Water Intake Calculator
Find a sensible daily water target for your weight and how active you are, and spread it across your day.
Above about 2,500 m (8,200 ft) you breathe out more water, so the target goes up.
About 20% for most diets; more with lots of fruit, vegetables and soup.
How your water target is worked out
A base from your weight, extra for exercise and heat, a share from food, then spread over your day.
- 1
Start with weight
70 kg × 35 ml
= 2,450 ml
35 ml a kg, age 30 - 2
Add your extras
30 min × 0.75 L/h
= + 375 ml
exercise only - 3
Food gives some
2,450 ÷ 0.8 × 20%
= 613 ml food
3.44 L in all - 4
Spread it out
2,825 ml ÷ 16 h
≈ 177 ml an hour
= 11.3 glasses (250 ml)
Read more: the formula, a worked example and reference intakes
Your body loses water all day through breathing, sweat and the toilet, and you replace it from drinks and food. There's no single right number for everyone, so this calculator uses a common weight-based rule, then adds extra for things that make you lose more water: exercise, heat, altitude, pregnancy, breastfeeding and alcohol. It's general guidance for healthy adults, not medical advice.
The formula
Base drinks = weight in kg × ml per kg (40 under age 30, 35 from 30 to 55, 30 over 55)
Total water = base ÷ 0.8, and food gives your food share of that (20% by default)
Drinks a day = total − food + exercise + climate + altitude + pregnancy or breastfeeding + alcohol
Exercise = hours × 0.5 L (light), 0.75 L (moderate) or 1 L (hard)
Worked example
A 30-year-old who weighs 70 kg and exercises at a moderate pace for 30 minutes a day: the base is 70 × 35 = 2,450 ml. Exercise adds 0.5 h × 0.75 L = 375 ml, so drinks come to 2,450 + 375 = 2,825 ml (2.83 L, about 95.5 US fl oz or 11.3 glasses of 250 ml). Food gives about 2,450 ÷ 0.8 × 20% = 612.5 ml, so total water is 2,825 + 612.5 = 3,437.5 ml (about 3.44 L). Awake from 07:00 to 23:00 is 16 hours, so that's 2,825 ÷ 16 ≈ 177 ml an hour, from 07:00 to the last drink at 22:00.
Extras we add
| What | Extra to drink |
|---|---|
| Exercise, light (walking, yoga) | 0.5 L an hour |
| Exercise, moderate (jogging, cycling) | 0.75 L an hour |
| Exercise, hard (running, sports, heavy work) | 1 L an hour |
| Warm (about 25–30 °C / 77–86 °F) | + 500 ml |
| Hot or humid (over 30 °C / 86 °F) | + 1000 ml |
| Altitude 2,500–3,500 m (8,200–11,500 ft) | + 500 ml |
| Altitude above 3,500 m (11,500 ft) | + 1,000 ml |
| Pregnant | + 300 ml |
| Breastfeeding | + 700 ml |
| Each alcoholic drink | + 250 ml (one glass) |
These extras are rough, rounded figures. Sweat rates vary a lot from person to person: in hot weather or long, hard sessions some people lose well over 1 litre an hour. Weighing yourself before and after exercise is the best way to know your own loss (each kilogram lost is about a litre of water).
Reference intakes
| Adults | US National Academies (total) | of which drinks | EFSA, Europe (total) |
|---|---|---|---|
| Men | 3.7 L | 3.0 L | 2.5 L |
| Women | 2.7 L | 2.2 L | 2.0 L |
| Pregnant | 3.0 L | 2.3 L | 2.3 L |
| Breastfeeding | 3.8 L | 3.1 L | 2.7 L |
These are "adequate intakes" for healthy adults in a mild climate, from the US National Academies (2004) and the European Food Safety Authority (2010). They are general guidance, not targets everyone must hit. Figures last reviewed on 10 Oct 2026.
Tea, coffee and other drinks
All drinks count, including tea, coffee, milk and juice. Moderate caffeine doesn't dry you out overall, but most health bodies suggest staying under about 400 mg of caffeine a day for adults (roughly 4 mugs of coffee) and less when pregnant. Water is still the best everyday choice because it has no sugar or calories.
Signs you're drinking enough
Pale straw-coloured pee and rarely feeling thirsty are good signs. Dark yellow pee, headaches and tiredness can mean you need more. Drinking far more than you need in a short time can also be harmful, so spread your drinks out rather than drinking litres at once.
This is general information, not medical advice. If you have kidney, heart or liver problems, or take medicine that affects fluid (such as diuretics), your doctor may want you to drink less or more, so ask them before changing how much you drink. Children's needs are different; follow local health guidance.
Common questions
How much water should I drink a day?
It depends on your weight, age and how active you are. This calculator starts at 35 ml per kg for adults aged 30 to 55 (40 ml under 30, 30 ml over 55), then adds extra for exercise, heat, altitude, pregnancy, breastfeeding and alcohol. A 30-year-old weighing 70 kg needs about 2,450 ml from drinks before exercise.
How much extra water do I need when I exercise?
The calculator adds 0.5 litres for each hour of light exercise, 0.75 litres for moderate and 1 litre for hard. Sweat rates vary a lot, so weighing yourself before and after exercise is the best way to know your own loss: each kilogram lost is about a litre of water.
Do tea and coffee count towards water intake?
Yes. All drinks count, including tea, coffee, milk and juice, and moderate caffeine doesn't dry you out overall. Most health bodies suggest adults stay under about 400 mg of caffeine a day, roughly 4 mugs of coffee, and less when pregnant.
Does food count towards my water intake?
Yes. Food gives part of your total water, about 20% by default in this calculator, so the drinks figure is your total minus what comes from food. For a 70 kg 30-year-old, food gives about 612 ml, so total water is about 3.44 litres with 30 minutes of moderate exercise.
How do I know if I'm drinking enough water?
Pale straw-coloured pee and rarely feeling thirsty are good signs. Dark yellow pee, headaches and tiredness can mean you need more. Water is still the best everyday choice, because it has no sugar or calories, and all drinks count towards your total.
Can you drink too much water?
Yes. Drinking far more than you need in a short time can be harmful, so spread your drinks across the day rather than drinking litres at once. If you have kidney, heart or liver problems, or take medicine that affects fluid, ask your doctor how much to drink.
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