Water intake calculator
A daily fluid target from body weight, activity and climate.
A rough guide; needs vary with health, medication and climate.
From all sources
Food supplies roughly the other 20%
About 35 mL per kg
Roughly 400 mL per 30 minutes
Spread across a 16-hour day
How it works
Fluid balance is simple in principle: replace what you lose. Losses come from urine (the bulk of it), breathing, sweat and the digestive tract, and they scale with body size, activity and heat.
baseline ≈ 35 mL per kg of body weight
plus ~400 mL per 30 minutes of exercise
plus ~20% in hot or humid conditions Food counts
The EFSA and US Institute of Medicine both express recommendations as total water, including food. Fruit and vegetables are largely water; soup, milk and yoghurt more so. Roughly 20% of intake typically comes from food, which is why “drink 3 litres” overshoots the actual drinking requirement.
Coffee and tea are not diuretics in practice
Caffeine has a mild diuretic effect, but the fluid in the drink more than compensates. Controlled studies find no meaningful difference in hydration between water and moderate coffee consumption. Alcohol is genuinely dehydrating.
A better indicator than any calculation
Urine colour. Pale straw suggests you are well hydrated; dark amber suggests you are not. Combined with thirst, it beats any weight-based formula for day-to-day purposes.
When formulas are not enough
Kidney disease, heart failure, pregnancy, breastfeeding, and several medications all change fluid requirements substantially — sometimes in the direction of restricting intake. Those are conversations for a clinician, not a calculator.