Body weight
The starting point. Larger bodies hold more water and lose more of it through the day. The base intake is roughly 31–35 ml of water per kilogram of body weight, varying by sex.
How much water should you drink?
Your daily hydration goal depends on your body, your day, and what you eat. This calculator uses the same formula as the Waterie app, so the number you see is the number you'll track.
kg
Sex
Activity level
Your daily hydration goal
2,350ml
per day
79.5 fl oz
≈ 9.4 glasses (250 ml each)
General guidance, not medical advice. Consult your doctor if you have kidney disease, heart failure, or any condition that restricts fluid intake.
How the calculation works
Five factors go into your daily goal. Most online calculators ignore the last one — and they're almost always too high as a result.
Sex
Men tend to carry more lean mass and higher basal water turnover than women, so the per-kilogram rate is slightly higher: 35 ml/kg for males, 31 ml/kg for females. We use 33 ml/kg as a middle point for anyone who selects other.
Age
Teens (14–17) have higher relative water needs while they grow, so their goal is nudged up. Adults 31–55 trend down slightly, and 56+ trends down further as lean mass and kidney concentrating ability decline.
Activity level
Sweat is the biggest single driver of extra water loss. We apply a multiplier from 1.0× (sedentary) to 1.4× (very active). If you had an unusually hot or intense training day, drink more than the calculated number — it represents a normal day at your activity level.
Food already gives you ~20% of your water
This is the factor most calculators skip, and it's why they give you numbers that feel impossibly high. Fruit, vegetables, soups, yogurt, and cooked grains all contribute real water. Research consistently puts the food share at around 20% of total daily water intake, so we multiply the drink-only target by 0.80. The goal shown above is the water you should drink from beverages — the rest comes from what you eat.
Frequently asked questions
Is the 8-glasses-a-day rule true?
No, it's a rough average at best. Eight 250 ml glasses is 2,000 ml, which happens to land close to the drink-only goal for an average adult — but your real number depends on your weight, sex, age, activity, and what you eat. A 55 kg sedentary adult and a 90 kg active adult can differ by more than a litre.
Does coffee or tea count?
Yes, both count toward your hydration. Caffeine has a mild, short-lived diuretic effect but habitual drinkers tolerate it, so the net fluid contribution is positive. The Waterie app uses the Beverage Hydration Index to count each drink with its real hydration value — coffee is about 85%, tea about 95%, water 100%.
Why is this number lower than other water calculators?
Because we subtract the ~20% of daily water you get from food. Other calculators give you the total daily water figure (the 35 ml/kg number) and present it as the amount to drink, which overcounts by a fifth. The number above is what to drink. Food closes the gap.
How much should I drink during exercise?
Add 400–800 ml per hour of moderate to hard exercise, more if it's hot or humid. The activity level selector covers your average day. For a specific training session on top, drink to thirst and refuel with electrolytes for sessions over an hour.
What about weather and climate?
Hot and humid weather can push your needs up by 10–20% or more. The Waterie app layers a weather-based adjustment on top of this baseline for Premium users, comparing today's temperature to your 14-day local baseline. This calculator gives you the neutral-weather starting point.
Can I drink too much water?
Yes. Hyponatremia happens when excessive fluid dilutes blood sodium, and it's dangerous. The calculator clamps results between 1.2 L and 3.6 L per day for this reason. If you want to drink significantly more than 3.6 L, especially without electrolyte replacement, talk to a doctor first.
Track this automatically
Waterie sets your goal the same way this calculator does — then helps you hit it. Every drink counts with its real hydration value. Your buddy reacts as you go. Reminders fire when you need them, not on a rigid schedule.
Download on the App Store