Where the numbers come from
The baseline of roughly 35 milliliters per kilogram of body weight is a long-standing clinical rule of thumb for adult fluid needs — about 2.5 liters for a 70 kg person before any exercise. Movement adds to it at around 12 ml per half hour of activity to cover sweat, and hot or humid weather multiplies the whole target by 15% because you lose more to perspiration even at rest. No formula beats your own signals, but this lands close to where dietitians start.
Reading the result
The target covers all fluid, not just plain water: coffee, tea, milk, and the roughly 20% of fluid that arrives in food all count, so the glass count is a ceiling of dedicated water drinking rather than a quota. Spread intake across the day, front-load a glass on waking, and let urine color be the daily check — pale straw means you are on track, dark amber means drink up.
Frequently Asked Questions
Is more always better?
No. Healthy kidneys shed a couple of glasses an hour at most, and forcing far beyond thirst can dilute blood sodium — a rare but real condition. People with kidney or heart conditions should follow the intake their care team sets.
Does coffee dehydrate me?
The mild diuretic effect is small and fades with regular drinking, so coffee and tea still contribute positively to your daily total. The old rule that caffeinated drinks do not count has been retired by the research.
Is this medical advice?
No — it is a population-level estimate. Pregnancy, kidney disease, certain medications, and endurance training all change real needs, so treat the result as a starting point and defer to your clinician.