"Drink 8 glasses a day" is easy to remember but ignores the two things that actually drive your fluid needs: how big you are and how much you sweat. A 50 kg office worker and a 90 kg runner do not need the same amount of water.
A practical formula
A widely used estimate for healthy adults is about 35 ml of fluid per kilogram of body weight per day. That works out to roughly 2.4 liters for a 70 kg person. On days you exercise, add roughly 350–700 ml depending on duration and sweat. General reference values point the same direction: about 3.7 liters of total daily fluids for men and 2.7 liters for women — including the water in food, which typically covers around 20% of intake.
Want the math done for you? Our free water intake calculator turns your weight and activity level into a daily target in liters, glasses, and ounces.
Quick reference by body weight
Using the ~35 ml per kilogram baseline for a moderately active adult (add more on training days):
- 50 kg (110 lb) → about 1.75 L, or 7 glasses
- 60 kg (132 lb) → about 2.1 L, or 8–9 glasses
- 70 kg (154 lb) → about 2.45 L, or 10 glasses
- 80 kg (176 lb) → about 2.8 L, or 11 glasses
- 90 kg (198 lb) → about 3.15 L, or 12–13 glasses
- For your exact number with activity factored in, use our free water intake calculator.
What counts toward the total?
More than you might think:
- Plain water — the best default: zero sugar, zero calories.
- Coffee and tea — despite the caffeine, they contribute net fluid.
- Milk, soup, and water-rich foods like fruit and vegetables.
- Sugary drinks technically hydrate too — but the added sugar makes them a poor way to get there.
Adjust for exercise, heat, and caffeine
Sweat is the big variable: add roughly 350–700 ml per hour of exercise, more in heat or humidity — weigh yourself before and after a hard session and replace each lost kilogram with about a liter over the following hours. Coffee and tea count toward your total; caffeine's diuretic effect at normal doses is far too small to cancel the fluid they deliver. Alcohol is the one drink that genuinely works against you.
Signs you're getting it right
You don't need lab tests. Pale-yellow urine, rarely feeling thirsty, and steady energy through the afternoon usually mean you're in range. Dark urine, headaches, and the 3 p.m. slump often point the other way. Drinking should be spread across the day — chugging a liter at bedtime mostly buys you a midnight bathroom trip.
Knowing the number is easy. Hitting it daily is the habit.
Most people don't fail at hydration because they miscalculated — they just forget. Tie water to existing routines (a glass with each meal, one before each workout) and make progress visible. That's exactly what BigDrink does on Apple devices: smart reminders, home screen widgets, one-tap logging on Apple Watch, and Apple Health sync across iPhone, iPad, Mac, and Apple Vision.