The short answer: roughly 30-35 ml per kg of body weight per day, adjusted for activity and heat - and food gives you about 20% of it for free. The famous "8 glasses" rule has no research behind it. Set your inputs below for a realistic target.
BODY WEIGHT
70 kg
ACTIVITY TODAY
CLIMATE
COFFEE OR TEA TODAY
DRINK ROUGHLY
2.3
litres · 79 oz
That's about 9 glasses of 250 ml across the day. Food covers roughly another 0.6 L, so this is what you actually drink.
SPREAD ACROSS THE DAY
Wake-10am
582 ml
10am-2pm
698 ml
2-6pm
698 ml
After 6pm
349 ml
Front-load the day and taper after dinner - late fluid is a common cause of 3 am bathroom wake-ups.
Do you really need 8 glasses a day?
No - the "8×8" rule has no identifiable scientific origin. Health authorities give total water intake figures of roughly 2.7 L/day for women and 3.7 L/day for men, and crucially those totals include water from food and all other drinks. Food typically supplies about 20%. Tea, coffee, milk and juice all count toward the rest - the idea that caffeine dehydrates you is largely a myth at normal intakes, though a big coffee day is worth an extra glass.
A simpler check than any calculator
Pale-straw urine and rarely feeling thirsty means you're fine. Dark yellow, headaches, or a dry mouth mid-afternoon means drink more. Thirst is a decent guide for most healthy adults under 65 - it becomes less reliable with age, which is why older adults benefit from drinking on a schedule rather than on demand.
Does drinking more actually change how you feel?
Mild dehydration measurably worsens fatigue and concentration - but so do six other things, and you can't feel which is which. Claritips logs your glasses alongside energy and mood in a 60-second daily check-in, then compares: "energy on 6+ glass days: 3.8★ vs 3.0★ below." Cheapest experiment on the list.
Yes. At normal intakes the diuretic effect is mild and more than offset by the fluid in the cup - reviews find no meaningful dehydration from moderate coffee or tea. Very high intakes are worth an extra glass, which is what the caffeine setting above adds.
Can you drink too much water?
Rarely, but yes - drinking litres in a short period can dilute blood sodium (hyponatraemia), which is dangerous. Spread intake across the day rather than downing a litre at once, and don’t force fluid past comfortable.
Does drinking water help with weight loss?
Modestly and indirectly: water before meals can reduce intake slightly, and thirst is sometimes misread as hunger. It is not a fat-burning mechanism, whatever the infographic says.
Why do I wake up at night to pee?
Often simple timing - a lot of fluid after dinner. Try shifting your last big drink to two hours before bed. If it persists despite that, or you’re up more than twice a night, it’s worth mentioning to a doctor.
Is this calculator medical advice?
No. It’s a rough population estimate from body weight, activity and climate. Pregnancy, breastfeeding, kidney or heart conditions and some medications change fluid needs substantially - follow your clinician’s guidance over any calculator.