How this is calculated
Inspect the versioned inputs.
| Constant | Value | Scope & source |
|---|---|---|
| rehydrationLow | 1 liters per kilogram of net mass deficit (approximation) | Sports recovery guidance particularly when recovery is under four hours. An illustrative estimate after sweat measurement, not a sauna-specific drinking prescription; chronic disease/fluid restrictions require individual advice. nata-fluid-2017 ↗ Verified 2026-11-08 |
| rehydrationHigh | 1.5 liters per kilogram of net mass deficit (approximation) | Sports recovery guidance particularly when recovery is under four hours. An illustrative estimate after sweat measurement, not a sauna-specific drinking prescription; chronic disease/fluid restrictions require individual advice. nata-fluid-2017 ↗ Verified 2026-11-08 |
| poundKg | 0.45359237 kg/lb | Exact unit definition or derived time/volume conversion. nist-unit-tables ↗ Verified 2026-11-08 |
Input defaults that are not sourced measurements are illustrative planning assumptions. Exact unit definitions, the 52-week convention and algorithm details are in the shared methodology.
Worked example
An example you can inspect.
80.0 → 79.5 kg, 0.25 L drunk and no urine over 30 minutes implies 0.75 L sweat and 1.5 L/hour. The remaining 0.5 kg deficit gives an illustrative 0.5–0.75 L context.
Make the measurements comparable.
Use a consistent weighing setup before and after the observation, and account for the drinks and urine measured during the same interval. Enter body mass in the displayed units and liquid quantities in the units specified by their controls. Changes in clothing, a different scale or an uncertain drink volume can materially affect the estimate. This is a field accounting method, not a direct measurement of sweat composition or an instrument-based diagnosis of hydration status.
Keep sweat loss and remaining deficit distinct.
The sweat estimate starts with the mass decrease, adds fluid drunk and subtracts urine. It answers a different question from the remaining net body-mass deficit. For example, drinking during the interval may replace part of what was lost, so a sweat-loss estimate should not simply be added again as a replacement requirement. The separate replacement context uses the remaining positive net deficit and the cited sports-medicine range. A non-positive deficit does not create an instruction to drink a negative amount.
Use the rate as an estimate of this interval.
Dividing estimated sweat loss by elapsed hours expresses an average rate for the entered observation. It does not forecast the next session or establish that a particular duration is appropriate. A negative implied sweat loss produces an input-check message because the measurements do not support the model. Review the entries rather than adjusting them merely to obtain a positive result. A reported fluid restriction withholds automatic replacement guidance; the relevant care plan and clinician remain the source for that decision.
Preserve uncertainty instead of hiding it.
Use sensible precision for measurements you can actually make. A displayed decimal does not mean the underlying scale and liquid measurements were that accurate. If a small measurement change substantially alters the estimate, treat the result as uncertain rather than choosing the more reassuring number. The output is context for the recorded interval and cannot establish electrolyte needs, replace a fluid-restriction plan or diagnose dehydration.
Questions, considered
Questions about this tool
How should I interpret the hydration & sweat-loss estimator result?
This is a sports measurement approximation, not a sauna drinking prescription or diagnosis. Fluid restrictions, kidney/heart disease, medications and illness need individual advice; comparable clothing and a reliable scale matter. Use before/after mass and fluid measurements to estimate sweat loss, with net-deficit replacement context and clear measurement limitations.
Which assumptions matter in the hydration & sweat-loss estimator?
Estimated sweat loss (L) = pre-mass (kg) − post-mass (kg) + drinks (L) − urine (L). Divide by hours for an estimated rate. Net mass deficit = pre − post. Short-recovery sports guidance of 1.0–1.5 L/kg applies to the remaining net deficit, not total sweat already partly replaced. A gain remains a gain. Negative inferred sweating is inconsistent input, not a reassuring output. Measurements never enter a URL or persistent storage.
Primary sources
Follow the source.
- National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active ↗
Table 2; Fluid Replacement; Rehydration After Exercise; PMID 28985128; DOI 10.4085/1062-6050-52.9.02
Sweat loss uses mass change plus drinks minus urine. With kilograms and liters, divide liters by hours for sweat rate. Short-recovery replacement is 100–150% of net fluid deficit, not necessarily total sweat produced. The statement addresses activity, so applying the measurement method to sauna is an explicit extrapolation. No universal intake prescription; avoid overdrinking and individualize with chronic disease/fluid restrictions.
Checked 2026-11-08