Your baseline water needs, covered in how much water you should drink a day, assume a fairly ordinary day. Exercise and heat change the maths, because sweat can take a lot of fluid out of you, fast.
How much do you lose?
Sweat rates vary enormously between people and conditions. During exercise, many people lose somewhere between half a litre and two litres an hour, more in heat and humidity. That's why a fixed "drink X ml per workout" rule doesn't work well.
The easiest way to find your number: weigh yourself just before and just after an hour of your usual exercise, without drinking during it. Each kilogram lost is roughly a litre of sweat. Do it once or twice in different weather and you'll know your range.
Before, during and after exercise
- Before: start well hydrated. Sports medicine guidance suggests drinking roughly 5–7 ml per kg of body weight a few hours beforehand, which is about 350–500 ml for a 70 kg person.
- During: for most sessions under an hour, sipping water to thirst is enough. For longer or very sweaty sessions, aim to replace some of what you lose without trying to replace all of it.
- After: replace what you lost over the next few hours. Because you keep producing urine, you need a bit more than you sweated out, around 1.25–1.5 litres per kilogram lost.
Do you need sports drinks?
For everyday workouts, water and your next meal are all you need. Sports drinks, which add sodium and carbohydrate, earn their place in long sessions (roughly over an hour), heavy sweating, or hot conditions, where you're losing salt as well as water.
Hot days
- Drink regularly through the day rather than waiting until you're thirsty.
- Carry water when you go out, and drink more if you're outdoors or active.
- Watch your urine colour. Pale straw is the goal.
- Check on older relatives and neighbours, who feel thirst less.
- If you've been told to limit fluids for a medical condition, ask your doctor how to manage hot weather.
Learn the signs of dehydration so you can catch it early.
Don't overdo it
Drinking far more than you sweat, especially during long endurance events, can dilute the sodium in your blood. This is called exercise-associated hyponatraemia, and it can be serious. Drink to replace losses, not as much as possible.
Letting your goal adjust automatically
Recalculating your goal every active or hot day is a chore, so Hydration Hero can do it for you:
- Health Power Up reads today's activity from Apple Health and adds to your goal: about 30 ml per 1,000 steps, 1.5 ml per active calorie and 8 ml per minute of exercise. The health data is processed on your phone.
- The weather feature adds water to your goal on hot or humid days, using your local temperature and humidity. Your location stays on your device.
Both are optional and off until you switch them on. On iPhone you can try the App Clip to see how it feels, with nothing to install.
Frequently asked questions
How much water should I drink during exercise?
It depends on how much you sweat, which varies from about half a litre to two litres an hour. For most sessions under an hour, sipping water to thirst is enough. For longer or sweaty sessions, replace some of your losses as you go and the rest afterwards.
How much water should I drink after a workout?
Weigh yourself before and after exercise. Each kilogram lost is about a litre of sweat, and you'll need around 1.25 to 1.5 litres per kilogram lost over the next few hours to fully rehydrate.
Do I need more water on hot days?
Yes. Heat and humidity increase sweating, so drink regularly through the day rather than waiting until you're thirsty, and aim for pale straw-coloured urine. If you've been told to limit fluids for a medical condition, ask your doctor.
Is there an app that adjusts my water goal for exercise and weather?
Hydration Hero does, if you switch the features on. Health Power Up adds water for steps, active calories and exercise minutes from Apple Health, and the weather feature adds water on hot or humid days.
Last updated 29 September 2026. This is general information, not medical advice. If you have a heart, kidney or liver condition, or have been told to limit fluids, follow your doctor's advice on how much to drink.