Estimate exercise fluid loss from pre- and post-workout weight, fluid consumed, and session duration, with sweat rate and rehydration targets.
Share this calculator
Also in Hydration
Health & Hydration
Accurate fluid loss measurement after exercise helps athletes and active individuals rehydrate correctly. Even mild dehydration of 1–2% of body weight can measurably impair physical and cognitive performance.
Each kilogram of body weight lost during exercise represents approximately 1 litre of fluid deficit — accounting for fluid consumed but not yet excreted. The formula adds any fluid consumed during the session because those drinks offset losses that would otherwise appear as greater weight reduction: Fluid loss (mL) = (pre-weight − post-weight) × 1,000 + fluid consumed (mL).
The American College of Sports Medicine recommends rehydrating with approximately 1.5 times the fluid lost to offset continued urine production and perspiration after exercise ends. Spreading rehydration over 2–4 hours rather than drinking it all at once improves fluid retention.
Frequently asked questions
For exercise under an hour in moderate conditions, water is generally sufficient. For sessions over 60–90 minutes, especially in heat, replacing sodium alongside fluid helps retain water and prevents hyponatraemia from drinking plain water without salt replacement.
Weight-based measurement is the most practical field method and is recommended by sports science bodies. Accuracy depends on consistent measurement conditions: empty bladder before both weighs, minimal clothing, no eating between measurements.
Related
These related calculators come from the same leaf category, nearby sibling categories, or the same top-level topic.
Estimate daily water intake from body weight, exercise time, and climate, with litres, fluid ounces, and cup targets.
Estimate daily hydration needs from body weight, wake time, exercise volume, climate, and sweat rate.
Estimate calories burned running by body weight, duration, and intensity — from a light jog to race-pace sprinting — with pace and distance included.
Estimate calories burned for any activity using MET values from the Compendium of Physical Activities, with weight and duration inputs.
