Water Intake Calculator
Everything runs in your browser. Your data never leaves your device.
General information only, not medical advice. Talk to a doctor or registered dietitian before making health decisions.
What this calculator does
This tool estimates how much water you should drink in a day. It starts from your body weight, adds extra for exercise, and adds more if you live or work in a hot or humid climate. The result is shown in litres, in 250 ml glasses and in fluid ounces, so it is easy to turn into a daily habit.
How to use it
- Choose your units and enter your weight.
- Enter how many minutes you exercise on a typical day. Leave 0 if you do not.
- Choose your climate.
- Read your daily estimate.
Method and sources
The estimate uses a common rule of thumb:
- Base: 33 ml of water for every kilogram of body weight.
- Exercise: about 350 ml for every 30 minutes of exercise.
- Hot or humid climate: an extra 500 ml.
Example: a 70 kg person who exercises 30 minutes in a temperate climate needs about 70 × 33 = 2,310 ml, plus 350 ml, which is roughly 2.7 litres or 11 glasses.
For comparison, the European Food Safety Authority sets an adequate total water intake, from drinks and food together, of about 2.0 litres for women and 2.5 litres for men.
Tips and cautions
Spread your drinking through the day and drink more when you feel thirsty, ill with a fever, or exercising in the heat. Pale yellow urine is a simple sign of good hydration. This is general information, not medical advice, so ask your doctor if you have a health condition.
Frequently asked questions
Is the "8 glasses a day" rule correct?
It is a simple rule of thumb, not a scientific limit. Needs depend on body size, activity, climate and diet, which is why this calculator uses your weight, exercise and weather.
Does tea, coffee or food count?
Yes. Tea, coffee, milk, juice and soup all contribute, and food supplies roughly a fifth of your fluid. The figure here is an estimate of the drinks you need, and plain water is a good main source.
Can you drink too much water?
Yes, although it is uncommon. Drinking very large amounts quickly can lower blood sodium. Spread your drinks through the day and stop if you feel unwell.
Who should not rely on this calculator?
People who are pregnant or breastfeeding, have kidney, heart or liver conditions, or are told by a doctor to limit fluids. Follow your doctor's advice instead.