Trail Running Hydration Calculator
How much water should you carry? Enter the conditions and the route and get a drinking range in mL/hour, a total to carry, and a sodium target. Free, instant, no account needed.
Conditions
What's the weather doing on the day?
Route & Runner
How far, how long, how much climbing.
Your Fluid Plan
How the Sweat Rate Estimate Works
The estimate starts from a low baseline sweat rate in cool conditions and rises linearly as temperature increases, then is scaled by effort level, body mass, humidity, and route steepness. Recommended intake is deliberately less than 100% of estimated sweat loss — targeting a zero deficit during exercise is neither realistic nor necessary, and ACSM's general fluid guidance for exercise is 400-800mL/hour. Individual sweat rates vary by up to a factor of three in real conditions — treat this as a starting plan and calibrate it by weighing yourself before and after a few long runs.
Frequently Asked Questions
How much water should I drink per hour when trail running?
400-800mL/hour is a reasonable range for most runners; the exact figure depends on temperature, effort, and body size. This calculator gives a narrower range based on the conditions you enter.
Why does the calculator not tell me to replace 100% of my sweat loss?
A mild fluid deficit during exercise is normal and expected; trying to fully replace losses in real time isn't realistic and pushes against absorption limits at high sweat rates. Sports-nutrition guidance generally targets 70-80% of losses.
How does elevation gain change how much water I need?
A steeper route demands more effort and heat production than a flat route of the same distance, which raises your sweat rate. The calculator bumps the estimate up when elevation gain per km is high.
How much sodium do I need on a long trail run?
Ultra-endurance nutrition literature commonly cites a range of 300-600mg/hour, which helps reduce the risk of exercise-associated hyponatremia.
Do I need to carry water for a one-hour trail run?
Usually not for short, cool runs — but even an hour of hard effort in the heat can produce meaningful sweat loss. Enter your conditions to see the estimate.
Is this hydration calculator free?
Yes — completely free, no account needed, all calculation happens in your browser.