Hydration Guide — How Much Water Do You Really Need?
Water is the most overlooked performance aid in sport. Losing just 2% of body weight as sweat — 1.4 kg for a 70 kg person — measurably reduces endurance, raises heart rate and makes exercise feel harder, and the effect grows with every extra percent. Yet most people drink by habit rather than by need: the same two glasses a day whether they train or not, in summer or winter, and then wonder why the afternoon session feels heavy.
The headline answer is that the European Food Safety Authority sets adequate intakes at 2.5 litres of water a day for men and 2.0 litres for women, from all drinks and food — and about 20% of that normally comes from food, so the drinks target is roughly 2.0 litres for men and 1.6 litres for women. Exercise adds 0.5–2 litres per hour of sweating on top, which is why a single fixed number is useless for active people. The right target is personal, and this guide shows how to work it out.
Use the calculator
Start with your own numbers: try our water intake calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: What to Eat After a Workout, Caffeine and Exercise, How to Recover Faster After Workouts. You can also browse the full guides section for the rest of the series.
The key points
Thirst is a late signal, not an early one. By the time you feel thirsty during exercise, you are usually already 1–2% dehydrated. That does not mean you should drink constantly — over-drinking is also a real risk in long events — but it does mean that waiting for thirst during a long session is a poor strategy. Drink to a plan during sessions over 60 minutes, and drink normally the rest of the day.
Sweat rate varies enormously between people. Two people doing the same workout can sweat 0.5 litres an hour or 2 litres an hour, depending on body size, fitness, heat and clothing. The only way to know yours is to weigh yourself before and after a session: each kilogram lost is roughly a litre of sweat. Do it once in warm conditions and once in cool conditions, and you have a personal hydration plan instead of a generic one.
Urine colour is the simplest daily check. Pale straw yellow means you are well hydrated; dark yellow means drink more; completely clear usually means you are drinking more than you need. Check the first urine of the day and one in the afternoon — together they track hydration better than any app, and they cost nothing.
Electrolytes matter when sessions are long or sweaty. Sweat contains about 1 g of sodium per litre, so a two-hour session can cost 2–4 g of salt. Water alone replaces the fluid but not the salt, and drinking large volumes of plain water during very long exercise can dilute blood sodium — a dangerous condition called hyponatraemia. For sessions over 90 minutes, use a sports drink or a salty meal afterwards; for shorter sessions, normal food covers the salt.
At a glance
| Situation | Extra fluid needed | What to drink |
|---|---|---|
| Rest day, cool weather | 0 (baseline ~2 L from drinks) | Water, tea, food |
| 1-hour gym session | 0.5–1 L during/after | Water |
| 2-hour run, warm weather | 2–3 L during/after | Water + electrolytes |
| Hot day, outdoor work | 1–2 L extra | Water, salty food |
| Illness with fever | 0.5–1 L extra | Water, oral rehydration |
| High altitude | 0.5 L extra | Water |
Baseline drinks of about 2.0 litres (men) or 1.6 litres (women) come from the EFSA adequate intakes of 2.5 L and 2.0 L total water, minus the roughly 20% that comes from food. Exercise fluid is additional to this baseline.
The formula behind it
There is no single formula for hydration, because sweat rate is individual, but two calculations are useful. First, sweat rate (L/hour) = (weight before − weight after + fluid drunk) ÷ hours — weigh yourself before and after a session in kilograms, add the litres you drank, and divide by the session length. Second, fluid to replace = weight lost (kg) × 1.5 litres, because ongoing urine and sweat losses mean you need about 50% more than the deficit to fully rehydrate. The EFSA adequate intakes (2.5 L men, 2.0 L women total water per day) are the baseline these additions sit on top of.
