Weigh yourself with minimal dry clothing before and after a representative run. Add fluid consumed, subtract urine produced, and account for the body-mass change. Divide the result by session time. This is an estimate for those conditions—not a fixed prescription for every run.
Use the same units
One kilogram of body-mass change is approximately one liter of water. If using pounds and fluid ounces, convert consistently. Record duration, temperature, humidity, sun, wind, pace, clothing, and route so the number has context.
Example
A runner loses 0.7 kg, drinks 0.4 L, produces no urine, and runs for one hour. Estimated fluid loss is about 1.1 L for that hour and those conditions. Measurement error, retained wet clothing, food, and unrecorded fluid can affect the result.
Do not turn one test into a universal target
Sweat rate changes with weather, effort, acclimation, body size, clothing, and the person. Drinking needs and sodium questions can be medically important; long events or health conditions deserve qualified guidance.
National Athletic Trainers’ Association position statement
The position statement describes body-mass change and fluid records as tools for estimating individual sweat losses and emphasizes individualized hydration planning.
What clothing tells you
A soaked shirt shows retained and transported moisture, not total sweat loss. Use the measurement to understand the session; use garment observations separately to judge cling, rub, weight, and drying.