"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.
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.
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.