Hydration Calculator
Estimate your daily fluid target and an in-session drinking rate (ml/hour) from bodyweight, training load, and conditions. Free, no signup.
Aim for roughly 2.5 L of fluid a day as a baseline, and drink 450 ml per hour while you train.
Session over an hour: add sodium/electrolytes — plain water alone isn’t enough.
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About this calculator
Generic apps treat hydration as one flat number, which is useless once you train hard. This tool splits it in two. Your daily baseline is roughly 30–35 ml per kg of bodyweight, nudged up on heavy training days. On top of that sits an in-session rate — how much to drink per hour while you train — driven by how much you sweat, which we estimate from climate and intensity if you don’t know your own rate. For sessions past an hour, plain water isn’t enough: you lose sodium in sweat, and replacing fluid without it can leave you flat or, in extreme cases, dangerously dilute your blood sodium. So we flag when to add electrolytes. Everything here is a guided estimate — the honest way to dial it in is to weigh yourself before and after a session and see how much fluid you actually lost.
Sources
Daily baseline ~30–35 ml/kg adjusted for load; in-session rate ~450–800 ml/hour by sweat rate/climate/intensity; sodium guidance for sessions over ~60 min. Guided estimate, not a prescription.
FAQs
Isn’t “8 glasses a day” enough?
For a sedentary person, maybe. For an athlete it ignores the biggest variable: sweat. A hard session in heat can cost one to two litres an hour, dwarfing your baseline. That’s why we separate everyday fluid from session replacement.
When do I need electrolytes, not just water?
Roughly past 60–90 minutes of sweaty training, or any long effort in heat. Sodium is the main one to replace. Drinking a lot of plain water on long sessions without sodium can actually be counterproductive and, rarely, dangerous (hyponatremia).
How do I find my real sweat rate?
Weigh yourself nude before and after a one-hour session (towel off, note any fluid drunk). Each kilogram lost is about a litre of sweat. Do it in a few conditions and you’ll have a personal number far better than any estimate.