Water intake calculator
Fluid needs scale with body size, activity and heat. This estimates the total, then separates what has to come from drinks — food supplies roughly a fifth of your intake.
- Drink, in cups
- 10.1
- Total fluid, in fluid ounces
- 101
- Baseline from body weight
- 89
- Added for exercise
- 13
- Glasses of 250 ml
- 10
A general estimate. Anyone with kidney or heart conditions, or who is pregnant or breastfeeding, has fluid needs set by their clinician rather than by a formula.
The formula
Baseline of about 35 ml per kilogram of body weight, plus roughly 500 ml per hour of exercise, multiplied by 1.1 in a warm climate or 1.2 in a hot one. Food is taken to supply a fifth of total fluid, so drinks cover the other four fifths.
Worked example
At 75 kg with 45 minutes of exercise in a temperate climate: 3 litres of total fluid, of which 2.4 litres from drinks — roughly ten glasses.
How to use it
- 1Count tea, coffee, milk and juice as fluid. Caffeine at normal intakes is mildly diuretic but still leaves you in net surplus.
- 2Add more on a hot day, at altitude, or when exercise is long or intense.
- 3Use thirst and urine colour as the real guide: pale straw is about right, dark amber means drink more.
Questions people ask
Is eight glasses a day actually right?
It is a reasonable coincidence rather than a rule. The figure has no strong research behind it, but it happens to land near what a moderately active adult of average size needs.
Can you drink too much?
Yes, though it is rare. Drinking several litres in a short time can dilute blood sodium dangerously, which is why endurance athletes are now advised to drink to thirst rather than on a schedule.
Last reviewed: 2026-10-20