Caffeine vs WaterHydration
How Much Water to Drink During Exercise (Before, During, After and in the Heat)
How much water to drink during exercise depends on your sweat rate. The ACSM guidance in plain numbers, a simple sweat test, and advice for hot weather.
The short answer
- Aim to lose less than 2% of your body weight during a workout (1.4 kg for a 70 kg person). That's the American College of Sports Medicine's (ACSM) target, not "drink as much as possible".
- Sweat rates vary from about 0.4 to 1.8 litres an hour for runners, so there's no single number. A practical starting point for long runs is 0.4–0.8 L (14–27 oz) an hour, sipped to thirst.
- Before: about 5–7 ml per kg of body weight at least 4 hours before a hard session (350–490 ml for 70 kg). After: about 1.5 L for every kg you lost.
- In the heat: about 1 cup (8 oz) every 15–20 minutes, and no more than 48 oz (about 1.4 L) an hour (CDC/NIOSH).
How much water to drink during exercise is one of those questions with a frustrating honest answer: it depends. A 55 kg yoga student and a 95 kg footballer training in July can differ by more than a litre of sweat an hour. But the sports medicine guidance turns into a simple plan, and it's as much about not overdrinking as about drinking enough.
The goal: lose less than 2% of your body weight
The ACSM's position on exercise and fluid replacement sets the target during exercise as preventing "excessive" dehydration, meaning more than 2% of body weight lost from water deficit. Beyond that, performance starts to suffer, especially in the heat. Here's what 2% looks like:
| Body weight | 2% loss | Drink before (5–7 ml/kg) | Top-up if urine is dark (3–5 ml/kg) |
|---|---|---|---|
| 50 kg (110 lb) | 1.0 kg | 250–350 ml | 150–250 ml |
| 60 kg (132 lb) | 1.2 kg | 300–420 ml | 180–300 ml |
| 70 kg (154 lb) | 1.4 kg | 350–490 ml | 210–350 ml |
| 80 kg (176 lb) | 1.6 kg | 400–560 ml | 240–400 ml |
| 90 kg (198 lb) | 1.8 kg | 450–630 ml | 270–450 ml |
| 100 kg (220 lb) | 2.0 kg | 500–700 ml | 300–500 ml |
The ACSM is equally clear that you shouldn't drink so much that you gain weight during exercise. Overdrinking is the more dangerous mistake, which we'll come to below.
Before a workout: start hydrated
If you've been eating and drinking normally and it's been 8–12 hours since your last session, you're probably already close to fully hydrated. For a hard session, a race or training in the heat, the ACSM suggests:
- Slowly drink about 5–7 ml per kg of body weight at least 4 hours before.
- If you're not peeing, or your urine is dark, drink another 3–5 ml per kg about 2 hours before.
- Have some salty food or a drink with sodium. It helps you keep the fluid and makes you thirstier.
Drinking early leaves time to pass any excess before you start. Trying to "super-hydrate" with extra water right before has no clear benefit and raises the risk of diluting your blood sodium.
How much water to drink during exercise
The ACSM doesn't give one number, because sweat rates vary too much between people, sports and weather. Its predicted sweat rates for running range from about 0.4 to 1.8 litres an hour, and measured rates across sports commonly fall between 0.5 and 2.0 litres an hour. Some practical rules:
- Under an hour, moderate effort, mild weather: start hydrated, sip water if you're thirsty, and drink normally afterwards. Your sweat losses will usually be well short of 2%.
- Long endurance sessions: the ACSM suggests marathon runners start at 0.4–0.8 L per hour, drinking to thirst, with the higher end for faster, heavier runners in warm weather and the lower end for slower, lighter runners in cool weather.
- Sip, don't gulp. Regular small drinks are absorbed and kept better than a large amount at once.
Work out your sweat rate in two minutes
This is the method the ACSM recommends, and all you need is a scale:
- Weigh yourself just before a typical workout, after using the toilet, in minimal clothing.
- Note how much you drink during the session.
- Towel off and weigh yourself again straight after.
- Sweat lost = weight before − weight after + fluid drunk. Each 1 kg lost is about 1 litre of sweat.
Example: you weigh 70.0 kg before a 60-minute run, drink 300 ml, and weigh 69.2 kg after. You lost 0.8 kg plus the 0.3 litres you drank, so your sweat rate is about 1.1 litres an hour in those conditions. You finished 1.1% down, comfortably under the 2% line. Repeat in hot weather: the number can be very different.
Exercising in the heat
Heat raises sweat rates and makes dehydration more likely to tip into heat illness. The U.S. National Institute for Occupational Safety and Health (NIOSH), part of the CDC, gives these guidelines for people working in the heat, which translate well to hot-weather training:
| Guideline | Amount |
|---|---|
| Drink regularly | 1 cup (8 oz / 240 ml) every 15–20 min |
| Per hour | 24–32 oz (about 0.7–0.95 L) |
| Upper limit | No more than 48 oz (about 1.4 L) per hour |
NIOSH also says water is almost always enough during work in the heat, as long as you eat regular meals to replace the salt you sweat out, and that most people need several hours after the heat to fully replace what they lost. Its "drink before you're thirsty" advice is for long shifts in the heat; for everyday workouts, thirst is a better guide (see below).
