Water Intake Calculator

The rule used

The calculator applies a common rule of thumb: 35 ml of water per kilogram of body weight per day. A 70 kg adult gets 70 × 35 = 2,450 ml (about 2.5 L); a 55 kg adult gets 1,925 ml.

Official reference values

The European Food Safety Authority (EFSA) sets adequate total water intake at about 2.5 litres a day for men and 2.0 litres for women. Those figures include all sources of water: around 20–30% usually comes from food — fruit, vegetables, soups, dairy — and the rest from drinks. In practice, that often means 1.5 to 2 litres of drinks a day for an adult in a temperate climate.

When you need more

• Exercise: sweat losses typically range from 0.5 to 1.5 litres per hour of intense activity. Weighing yourself before and after a session shows how much you lost.
• Heat and altitude: hot weather can raise needs considerably, even without exercise.
• Fever, vomiting or diarrhoea: fluid losses increase quickly; oral rehydration solutions are designed for this situation.
• Pregnancy and breastfeeding: EFSA recommends about 300 ml more per day during pregnancy and 600–700 ml more while breastfeeding.

Signs to watch

The simplest indicator is urine colour: pale yellow generally means you are well hydrated, while dark yellow suggests you should drink more. Thirst, headaches, fatigue and dizziness are other early signs. Older adults feel thirst less and are more at risk of dehydration.

Drinking far more than you need is rarely a problem for healthy people, but drinking very large amounts in a short time — for example several litres during an endurance event without salt intake — can dangerously lower blood sodium. People with heart or kidney disease may be advised to limit fluids: follow your doctor's instructions rather than this estimate.

Formulas and content last reviewed: October 2026 · About