Water Intake Calculator
Estimate how much water to drink each day in litres, cups, and fluid ounces, based on your body weight and activity level.
Water makes up around 60% of your body and drives nearly every process in it — temperature control, circulation, digestion, and joint lubrication. This daily water intake calculator estimates your fluid target from your body weight and activity level, and shows it in litres, cups, and fluid ounces.
How to calculate water intake
The calculator uses a weight-based baseline plus adjustments for your activity, climate, and life stage:
- Baseline: about 35 ml per kg of body weight.
- Activity add-on: extra fluid to replace what you lose through sweat, from none when sedentary up to about a litre a day for hard training.
- Climate: add fluid in hot or humid conditions, where sweat and breathing losses climb.
- Pregnancy / breastfeeding: the Institute of Medicine sets higher adequate intakes — roughly +300 ml in pregnancy and +700 ml when breastfeeding.
For a 70 kg moderately active person in a temperate climate that’s roughly 2,450 ml baseline + 500 ml for activity ≈ 2.95 litres (about 100 fl oz, or 12–13 cups) per day. That weight-based baseline is the core of the estimate — everything else adjusts around it.
What counts toward it
You don’t have to drink the whole amount as plain water:
- All fluids count — water, milk, tea, coffee, and juice all contribute.
- Food provides roughly 20% of your water, especially fruit and vegetables.
So the figure above is a total fluid target; a good chunk arrives through the food and drinks you already have.
Drink more when you need to — but not too much
Your needs rise in heat and humidity, at altitude, during illness with fever or diarrhoea, and during pregnancy or breastfeeding. The best everyday guide is your body: aim for pale-straw-coloured urine and drink when you’re thirsty.
Overdoing it is rarely a worry for most people, but it isn’t impossible: drinking far more than you lose can dilute blood sodium (hyponatraemia), which is why endurance athletes are advised to drink to thirst rather than force fluids during long events. For everyday life, matching your intake to thirst and the estimate above is plenty.
Frequently asked questions
How much water should I drink a day?
A common guideline is about 30–35 ml per kg of body weight, plus extra for exercise — roughly 2.5–3.5 litres for most adults. This calculator personalises it to your weight and activity. The old "8 glasses a day" rule is a reasonable floor but isn't tailored to you.
How do I calculate my water intake?
Start with your weight: multiply kilograms by 30–35 to get your baseline in millilitres (a 70 kg adult lands near 2,450 ml). Then add roughly 500 ml to 1 litre for exercise, plus extra in hot weather or during pregnancy and breastfeeding. This calculator does all of that for you in one step.
How do I calculate water intake by weight?
A common rule of thumb is about 30–35 ml of water per kilogram of body weight per day. For a 70 kg adult that's roughly 2.1–2.5 litres before any exercise. This calculator starts from that weight-based baseline, then adds fluid for your activity level and climate.
Does food count toward my water intake?
Yes. About 20% of your total water typically comes from food, and all drinks — including tea, coffee, and milk — count toward hydration. This calculator estimates total fluid needs; you don't have to drink every millilitre as plain water.
How do I know if I'm drinking enough?
Urine colour is the simplest check: pale straw means you're well hydrated, while dark yellow suggests you need more. Thirst is also a reliable everyday guide for healthy adults.
When do I need more than the calculator shows?
Increase your intake in hot or humid weather, at altitude, during illness with fever, vomiting, or diarrhoea, and during pregnancy or breastfeeding. Endurance exercise and heavy sweating also raise your needs substantially.