Published 19 October 2026
Most adults need roughly 2–3 litres of water a day, but the right number depends on your body weight and how active you are. This calculator starts from a research-backed base of about 35 ml per kilogram of body weight, then adjusts for activity and heat. Enter your weight below.
1. Enter your current body weight and choose kilograms or pounds.
2. Select the activity level that best matches your typical day.
3. Tick the hot climate box if you live or work somewhere warm, or it's summer.
4. Press Calculate My Intake to see your recommended daily total in litres, millilitres and standard 250 ml glasses. The result updates automatically as you change any input.
The calculator is built on a widely used hydration rule of thumb: about 35 ml of water per kilogram of body weight per day. So a 70 kg person starts with a base target of 2,450 ml (2.45 litres) before any adjustments.
This base aligns well with the adequate intakes published by the European Food Safety Authority (EFSA): roughly 2.0 litres per day of total water for women and 2.5 litres for men, counting water from both food and drinks. It is worth remembering that around 20–30% of your daily water typically comes from food — fruit, vegetables, soups and even bread all contribute — so your drinking target is somewhat below the total figure.
From that base, the calculator adds an activity adjustment because exercise increases fluid loss through sweat and respiration: +0.3 L for light activity, +0.6 L for moderate activity, and +1.0 L for very active days. Hot weather raises sweat rates further, so ticking the hot climate option adds another 0.5 L.
These are evidence-informed estimates for healthy adults, not precise medical targets — your actual needs also shift with illness, breastfeeding, altitude and individual differences in sweat rate.
Example 1 — 70 kg, moderately active. Base: 70 × 35 ml = 2,450 ml. Activity bonus: +600 ml. Total: 3.05 litres per day. In practical terms, that is about 12 glasses of 250 ml — roughly one glass every hour and a quarter of the waking day.
Example 2 — 55 kg, sedentary. Base: 55 × 35 ml = 1,925 ml. No activity bonus, no heat bonus. Total: 1.93 litres per day, or about 8 standard 250 ml glasses. This sits just below the EFSA reference for women, which makes sense: food supplies roughly 20–30% of daily water, so about 1.3–1.5 litres of actual drinking fluid would cover the rest comfortably.
Both examples show why one-size-fits-all targets fail: the two people differ by more than a litre a day — roughly four glasses.
You don't need a lab test to check your hydration. The most practical everyday indicators are:
Pale-yellow urine. Light straw colour is the classic sign of good hydration; dark amber means drink more water soon. If your urine is consistently colourless, you may be drinking slightly more than needed.
Steady energy. Even mild dehydration can bring on tiredness and difficulty concentrating by the afternoon, so stable energy levels are a good sign.
Infrequent headaches. Dehydration is a common trigger for headaches, so if they're rare and your water intake is on target, you're probably doing well.
For many average-sized, moderately active adults, 2 litres a day is a reasonable baseline — it is close to the EFSA adequate intake of about 2.0 litres of total water per day for women. But it is not a universal target: larger people, very active people, and anyone in hot weather typically need 2.5 to 3.5 litres or more. Use the calculator above for a personalised target.
Yes, although it is rare. Drinking extreme volumes very quickly can dilute the sodium in your blood, a condition called hyponatraemia (water intoxication), which can cause nausea, confusion and, in severe cases, seizures. It mostly affects endurance athletes drinking far beyond thirst and people with certain kidney conditions. Sticking to your calculated target and drinking to thirst keeps you safe.
Yes. Despite their mild diuretic effect, coffee and tea are still mostly water, and research shows moderate caffeine intake (up to around 300 mg, roughly 3–4 cups of coffee) does not cause net dehydration in habitual drinkers. They count towards your daily total, although plain water should still be your main source.
Urine colour is the simplest reliable guide: pale straw or light yellow generally means good hydration, while dark amber suggests you should drink more. Completely clear urine most of the time can mean you are overdoing it slightly. Other signs of good hydration include steady energy through the day and infrequent headaches.
Yes. Exercise increases fluid loss through sweat and breathing. As a rough guide, add about 300–500 ml for 30–60 minutes of moderate exercise, and up to 1 litre for intense or prolonged sessions over an hour. Drink before, during and after the workout rather than all at once, and include electrolytes for sessions longer than about 90 minutes.
Yes. In hot weather your body sweats more to cool itself, and fluid needs can rise by around 500 ml to 1 litre per day depending on the heat and your activity. Tick the hot climate option in the calculator above and it will add 0.5 litres to your target automatically.
The 8×8 rule (eight 8-ounce glasses, about 1.9 litres) has no single scientific origin — it is usually traced to a 1945 US Food and Nutrition Board note that was taken out of context. Research since then shows there is no magic number for everyone; needs vary by body size, diet, activity and climate. EFSA's adequate intakes — about 2.0 litres of total water per day for women and 2.5 litres for men, from food and drinks combined — are a better evidence-based starting point.
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