How Much Water Should You Drink When Exercising?

There's no single right amount, because how much you sweat depends on you, your workout and the weather. Sports medicine guidance comes down to this: start exercise well hydrated, try not to lose more than about 2% of your body weight while you exercise, never drink so much that you gain weight, and replace what you lost afterward. Some experts prefer simply drinking to thirst. Weighing yourself before and after a workout is the simplest way to find your own numbers.

Why there's no one number for everyone

The US National Academies say that people who are physically active or exposed to heat need more water than the reference values for everyday life. Very active people who are continually exposed to hot weather often need 6 liters (about 203 fl oz) or more of total water a day.

Sweat losses aren't the same for everyone or every session, which is why the American College of Sports Medicine (ACSM) recommends "customized fluid replacement programs" rather than a fixed amount. A half-hour walk on a cool morning and a two-hour run in summer are very different situations.

If a doctor has told you to limit fluids (for example for heart failure or kidney disease), follow their limit rather than general advice, and ask them how it applies to exercise.

Before you exercise

The ACSM advises starting exercise well hydrated. For most people that simply means drinking normally through the day. If you need to top up, the ACSM suggests doing it "at least several hours before" you start, rather than drinking a lot right before you set off.

During exercise

The 2% guideline

The ACSM's goal for drinking during exercise is to prevent "excessive (>2% body weight loss from water deficit) dehydration," which can affect performance. The National Athletic Trainers' Association (NATA) gives the same target: a body mass loss of less than 2% by the end of exercise.

For a person who weighs 70 kg (154 lb), 2% is 1.4 kg (about 3 lb). Whether you get anywhere near that depends on how long and hard you go, how hot it is, and how much you sweat.

Drink to thirst, or follow a plan?

Here the experts genuinely disagree, and it helps to know both sides.

Drinking to thirst. A 2017 review on exercise-associated hyponatremia (low blood sodium during or after exercise) recommends exactly that: "Drink according to thirst." The authors call it "the most individualized hydration strategy" and say that "following thirst as a real-time guide appears safe and effective." They also note that fluid deficits up to about 3% of body mass "can be well tolerated."

Planned drinking. The ACSM recommends a customized plan that keeps losses under 2% of body weight. NATA points out that in competitive athletes, "thirst may be delayed," and that beyond age 50, thirst sensitivity decreases when the body is short of water.

Where they agree. NATA, on the planning side, and the hyponatremia review, on the thirst side, both warn against drinking more than you lose. That's the point that matters most for safety.

One practical reading of both views: for everyday workouts, drinking when you're thirsty keeps things simple. If you compete or are over 50 (where NATA says thirst may lag), or you train for long periods, it can help to know your sweat rate and have a rough plan. Heat changes things too; see how to stay hydrated in hot weather.

Electrolytes in longer sessions

The ACSM's aim covers more than water: it also wants to avoid "excessive changes in electrolyte balance." It says drinks with electrolytes and carbohydrate "can provide benefits over water alone under certain circumstances." NATA recommends that sodium before and during exercise "should be individualized based on specific losses and needs." If you train long and sweat heavily, a coach or sports dietitian can help you work out what suits you.

After exercise

The ACSM advises replacing any fluid and electrolyte deficit once you've finished. NATA is more specific: "Up to 150% of the estimated fluid deficit needs to be consumed." Weighing yourself is the easiest way to estimate that deficit.

Spread your recovery drinks out rather than drinking them all at once. The National Academies note that healthy kidneys can clear at most about 0.7 to 1.0 liters (about 24 to 34 fl oz) of water an hour.

How to work out your sweat rate

The ACSM notes that "individual sweat rates can be estimated by measuring body weight before and after exercise." Here's a simple way to do it:

Weigh yourself just before exercise, after using the bathroom, in as little clothing as possible.
Exercise as usual and note how much you drink.
Towel off and weigh yourself again in the same clothing.

Every kilogram (2.2 lb) of body weight you lose is roughly a liter (about 34 fl oz) of fluid. Say you weigh 70.0 kg (154.3 lb) before an hour's run and 69.2 kg (152.6 lb) after, and you drank 500 ml (about 17 fl oz) along the way:

You lost 0.8 kg (about 1.8 lb), a little over 1% of your body weight, so you stayed inside the 2% guideline.
Your total sweat loss was roughly 0.8 + 0.5 = 1.3 liters (about 44 fl oz) in that hour.
Under NATA's guidance, you'd drink up to 1.2 liters (about 41 fl oz) afterward, which is 150% of the 0.8 liter deficit.

Repeat it in different conditions, such as a cool day and a hot one, or an easy session and a hard one, and you'll get a feel for your own range.

If you weigh more after exercise than before, you drank more than you lost. That's the one result to avoid.

Don't drink more than you sweat

NATA is clear on this: "Physically active individuals should not consume fluid that exceeds their exercise-related body mass losses," and "Individuals should not gain body mass (or body weight) during exercise."

The reason is exercise-associated hyponatremia, where blood sodium falls too low. According to the 2017 review, "overdrinking beyond thirst" is one of the most common causes, together with the hormone vasopressin making the body hold on to water. Pain, nausea, heat, exercise itself and some medicines, such as anti-inflammatory painkillers (NSAIDs), can all raise vasopressin. Most athletes who develop symptoms have gained weight during the event, and the review notes that such weight gains "can only occur through ingestion of fluids in excess of body losses."

This is rare: the National Academies call dangerous overdrinking highly unusual. But it's the main reason not to force down water on a schedule you've never tested. To learn more about the warning signs, see can you drink too much water.

FAQ

How much water should I drink during a workout?

There's no fixed amount. Never drink so much that you gain weight. Either drink when you're thirsty or, as the ACSM and NATA suggest, follow a plan based on your own sweat rate that keeps losses under about 2% of your body weight.

Should I drink water before a workout?

Start well hydrated, which for most people means drinking normally during the day. If you need to top up, the ACSM suggests doing it at least several hours before you exercise.

How much water should I drink after exercise?

NATA says to drink up to 150% of the fluid you lost. Weigh yourself before and after: each kilogram (2.2 lb) lost is roughly a liter (about 34 fl oz) of fluid. For when to worry about fluid loss, see signs of dehydration.

Do I need a sports drink instead of water?

Not always. The ACSM says drinks with electrolytes and carbohydrate can help more than water alone in certain situations, and NATA says sodium should match each person's losses and needs.

Sources

This article is general information, not medical advice. If you have a health condition or your doctor has given you a fluid limit, follow their advice.