Worked example — a 70 kg runner does a 90-minute run in warm weather. Before: 70.0 kg. After: 68.8 kg. Drank 0.5 L during the run. Sweat loss = (70.0 − 68.8) + 0.5 = 1.7 L over 1.5 hours, so the sweat rate is about 1.1 L per hour. To replace it, the target is 1.2 kg lost × 1.5 = 1.8 L over the next few hours, with food for the salt — roughly 2 g of sodium, which a normal salty meal provides. On a rest day the same person needs only the baseline of about 2 litres from drinks. The difference between the two days is 1.8 litres, which is why a single daily target fails active people.
Your plan, step by step
- Weigh yourself before and after one typical session to find your sweat rate — it takes two minutes and replaces all guesswork.
- Set a baseline of about 2 litres of drinks a day (men) or 1.6 litres (women), from water, tea, coffee and food moisture.
- During sessions over 60 minutes, drink to your sweat rate in small, regular sips rather than large gulps.
- For sessions over 90 minutes, add electrolytes — a sports drink, or salted food afterwards.
- Check urine colour twice a day and adjust: dark yellow means more fluid, completely clear means less.
How to use this guide
Use the sweat-rate test once per season — your rate changes with fitness and temperature — and build your drinking plan around it. Keep a bottle at your desk and one in your gym bag, and let urine colour be the daily check rather than an app. If your sessions feel strong and your urine is pale yellow, the plan is working; if sessions feel heavy and your urine is dark, drink more before you change anything else.
Common mistakes
Drinking a fixed '8 glasses' regardless of training. Eight glasses is a reasonable baseline for a sedentary day and far too little for a two-hour run in the heat. Hydration needs scale with sweat, not with a slogan — measure your sweat rate and adjust.
Over-drinking during long events. Drinking far more than you sweat during a marathon or a long ride can dilute blood sodium and cause hyponatraemia, which is dangerous. Drink to your measured sweat rate, not to a fear of dehydration.
Ignoring salt after sweaty sessions. Replacing two litres of sweat with plain water leaves a salt deficit that causes cramps, headaches and poor recovery. Eat a salty meal after long sessions, or use an electrolyte drink during them.
What this guide does NOT tell you
Hydration guidance on this page is for healthy adults; people with kidney disease, heart failure, or conditions or medicines that affect fluid balance must follow their clinician's fluid advice, which may be very different. Endurance athletes should be aware of hyponatraemia risk in events over four hours. This guide is general information, not medical advice, and it does not replace individual guidance for anyone with a medical condition.
Frequently asked questions
How much water should I drink a day?
The EFSA adequate intake is 2.5 litres of total water a day for men and 2.0 litres for women, including water from food — so roughly 2.0 litres and 1.6 litres from drinks. Exercise adds 0.5–2 litres per hour of sweating on top. The best personal check is urine colour: pale straw yellow means you are drinking enough.
Does coffee dehydrate you?
No, not in normal amounts. Caffeine is a mild diuretic, but the water in coffee, tea and other caffeinated drinks more than compensates — studies find coffee hydrates about as well as water at intakes up to four or five cups a day. Very high caffeine intakes and alcohol are different: alcohol genuinely increases fluid loss, which is part of why hangovers involve dehydration.
Should I drink during a workout?
For sessions under 60 minutes, drinking before and after is usually enough. For sessions over 60 minutes, drink during the session in small, regular sips, aiming to replace most of what you sweat — your measured sweat rate tells you how much. For sessions over 90 minutes, add electrolytes as well as water.
What is the best drink for hydration?
Water is the best drink for most sessions. Sports drinks earn their place only in sessions over 90 minutes, when you need carbohydrate and salt as well as fluid. Coconut water, milk and oral rehydration solutions also hydrate well. Sugary soft drinks hydrate too, but they add calories and sugar you probably do not need.
Can you drink too much water?
Yes. Drinking far more than you sweat, especially during long endurance events, can dilute blood sodium and cause hyponatraemia — confusion, nausea, seizures, and in rare cases death. It is a real risk in marathons and ultramarathons. Drink to your measured sweat rate and to thirst, not to a fixed large volume, and include salt for long sessions.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.