |
This athlete should target 675 mL/h during exercise (replacing about 75 percent of sweat losses in-flight), with the remaining 225 mL/h restored post-exercise.
Conditions affect sweat rate: run the test twice
Sweat rate is not a fixed number. It changes with:
- Temperature: Sweat rate increases roughly 100 to 150 mL per hour for every 10 degrees Fahrenheit above mild temperature. A 900 mL/h rate at 65F might be 1,300 mL/h at 85F.
- Humidity: High humidity reduces evaporative cooling, driving more sweat output at the same temperature.
- Exercise intensity: Higher intensity increases sweat rate. A Zone 2 sweat rate is lower than a race-pace sweat rate.
- Acclimatization: Acclimatized athletes sweat earlier and more efficiently. A summer-acclimatized athlete will sweat more at the same temperature than in spring.
Run the sweat test at the conditions most relevant to your target race. A winter sweat test used for a summer race will significantly underestimate fluid needs.
How to use your sweat rate
Once you have a sweat rate in mL/h:
- Multiply by 0.75 to get your in-flight drinking target (replace 75 percent during exercise).
- Cap at 800 mL/h. Drinking more than 800 mL/h of plain water significantly increases hyponatremia risk regardless of sweat rate.
- Combine with your sodium target from the sodium calculator to build a complete hourly hydration plan.
| Sweat rate | In-flight target (75%) | Notes |
|---|---|---|
| 500 mL/h | 375 mL/h | Low sweater. About 6 oz per 15 min. |
| 800 mL/h | 600 mL/h | Average. About 10 oz per 15 min. |
| 1,200 mL/h | 800 mL/h (capped) | Heavy sweater. Sodium becomes critical. |
| 1,500 mL/h | 800 mL/h (capped) | Very heavy sweater. Use high-sodium drink. |
The sweat rate calculator automates this math and outputs both the hourly drinking target and a body-mass-loss percentage to flag dangerous dehydration levels.
Frequently asked questions
What is a normal sweat rate for endurance athletes?
Most endurance athletes sweat between 500 and 1,500 mL per hour during exercise, with an average around 800 to 1,000 mL per hour in moderate conditions. Individual variation is large: a light sweater in cool weather may lose 400 mL per hour, while a heavy sweater in a hot race can exceed 2,000 mL per hour. Sweat rate also shifts substantially with temperature, humidity, and exercise intensity, so a single number is only useful in the specific conditions where it was measured.
How accurate is the naked weight sweat test?
It is accurate enough to be the gold standard for field testing. The main sources of error are incomplete bladder emptying before weighing, skin wetness at the post-exercise weigh-in, and respiratory water loss (which is included in the weight change but is not sweat). For practical planning purposes, the test gives a reliable estimate within about 10 percent, which is sufficient to set a meaningful hydration target. Repeat the test in different conditions to build a fuller picture.
Does sweat rate change with fitness or heat acclimatization?
Yes. Fit, acclimatized athletes start sweating earlier and at a higher rate than unfit or unacclimatized athletes at the same exercise intensity and ambient temperature. This is an adaptation: more efficient cooling through higher sweat output keeps core temperature lower and protects performance. It also means a sweat test done before a heat-training block will underestimate sweat losses after acclimatization, so retest after major fitness or environmental changes.
Should I drink to replace 100 percent of my sweat losses?
No. Replacing 75 to 80 percent of sweat losses during exercise is a practical and physiologically sound target. Attempting to match losses exactly is difficult in the field and risks overdrinking, which dilutes blood sodium and raises hyponatremia risk. The remaining 20 to 25 percent is comfortably replaced in the hours after exercise. The hard ceiling is around 800 mL per hour of plain water, regardless of sweat rate, because the gut cannot absorb fluid faster than that.