Water or a sports drink?
For most gym sessions, runs and classes, water plus your normal meals is enough. The ACSM notes that drinks with electrolytes and carbohydrates can help "under certain circumstances": long, hard sessions, heavy sweating, hot weather, or when you need energy as well as fluid. For those, the guidance for a sports drink is about 20–30 mEq/L of sodium and 5–10% carbohydrate. Gels, bars and salty snacks can do the same job alongside water.
After a workout: replace what you lost
If you have time before your next session, normal meals and drinks will get you back to fully hydrated. If you need to recover quickly, the ACSM suggests drinking about 1.5 litres for every kilogram of body weight you lost, spread over time rather than all at once. The extra 50% covers the fluid you'll pee out while rehydrating. Salt with your food or fluids helps you keep it.
Everything you drink after exercise counts, including milk, which was one of the most hydrating drinks in a 2016 comparison of 13 drinks. The ACSM does advise going easy on alcohol after exercise if rehydration is the goal. What counts as water intake covers the rest.
Don't overdo it: exercise-associated hyponatremia
Drinking far more than you sweat can dilute the sodium in your blood. This condition, exercise-associated hyponatremia, has caused deaths at marathons and other endurance events. In marathons, it's more likely in smaller runners who run slowly, sweat less and drink heavily before, during and after the race.
A 2015 international consensus statement concluded that the best way to prevent it is simply to drink when you're thirsty, rather than following a forced drinking schedule. A clear sign you've overdrunk: you weigh more at the end than at the start.
What about caffeine and pre-workout?
Caffeine doesn't dehydrate you during exercise. A 2015 meta-analysis found caffeine's small diuretic effect largely disappears when you're active, and the ACSM says it's doubtful caffeine during exercise raises urine output. That's the hydration side. The sleep side is different: a typical pre-workout scoop has about 200 mg of caffeine, twice a standard coffee, and an evening workout with a pre-workout can leave a lot of caffeine in you at bedtime. Check when to stop caffeine before bed or try the bedtime caffeine calculator. Hidden sources of caffeine covers other pre-workout products.
Using your Apple Watch for workout hydration
Your Apple Watch doesn't measure sweat, but it can help. The workout summary shows duration and heart rate, which you can compare with a before-and-after weigh-in to learn your sweat rate for different kinds of sessions.
In Caffeine vs Water, you can log water straight from the Apple Watch app, and your daily goal already includes extra for an active lifestyle and hot weather. It's an estimate of daily needs, not a live sweat measurement. More on the Watch side is in caffeine tracking on Apple Watch. For your baseline daily target, start with how much water you should drink a day, and if you're not sure whether you're low, read up on the signs of dehydration.
Frequently asked questions
How much water should I drink during exercise?
Enough to keep your weight loss under about 2% of body weight, without gaining weight. For long endurance sessions, a common starting point is 0.4–0.8 litres an hour, drinking to thirst. For shorter workouts in mild weather, start hydrated, sip to thirst and drink normally afterwards.
How much water should I drink before a workout?
The ACSM suggests about 5–7 ml per kg of body weight at least 4 hours before a hard session, which is 350–490 ml for a 70 kg person. Drink another 3–5 ml per kg about 2 hours before if your urine is still dark.
How much water should I drink after exercise?
If you have time, normal meals and drinks are enough. To recover quickly, drink about 1.5 litres for every kilogram of body weight you lost, spread out over time, with some salt.
How much water should I drink when exercising in the heat?
CDC/NIOSH guidance for work in the heat is 1 cup (8 oz) every 15–20 minutes, about 24–32 oz an hour, and no more than 48 oz (about 1.4 litres) an hour.
Can you drink too much water during exercise?
Yes. Drinking much more than you sweat, especially during long events, can dilute your blood sodium (exercise-associated hyponatremia), which can be life-threatening. A 2015 consensus statement recommends drinking to thirst.
Do I need a sports drink for hydration during workouts?
Usually not. Water plus normal meals covers most workouts. Sports drinks with sodium and carbohydrate can help in long, intense sessions, heavy sweating or hot weather.
Sources
- American College of Sports Medicine; Sawka MN, et al. Exercise and fluid replacement. Med Sci Sports Exerc, 2007.
- CDC / NIOSH. Heat stress: hydration, 2017.
- Hew-Butler T, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med, 2015.
- Zhang Y, et al. Caffeine and diuresis during rest and exercise: a meta-analysis. J Sci Med Sport, 2015.
- Maughan RJ, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr, 2016.
General wellness information, not medical advice. If you're pregnant, have a heart condition, or take medication that affects caffeine, check your limits with a doctor.