How much water should you drink a day?
The old “eight glasses a day” rule is easy to remember but ignores the things that really change how much you need: your size, how much you sweat, the weather and whether you are pregnant or breastfeeding. This water intake calculator takes all of those into account and gives you a daily target in litres, glasses of 250 ml and US cups of 8 fl oz.
It starts from a per-kilogram baseline, adds water for exercise and hot, humid weather, and allows for the extra needs of pregnancy and breastfeeding. It also shows how much of your total water normally comes from food, so the target you see is what to drink.
The result is an estimate for healthy adults, not medical advice. Thirst and the colour of your urine remain the best day-to-day guides.
How to use the Water Intake Calculator
- Choose metric (kg, litres) or imperial (lb, fl oz).
- Enter your body weight.
- Enter the minutes of sweaty exercise you do on a typical day.
- Pick your climate, and select pregnancy or breastfeeding if it applies.
- Read your daily target and the breakdown that shows where each part comes from.
Water Intake Calculator features
Weight-based baseline
Uses about 35 ml per kilogram of body weight for total water, so a larger body gets a larger target.
Exercise and heat allowances
Adds roughly 350 ml for every 30 minutes of exercise and 500 ml for hot, humid weather.
Pregnancy and breastfeeding
Adds 300 ml in pregnancy and 700 ml while breastfeeding, in line with European Food Safety Authority advice.
Litres, glasses and cups
See your target as litres or fluid ounces, 250 ml glasses and 8 fl oz cups, plus how much to drink per waking hour.
Transparent breakdown
Every addition is listed, so you can see exactly how the number was built.
How the calculation works
Baseline: healthy adults need roughly 30–35 ml of total water per kilogram of body weight a day. The calculator uses 35 ml/kg and assumes about 20% of it comes from food — fruit, vegetables, soups, rice and dal are all mostly water — leaving 28 ml/kg to drink.
Exercise: sweat losses vary from about 0.5 to 2 litres an hour, so a moderate allowance of 350 ml per 30 minutes is added. Climate: 500 ml is added for hot, humid weather. Pregnancy and breastfeeding: 300 ml and 700 ml a day respectively.
Example: a 70 kg adult who exercises for 30 minutes in a temperate climate needs 35 × 70 = 2,450 ml of total water, of which about 490 ml comes from food. Add 350 ml for exercise and the target is about 2.3 litres to drink — roughly nine 250 ml glasses.
What the official guidelines say
Reference values are given as adequate intakes of total water from all drinks and food. The European Food Safety Authority (EFSA, 2010) suggests 2.0 litres a day for women and 2.5 litres for men. The US National Academies (2004) set higher figures of 2.7 litres for women and 3.7 litres for men, and estimate that about 80% comes from drinks — around 2.2 and 3.0 litres. For pregnancy they suggest 3.0 litres and for breastfeeding 3.8 litres in total.
Our per-kilogram method usually lands between these two sets of figures. If your result is far from them — for example because you are very tall or heavy — use the guidelines as a sanity check.
Practical hydration tips
- Pale straw-coloured urine usually means you are drinking enough; dark yellow suggests you need more.
- Water, milk, tea and coffee all count. Moderate caffeine has only a mild diuretic effect.
- Spread drinks through the day rather than gulping a litre at once. Your kidneys can only clear roughly 0.8–1.0 litre an hour, and drinking far beyond that can dilute blood sodium (hyponatraemia).
- On long, hot workouts, replace salt as well as water. The calories burned calculator helps you plan longer sessions.
- If you are pregnant, the pregnancy due date calculator helps you track which trimester you are in.
Water Intake Calculator FAQ
How much water should I drink a day based on my weight?
A common rule is about 30–35 ml of total water per kilogram of body weight, with around 80% of that from drinks. For a 70 kg person that is roughly 2 litres to drink before adding exercise or hot weather. The water intake calculator does the maths for you.
Is 8 glasses of water a day right for everyone?
Eight 250 ml glasses (2 litres) is a reasonable average, but smaller, less active people may need less and larger or very active people more. Food, especially fruit and vegetables, also supplies water.
Do tea and coffee count towards my water intake?
Yes. Although caffeine is a mild diuretic, the water in tea and coffee more than makes up for it in regular drinkers. Sugary drinks and alcohol count as fluid but are best kept to a minimum.
How much more water do I need when I exercise?
Roughly 350 ml for every 30 minutes, more in heat or for heavy sweaters. Weighing yourself before and after a workout shows your own losses: each kilogram lost is about a litre of sweat.
Can I drink too much water?
Yes, though it is rare in everyday life. Drinking several litres in a short time — sometimes seen in endurance events — can cause hyponatraemia, a dangerous drop in blood sodium. Drink steadily and to thirst.
How much water should I drink when pregnant or breastfeeding?
European guidance adds about 300 ml a day in pregnancy and 700 ml while breastfeeding. Your midwife or doctor may advise differently if you have swelling, high blood pressure or other conditions.