How much water should you drink during exercise? A sweat-rate guide

Most hydration advice for exercise is written as if every workout were a marathon in July. Drink before you are thirsty, drink on a schedule, drink a sports drink, drink more than you think.

For most sessions, almost none of that is needed. For a gym session or run under an hour, drinking when you are thirsty is enough. For longer or hotter sessions, there is a real target, and the simplest way to find yours is a bathroom scale.

There is also a risk the "drink more" message hides. For people on long, slow events, drinking too much is a more dangerous mistake than drinking too little.

This post sits between the Workout nutrition cluster and the Hydration cluster. The daily baseline, outside exercise, lives in how much water should you drink per day.

The quick answer, by session

The ranges are deliberately wide. Sweat rate varies more between people than almost any other number in sports nutrition, which is why the most useful step is measuring your own.

The 2% rule

The American College of Sports Medicine position stand on fluid replacement sets the goal of drinking during exercise as preventing a water loss of more than 2% of body weight. It also says plainly that sweat rates vary so much between people that individual plans are the right approach, rather than one number for everyone.

A 2014 review by Cheuvront and Kenefick supports that threshold for endurance exercise: performance drops reliably from about 2% dehydration. For strength and power, the effect is smaller and inconsistent, with no clear threshold.

Here is what 2% looks like at different body weights.

How to find your sweat rate

This takes one session and a bathroom scale. It is the single most useful thing you can do for hydration during exercise, because it replaces guesswork with your own number.

1. Weigh yourself before the session, after using the toilet, in minimal clothing.
2. Train for about an hour at your normal intensity. Note how much you drink, if anything.
3. Towel off and weigh yourself again, in the same minimal clothing, before drinking or using the toilet.
4. Calculate: weight lost in kg, plus litres drunk, equals litres of sweat per hour.

A worked example:

This person stayed well under 2%, so they drank enough. If they had drunk nothing, they would have lost 1.3 kg, or 1.7% of body weight, still under the line for a one-hour session.

Sweat rates commonly fall between about 0.5 and 2 litres per hour, and the same person can sit at both ends depending on heat, humidity and intensity. Do the test once in cool weather and once in hot weather, and you will have the range that matters.

Why overdrinking is the bigger risk on long events

Exercise-associated hyponatremia happens when blood sodium drops too low, usually because someone has drunk much more than they sweated over several hours. The 2015 international consensus statement on the condition names overdrinking as the main cause and recommends drinking to thirst as the primary strategy.

The people most at risk are not elite athletes. They are slower participants in long events, such as marathon runners finishing in four hours or more, who have time to drink at every station while sweating relatively little.

Warning signs include headache, nausea, bloating, swelling of the hands and confusion during or after a long event. Severe hyponatremia is a medical emergency. If you are doing a long event, the practical rules are:

• Drink to thirst, not to a schedule set by the aid stations.
• Do not gain weight during the event. If your sweat test shows you finish heavier than you started, you are drinking too much.
• Do not force down water in the last hours because you are worried about dehydration.

Do you need a sports drink?

For most sessions, no. Sports drinks are designed for longer endurance work, where they deliver three things water does not:

1. Carbohydrate, which matters from around 60 to 90 minutes. The pre-workout meal guide covers the 30 to 60g per hour that extends endurance.
2. Sodium, which replaces what you sweat out and helps you hold on to fluid. The electrolytes post covers the 400 to 1,500mg of sodium per litre of sweat and when it is worth replacing.
3. Flavour, which sounds trivial but makes people drink more when they need to.

They also cost calories, which matters if you are training to lose weight.

A 500ml sports drink is a meaningful share of what a moderate one-hour gym session burns. On a long ride or run, that is the point: the carbs are fuel. In a 45-minute weights session, they are just calories you drank back. Water does the job, and the diet soda vs water post covers what happens when people swap to sweet drinks around exercise.

Before and after the session

Before

Start the session already hydrated. For most people that simply means drinking normally through the day, with pale straw-coloured urine as a rough check. The signs of dehydration post covers the less obvious ones.

The ACSM suggests that if you need to top up before a long or hot session, drink around 5 to 7ml per kg of body weight about four hours beforehand. For a 70 kg person that is roughly 350 to 490ml, one large glass. That leaves time for your body to absorb what it needs and pass the rest before you start.

After

For a normal gym session, drinking with your next meal is enough. If you lost a meaningful amount of weight during a long or hot session, the ACSM guidance is to drink about 1.5 litres for every kilogram lost over the following hours. The extra half litre covers the fluid you lose as urine while you rehydrate. Eating a normal meal with some salt helps you hold on to it.

Coffee and tea count. At normal intakes, caffeine does not cause net fluid loss, which the coffee, tea and sparkling water post covers in detail.

Mistakes to avoid

Drinking on a schedule in a short session

A litre of water forced down during a 45-minute weights session will not improve anything and may leave you uncomfortable. Drink when you want to.

Treating every workout like an endurance event

Sports drinks, electrolyte powders and hydration plans are tools for long or hot sessions. For most gym training, water and thirst are the plan.

Never measuring

People guess their sweat rate badly in both directions. One weigh-in before and after a session is free and tells you more than any calculator.

Drinking at every aid station on a long, slow race

This is the pattern behind most cases of exercise-associated hyponatremia. Drink to thirst, and know your sweat rate from training.

Forgetting the calories in what you drink

Two 750ml sports drinks across a day of training is 360 kcal. That is worth it on a long ride. It is not worth it on a rest day or a short session.

Bottom line

For a gym session or run under an hour, drink when you are thirsty. That is enough for almost everyone.

For longer or hotter sessions, aim to keep weight loss during the session under 2% of body weight, which is 1.5 kg for a 75 kg person. Find your own sweat rate with a scale before and after one session, and add sodium and carbs once you go past 60 to 90 minutes.

On long, slow events, the bigger danger is drinking too much, not too little. Drink to thirst, and do not finish heavier than you started.

Next in this cluster: BCAAs vs EAAs, caffeine before workouts, and the pre-workout meal guide. For the daily baseline outside training, see how much water should you drink per day.