Water Intake Calculator: How Much Should You Actually Drink?
The old eight-glasses rule ignores your size and your day. A weight-based estimate with an activity adjustment is a better starting point.
What is the Water Intake Calculator?
This tool starts from a simple rule of thumb — about 35 millilitres of water per kilogram of body weight per day — then adds extra for activity: nothing for light days, half a litre for moderate activity, and a full litre for high activity. It also converts the total into 250 ml glasses. The result is general information only, not medical advice, and hot weather, illness, and diet all change real needs.
How to use it
- Enter your weight in kilograms.
- Choose your activity level for the day.
- Read your daily water target in litres and glasses.
Worked example
A 70 kg person with moderate activity needs 2.95 litres per day (2.45 litres base + 0.5 litres), which is about 12 glasses of 250 ml.
How much water should I drink per day?
A common rule of thumb is about 35 ml per kilogram of body weight per day, so a 70 kg person starts at roughly 2.45 litres, plus more on active or hot days. Thirst, urine colour, and how much you sweat are better day-to-day guides than any fixed number, and most people do not need to force fluids beyond comfortable thirst.
Frequently asked questions
Does food count toward water intake?
Yes — food typically supplies around a fifth of daily water, so this target is a guide for total fluids, not a strict drinking quota.
Should I drink more on hot days?
Usually yes; heat and sweating raise losses, so treat this figure as a baseline and drink more when you are sweating heavily.