Hydration during exercise sits in an awkward middle ground: drink too little and your performance tanks and you risk heat illness, drink too much and you can dilute your blood to dangerous levels. The advice you’ve heard — “drink as much as you can” — is actually wrong and occasionally harmful. The real answer is to know your own sweat rate and drink to roughly match it. This guide shows you how to measure that, how much to aim for, what to drink, and how to stay on the safe side of both extremes.
Quick answer
- Most people need about 0.4–0.8 L of fluid per hour during exercise — but it’s individual.
- Find your real number by weighing yourself before and after a session: ~1 kg lost ≈ 1 L of sweat.
- Keep total weight loss under ~2% of body weight to protect endurance performance.
- After exercise, drink ~1.5 L for every 1 kg lost, spread over a few hours.
- Don’t overdrink plain water. Drinking far more than you sweat can cause hyponatremia (dangerously low blood sodium).
- Add sodium for sessions over ~1 hour or heavy sweating.
Why hydration during exercise matters
When you exercise, you sweat to cool yourself, and sweating drains body water. Lose enough and your blood volume drops, your heart has to work harder, your core temperature climbs, and your endurance falls off. The research consistently shows that dehydration beyond about 2% of body weight impairs aerobic performance, especially in the heat.1
But the answer isn’t to drink as much as possible. Overdrinking is its own problem, and we’ll get to it. The goal is to replace what you lose — not more, not dramatically less.
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Generic guidelines are a starting point, but sweat rates vary enormously — from under half a liter an hour to over two liters an hour depending on the person, the intensity, and the conditions.2 Here’s how to find yours:
- Weigh yourself naked (or in minimal dry clothing) right before a one-hour workout.
- Train as normal for one hour, noting how much fluid you drink during it.
- Towel off and weigh yourself again right after.
- Do the math: (pre-weight − post-weight) + fluid you drank = your sweat loss for that hour. Each 1 kg ≈ 1 L of sweat.
Repeat it in different conditions — a cool day and a hot day will give very different numbers. Now you know roughly how much to replace per hour for a given type of session.
How much to drink during exercise
With your sweat rate in hand:
- Short or easy sessions (under ~1 hour): drink to thirst. Plain water is usually fine.
- Longer or hot sessions: aim to replace enough to keep your weight loss under about 2%. For most people that lands around 0.4–0.8 L per hour, sipped at regular intervals rather than chugged.3
- Heavy sweaters in extreme heat may need more — your measured rate is the guide.
Cool, flavored drinks tend to get consumed more readily than warm plain water, which matters when you’re trying to actually hit your target.1 Start drinking early rather than waiting until you’re parched.
Before and after
Before: Show up hydrated. Drinking about 500 ml of fluid roughly two hours before exercise tops you up and leaves time to pee out the excess.3 Don’t overdo it right before — you’ll just slosh.
After: Rehydration is more than matching the loss, because you’ll urinate some of it away. Drink about 1.5 liters for every 1 kg of body weight lost, over the next few hours, and include some sodium to help you hold onto it.4 Our ways to rehydrate guide covers the practical options, and how much water should you drink per day sets your daily baseline.
The overdrinking danger: hyponatremia
This is the part the “drink, drink, drink” crowd gets dangerously wrong. If you drink far more fluid than you sweat — especially plain water over many hours — you can dilute the sodium in your blood to dangerously low levels, a condition called exercise-associated hyponatremia (EAH). It causes nausea, headache, confusion, and in severe cases brain swelling, seizures, and death.
It’s not rare in endurance events: studies of marathon runners report symptomatic and asymptomatic EAH in roughly 7–15% of participants, with the biggest risk factors being overdrinking, longer race times, and being female.5 The takeaway is blunt: more is not better. Drink to replace your losses, not to flood yourself “just in case.”
Suggested read: Carb Loading: The Glycogen Protocol for Endurance
| Status | Cause | Risk |
|---|---|---|
| Well hydrated | Intake ≈ sweat losses | Optimal |
| Dehydrated (>2% loss) | Drinking too little | Reduced performance, heat illness |
| Hyponatremic | Drinking well beyond sweat losses | Nausea, confusion, seizure, death |
Do you need a sports drink or is water fine?
For most workouts under an hour, plain water plus a normal diet does the job. Once you go longer or sweat heavily, adding sodium (and some carbohydrate for fuel) helps you retain fluid and lowers EAH risk. We break this down fully in electrolytes for sweating, and you can compare products in electrolyte drinks and electrolyte water. A normal diet supplies most of what you need — see how much sodium per day for context on intake.
Hydration in the heat
Hot conditions raise your sweat rate, so your fluid needs go up — but the 2% ceiling and the no-overdrinking rule both still apply. Heat also raises the stakes: poor hydration plus high core temperature is the setup for heat illness. If you train in summer, pair this with heat acclimatization (which actually increases your sweat rate, so plan for more fluid) and learn the warning signs in signs of heat exhaustion. The whole hot-weather approach lives in exercising in heat, and a light dynamic warm-up is a good way to start a hot session without overheating early.
A simple hydration protocol
- Arrive hydrated — ~500 ml about two hours before.
- Know your sweat rate from the weigh-in/weigh-out test.
- During exercise, sip ~0.4–0.8 L/h (adjust to your number), starting early.
- Add sodium if the session is over an hour or you’re sweating heavily.
- Don’t exceed your sweat rate — overdrinking is a real risk, not a safety net.
- After, replace ~1.5 L per kg lost with some sodium.
- Check your pee — pale yellow over the day is a decent rough gauge.
Bottom line
Good hydration during exercise is about matching, not maxing. Find your sweat rate with a simple before-and-after weigh-in, then drink enough to keep your weight loss under about 2% — usually 0.4–0.8 L per hour, individualized. Add sodium when sessions run long or you sweat heavily, and rehydrate afterward with around 1.5 L per kilogram lost. Above all, don’t fall for “drink as much as you can”: overdrinking plain water can cause hyponatremia, which affects a meaningful slice of endurance athletes and can be fatal. Replace what you lose, lean on sodium for long efforts, and you’ll perform better and stay safe. For the related pieces, see electrolytes for sweating, exercising in heat, heat acclimatization, and ways to rehydrate.
Convertino VA, Armstrong LE, Coyle EF, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 1996;28(1):i-vii. PubMed | DOI ↩︎ ↩︎
Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Med. 2017;47(Suppl 1):111-128. PubMed | DOI ↩︎
Montain SJ. Hydration recommendations for sport 2008. Curr Sports Med Rep. 2008;7(4):187-192. PubMed | DOI ↩︎ ↩︎
McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers’ Association position statement: fluid replacement for the physically active. J Athl Train. 2017;52(9):877-895. PubMed | DOI ↩︎
Klingert M, Nikolaidis PT, Weiss K, Thuany M, Chlíbková D, Knechtle B. Exercise-associated hyponatremia in marathon runners. J Clin Med. 2022;11(22):6775. PubMed | DOI ↩︎
