Baseline = 35 ml per kg of body weight (30 ml/kg after age 65). Exercise adds about 12 ml per minute of activity. Total ÷ 250 ml gives the number of glasses.
U.S. National Academies (2005) Dietary Reference Intakes for water; ACSM fluid-replacement guidance.
A starting estimate only. Climate, sweat rate, illness, and diet shift real needs substantially — thirst and pale-yellow urine are better daily signals.
People with heart or kidney conditions on a fluid restriction, and anyone advised by a clinician to limit fluids — follow that advice instead.
Frequently asked questions
A practical starting point is about 35 ml per kilogram of body weight, plus extra for exercise. For a 70 kg adult that's roughly 2.5 litres before training. Thirst and urine colour are your best day-to-day guides.
It's a rough average, not a rule. Your needs scale with body size, activity, climate, and diet. This calculator personalises the number to your weight and training instead of using a one-size-fits-all figure.
Roughly 350–700 ml per hour of exercise to replace sweat losses, more in heat. This tool adds about 12 ml per minute of activity on top of your baseline.
Yes. Around 20% of daily fluid typically comes from food, and tea, coffee, and milk all count toward hydration. The target here is total fluid, not just plain water.
Fluid needs per kilogram ease slightly with age, and older adults have a blunted thirst response, so the calculator uses 30 ml/kg after 65. The bigger risk in older age is under-drinking because you don't feel thirsty.
Rarely, but yes — drinking far more than you lose can dilute blood sodium (hyponatraemia), which is dangerous. Spread intake through the day rather than forcing large volumes at once.