Forget “eight glasses” — the right amount of water scales with the body it has to serve. This calculator builds your daily target from your weight (35 ml per kilogram) plus an honest top-up for exercise, and shows it in litres, glasses and fluid ounces.
Thirst and pale-yellow urine beat any fixed number. Food provides roughly a fifth of daily water. Hot climate or illness? You will need more — and medical conditions can change everything, so ask a professional.
How the target is built
Base: 35 ml × your weight in kg. At 80 kg that is 2,800 ml — 2.8 litres. Activity top-ups: +350 ml for a light 30-minute workout, +600 ml for a solid hour, +900 ml for long or hot sessions. Sweat rates vary enormously, so treat these as sensible averages, not measurements.
For context, the National Academies’ adequate intake for total water — drinks plus the water inside food — is about 3.7 litres a day for men and 2.7 litres for women, with food typically providing around a fifth. Your drinking target landing a little under those totals is exactly what you would expect.
The two checks that beat any calculator
- Thirst. For healthy adults in normal conditions, drinking to thirst is a reliable guide — your body is not shy about asking.
- Urine colour. Pale yellow means you are on track. Consistently dark means drink more; constantly crystal-clear can mean you are overdoing it.
When the standard numbers do not apply
Heat, altitude, illness (especially anything causing fluid loss), pregnancy and breastfeeding all raise needs; some medical conditions and medications require restricting fluids. In any of those situations, a professional’s guidance replaces a web calculator — including this one. More in our guide How Much Water Should You Drink Each Day?.