Hydration Calculator
Turn run duration, heat and effort into a starting number for how much fluid to carry, and how many flasks that actually means. Nothing is stored or sent anywhere.
Read this before you trust the number
Sweat rate varies enormously between people, by a factor of three or more between two runners of the same size in the same conditions. This calculator gives you a planning baseline built from the usual per-hour ranges, adjusted for heat and effort. It is a starting point to personalise, not a prescription.
The only way to get your real number is to measure it: weigh yourself naked before a run, weigh yourself again straight after towelling off, and add back anything you drank. Every kilogram lost is roughly a litre of fluid you did not replace. Do that two or three times in different conditions and you will know more about your own hydration than any calculator can tell you. The method is explained in how much water to carry on a trail run.
Two warnings. Drinking far more than you sweat is genuinely dangerous on long efforts: overdrinking plain water dilutes blood sodium, and hyponatremia hurts and occasionally kills people in ultras. And this is general information from a gear site, not medical advice. If you have a medical condition that affects fluid or electrolyte balance, ask a doctor, not a website.