Sweat Rate & Hydration Plan Calculator
Use a weigh-in sweat test to estimate your sweat rate and turn it into a target-based drinking plan for a future session. The calculator accounts for fluid you drank and urine you produced during the test.
Fluid needed to stay within target (L)0.8
- Fluid needed to stay within target (US fl oz)
- 27.05
- Measured sweat loss (L)
- 1.1
- Sweat rate (L/h)
- 1.1
- Sweat rate (US fl oz/h)
- 37.2
- Scale-weight change
- 0.86%
- Projected sweat loss (L)
- 2.2
- Target average drinking rate (mL/h)
- 400
- Target average drinking rate (US fl oz/h)
- 13.53
- Modeled no-weight-gain ceiling (L/h)
- 1.1
- Projected ending body-mass loss
- 2%
- Planning interpretation
- The target plan stays within the selected ending body-mass-loss limit and below the modeled no-weight-gain ceiling.
Metric and US equivalents
| Quantity | Liters | US fluid ounces |
|---|---|---|
| Measured sweat loss | 1.1 | 37.2 |
| Projected sweat loss | 2.2 | 74.39 |
| Fluid needed within target | 0.8 | 27.05 |
Metric fluid inputs are milliliters; US customary fluid inputs are US fluid ounces.
Uses the field approximation that 1 kilogram of short-term mass change equals 1 liter of water.
How to use this calculator
- Choose the measurement system you used for the weigh-in and fluid measurements.
- Enter body weight before and after exercise under the same scale and clothing conditions.
- Add the fluid consumed, urine produced, and the actual test duration.
- Enter the future workout duration and the maximum ending body-mass loss you want to plan around.
How the sweat rate is calculated
A practical sweat-rate test compares body weight before and after exercise, then adjusts for fluid that crossed the boundary during the test. If you drank during the test, that fluid would hide part of the sweat loss on the scale. If you urinated, that volume should not be counted as sweat.
The calculator converts all entries to metric units first. Pounds are divided by 2.2046226218 to get kilograms. US fluid ounces are multiplied by 0.0295735296 to get liters. Metric fluid entries are treated as milliliters and divided by 1000.
The measured sweat loss is:
S = W0 - W1 + F - U
- W0 is pre-exercise body mass in kilograms.
- W1 is post-exercise body mass in kilograms.
- F is fluid consumed during exercise in liters.
- U is urine produced during exercise in liters.
The sweat rate is R = S / t, where t is the test duration in hours. Scale-weight change is shown separately as 100 x (W0 - W1) / W0, because the scale alone does not include drinks or urine.
How the hydration plan is built
For the planned workout, projected sweat loss is P = R x T, where T is the future duration in hours. The selected body-mass-loss target creates an allowable fluid deficit: A = p x W0. This uses the field approximation that 1 kilogram of water equals 1 liter. The planned fluid need is Q = max(0, P - A). The average drinking rate is q = Q / T.
A 0% target replaces the modeled sweat loss. A 2% target allows a larger ending deficit, so the planned drinking volume is lower. If the allowable deficit is larger than projected sweat loss, the calculator shows zero required fluid for the target rather than forcing intake.
What changes the result most
The largest drivers are the weight change, test duration, and how representative the test was. A short test can make small scale or bottle-measurement errors look like a large hourly sweat rate. Heat, humidity, clothing, acclimatization, intensity, and sport all change sweat rate, so one cool easy run should not be used as the plan for a hot race or a long ride.
What this calculator leaves out
The model treats short-term body-mass change mainly as water. It does not estimate respiratory water loss, fuel mass changes, unmeasured food, spilled drinks, sweat trapped in clothing, or sweat sodium concentration. It also does not decide whether you should drink to a target; it only converts your test into a planning estimate that should be checked against thirst, stomach tolerance, event access, and medical guidance when relevant.
Worked example
Suppose an athlete weighs 70.0 kg before a 60-minute workout and 69.4 kg afterward. During the test they drink 500 mL and do not urinate. The measured sweat loss is 70.0 - 69.4 + 0.5 - 0 = 1.1 L, so the sweat rate is 1.1 L/h, or about 37.2 US fl oz/h.
For a planned 120-minute session, projected sweat loss is 2.2 L. With a 2% ending body-mass-loss target, the allowable deficit is 0.02 x 70.0 = 1.4 L. The fluid needed to stay within the target is 0.8 L, which is an average of 400 mL/h, or about 13.5 US fl oz/h. The modeled ending body-mass loss is 2.0%.
Common questions
How do I calculate my sweat rate?
Weigh yourself before and after exercise under similar clothing and scale conditions. Add the fluid you drank, subtract any urine produced, then divide the adjusted sweat loss by the test duration in hours.
Why are drinks and urine included in the calculation?
Drinks add mass during the test, so they hide some sweat loss on the scale. Urine removes fluid that is not sweat, so it should be subtracted to avoid overstating sweat loss.
Should I replace 100% of my sweat during exercise?
Not always. Many plans aim to limit body-mass loss rather than replace every drop, because stomach tolerance, thirst, event logistics, and hyponatremia risk matter. A 0% target models full replacement, while a higher target allows some ending deficit.
What does gaining weight during a workout mean?
Weight gain usually means fluid intake exceeded measured losses during that session, or the measurements were inconsistent. Drinking enough to gain weight during exercise can increase hyponatremia risk, so recheck the test before using it as a plan.
Can I use one sweat rate for both running and cycling?
Use caution. Sport, intensity, airflow, clothing, and equipment can change sweat rate, even in the same weather. Repeat the test under conditions that match the session you are planning.