Estimate how much you sweat per hour and how much to drink, sized for running in the heat rather than a temperate climate.
Most hydration guidance is written for runners in climates where an 18 degree morning is normal. Run the same session in 30 degrees at 85 percent humidity and sweat rates can nearly double, because humidity stops sweat evaporating and your body just produces more of it. The eight glasses a day rule tells you nothing useful about a two hour long run in Bangi.
The first is your sweat rate, in litres per hour. The second is how much of it you can actually absorb while running, which for most people is 400 to 800 ml per hour before the stomach starts complaining. If your sweat rate is well above what you can take in, you are not going to fully replace it during the run, and that is normal. Plan to start well hydrated, replace what you comfortably can, and finish the job afterwards.
Dark urine before a run means you started behind. A heavy, sloshing stomach means you are drinking faster than you absorb. A headache and swollen fingers late in a long run can mean too much plain water and not enough sodium. None of this shows up in a calculator, so treat the output as a plan and your body as the correction.
Most runners here sweat between 1.0 and 1.8 litres per hour once the sun is up. Replacing roughly 60 to 80 percent of that during the run works for most people, which usually lands between 500 and 900 ml per hour. Use the calculator above for a number based on your own weight rather than a generic figure.
Weigh yourself with minimal clothing right before a run, note how much fluid you drink during it, then weigh yourself again straight after, towelled dry. Every kilogram lost is roughly one litre of sweat. Add what you drank, divide by the hours you ran, and you have your rate. Do it twice: once on a cool morning and once in the heat.
Yes, and it is more dangerous than mild dehydration. Drinking beyond your sweat rate dilutes blood sodium, a condition called hyponatremia, which in severe cases is life threatening. The simple check: you should finish a run slightly lighter than you started, never heavier.
For runs under an hour, plain water is fine. Beyond 90 minutes in the heat, especially if you are a salty sweater with white marks on your cap, adding sodium helps you retain what you drink. Sports drinks, electrolyte tablets and salted food all work.
Temperature, humidity, effort, clothing, acclimatisation and how well you slept all move it. Someone newly arrived from a cool climate will sweat less efficiently than a local runner for the first couple of weeks. The number is a moving target, so measure it more than once.
Turn any distance and finish time into pace per km, pace per mile and speed.
Open tool →Break a goal finish time into every kilometre split, ready to print.
Open tool →Predict your finish time at another distance from one recent race result.
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