“Eight glasses a day” is easy to remember — and wrong for most people. How much water you need depends on your weight, how active you are and how hot it is, not on a single magic number. A better starting point is weight-based: about 35 ml per kilogram of body weight a day, plus more when you sweat.
Your personal starting point
At 35 ml/kg, a 70 kg adult needs about 2.5 litres a day and an 80 kg adult about 2.8 litres — roughly 11 glasses, or 95 fluid ounces. That figure includes water from food (about a fifth of intake) as well as drinks, so do not panic if your bottle count looks lower. Use the Water Intake Calculator for your own target.
When you need more
- Exercise: add roughly 0.5 litres per hour of moderate exercise, more in heat.
- Heat and humidity: sweat losses rise sharply; drink before you feel thirsty.
- Illness: fever, vomiting or diarrhoea increase losses and need replacing.
- Pregnancy and breastfeeding: needs rise, typically by 0.3–0.7 litres a day.
Thirst, urine and other checks
Thirst is a decent guide for most healthy adults in normal conditions — drink when you are thirsty, and a little more around exercise and heat. Pale yellow urine usually means you are well hydrated; dark yellow means drink more; completely clear urine all day can mean you are overdoing it. Headaches, fatigue and constipation are sometimes early signs of under-drinking, but they have many causes.
Can you drink too much?
Yes, though it is rare outside endurance events and forced over-drinking: drinking far more than you lose can dilute blood sodium (hyponatraemia). If you exercise for hours, include electrolytes or salty food rather than plain water alone, and do not force litres beyond thirst without reason.
Does coffee count?
Yes — tea, coffee, milk, juice, soups and water-rich foods all contribute. Caffeine in normal amounts does not dehydrate you enough to cancel the fluid in the drink. Water itself is still the simplest, cheapest default.
See the methodology for the 35 ml/kg basis.