Sweat Rate Calculations
What is Sweat Rate?
Sweat rate is the volume of fluid your body loses through perspiration during exercise, typically expressed in litres per hour. It reflects the balance between fluid lost via sweat and any fluids you ingest.
Why Measure Your Sweat Rate?
Knowing your personal sweat rate helps tailor your hydration strategy to prevent both dehydration, which impairs performance and heats tolerance—and over‑drinking, which risks hyponatraemia.
How to Measure Your Sweat Rate
Weigh yourself (in kilograms) immediately before exercise, wearing minimal clothing.
Complete a training session of known duration without drinking (or note any fluid consumed).
Dry off, weigh yourself again immediately post‑session.
Calculate sweat rate:
Sweat rate (L/h)=(Δbody mass (kg)+fluid consumed (mL)1000)duration (h)Sweat rate (L/h)=duration (h)(Δbody mass (kg)+1000fluid consumed (mL))
This method, adapted from Jeukendrup and detailed by Precision Hydration, provides a reliable field measure of your fluid losses Precision Fuel & Hydration.
Using This Calculator
Enter your pre‑ and post‑session mass, fluid intake, duration, ambient temperature, humidity and altitude. The tool returns your sweat rate (L/h), percentage body‑mass lost and brief environment‑specific notes to guide your fluid plan.
Accounting for Temperature & Humidity
High temperatures (≥ 25 °C) and elevated humidity (≥ 60 %) reduce evaporative cooling, increasing sweat rate and fluid needs, so ensure you aim to replace at least your calculated sweat losses.
Accounting for Altitude
At moderate to high elevations (≥ 1 500 m), “insensible” losses via respiration rise and altitude‑induced diuresis can further increase fluid needs by 0.2–0.5 L/day. Plan to add extra fluids when training or racing at 2 000–3 000 m to maintain hydration and performance.