How to estimate a hydration deficit
Use sweat rate, session length and fluid intake.
Estimate your sweat rate
Weigh in before and after a similar session; each kilogram lost is about one litre, plus what you drank.
Enter the session
Use the planned duration and the sweat rate you measured or estimated.
Plan the replacement
Spread the replacement volume over the next few hours rather than drinking it all at once.
What does this tool do?
Sweat losses depend on intensity, heat, clothing and individual sweat rate, so a body-weight check is more useful than a fixed daily target. This tool converts session length and sweat rate into a deficit, then shows a replacement volume.
The Formula
Sweat loss (L) = sweat rate (L/h) × duration (h); deficit = sweat loss − fluid consumed; replacement ≈ 125% of deficitFrequently Asked Questions
How do I measure my sweat rate?
Weigh yourself before and after a session. Each kilogram lost is about one litre of sweat; add the fluid you drank and divide by the session length in hours.
Should I replace 100% of the deficit?
Sports guidance often suggests 125–150% spread over several hours because urine losses continue. Drinking to thirst is a reasonable default outside endurance events.