Fuelling · Free tool
Running Hydration Calculator
Enter how long you expect to be out, your body weight, and your sweat rate — typed in, or worked out from a simple sweat test. We estimate how much fluid you will lose, the minimum to drink each hour to finish under a 2% body-weight loss, and how many bottles that is.
The 2% rule
Runners lose between half a litre and two litres of sweat an hour, and nobody replaces all of it mid-run. The aim is to keep the deficit under about 2% of body weight — around 1.4 kg for a 70 kg runner — because that is where research shows heart rate, perceived effort and pace start to suffer, especially in heat.
That turns hydration into simple arithmetic: total sweat loss minus the loss you can afford is what you have to drink, spread evenly across the run. For a two-hour effort many runners need nothing at all; for a hot four-hour marathon the same maths can demand 600–800 ml an hour.
Measure your sweat rate
Weigh yourself naked before and after a one-hour run, note how much you drank, and add it back: every kilogram down is a litre of sweat. Switch to the sweat-test mode above and the calculator does the sum. Repeat it on a warm day and a cool one — the same runner can lose twice as much in summer.
Sweat rate is personal and stable enough to plan around, which is why a measured number beats the "drink 500 ml an hour" rule of thumb. If you cannot test, the presets cover a light, moderate and heavy sweater.
Do not overdrink
More is not safer. The gut absorbs at most about a litre an hour, and drinking beyond your losses dilutes blood sodium, which causes exercise-associated hyponatraemia — the most dangerous medical problem at big-city marathons. If the calculator asks for more than a litre an hour, the answer is to start well hydrated, accept some deficit, and slow down, not to force fluid.
Pair the fluid with sodium on long or hot efforts: the sodium calculator turns the same sweat rate into a capsule or electrolyte plan.
Frequently asked questions
How much water should I drink per hour while running?
Enough to keep your body-weight loss under about 2%. For most runners on efforts over two hours that lands between 400 and 800 ml an hour; on shorter runs you may not need to drink at all. Your own number depends on your sweat rate, which this calculator uses directly.
How much should I drink during a marathon?
A 70 kg runner sweating a litre an hour over four hours loses 4 L and can afford to lose 1.4 kg, so needs to drink about 2.6 L — roughly 650 ml an hour, or a cup or two at every aid station. Cooler weather and a lower sweat rate cut that sharply.
Should I just drink to thirst?
For most runs, yes — thirst is a reliable guide and protects against overdrinking. A plan matters for long, hot efforts where thirst lags behind losses and aid stations are spaced out. Use the calculator to set a ceiling and a rhythm, then let thirst fine-tune it.