Get a personalized daily water goal based on your body weight, activity and climate — shown in ounces, cups, liters and 8-oz glasses, with a simple visual.
A water intake calculator estimates how much water you should drink each day from your body weight and activity level. A common guideline is about half an ounce to one ounce per pound of body weight, with more needed during exercise or hot weather.
Personalized daily hydration goal
Based on your body weight, activity and climate — not a generic "8 glasses" rule.
Ounces, cups, liters, 8-oz glasses and water bottles, all at once.
Runs entirely in your browser — nothing is uploaded.
The famous "8 glasses a day" rule (about 64 oz) is a handy starting point, but real hydration needs depend on your body weight, activity level, climate and overall health. A common evidence-based guideline is roughly half to one ounce of water per pound of body weight (about 30–35 ml per kg), with extra fluid to replace what you lose through exercise and heat.
This calculator estimates a baseline from your weight, then adds about 12 oz (350 ml) for every 30 minutes of exercise and adjusts for a hot or cold climate and your general activity. The U.S. National Academies suggest a total daily fluid intake of about 125 oz (3.7 L) for men and 91 oz (2.7 L) for women from all food and drink combined — roughly 80% of that comes from beverages and 20% from food.
Signs you are well hydrated include pale-yellow urine and rarely feeling thirsty. Dark urine, headaches, fatigue and dizziness can signal you need more. You do not have to hit your goal with plain water alone — milk, tea, coffee, fruit and vegetables all count toward fluid intake. Spread your drinking through the day rather than gulping it all at once, and drink a little extra when you exercise, fly, or spend time in the heat.
A note of caution: drinking too much water in a short time can dilute blood sodium (a rare condition called hyponatremia), so there is no need to vastly exceed your goal. These figures are general guidance, not medical advice — people with kidney, heart or other conditions, and pregnant or breastfeeding women, should follow their doctor's recommendations.
Daily water need scales with body mass, because larger bodies lose more through respiration, perspiration and urine. The common clinical rule of thumb is 30–40 ml per kilogram, which for most adults lands near the familiar two-to-three litres — though the popular "eight glasses a day" has no research basis and is an oddly persistent invention. Two adjustments matter more than the base figure: exercise adds roughly 350–700 ml per hour of activity depending on intensity and heat, and around 20% of daily water intake arrives in food rather than drink. Thirst is a reliable guide for healthy adults, and urine colour is a better daily check than any arithmetic.
Baseline (ml) = body weight (kg) × 35Total = Baseline + (500 ml × hours of exercise)Roughly 20% typically comes from foodwhere:
Assumptions: General guidance for healthy adults. Kidney disease, heart failure and some medications require individualised fluid targets set by a clinician, sometimes restricting rather than encouraging intake. Pregnancy and breastfeeding raise requirements.
Baseline need plus a single hour of moderate exercise, then adjusted for water from food.
ResultAbout 2.7 litres to drink (3.4 litres total intake)
On a rest day the same person needs about 2.3 litres of drink. In hot weather or at altitude, add more. Pale straw-coloured urine is the practical confirmation that the number is roughly right for you.