How it’s calculated
The European Food Safety Authority (EFSA) considers 2.0 liters a day adequate for women and 2.5 liters for men. These are amounts of total water — what you drink plus the water in food. The US National Academies estimate that about 80% of total water typically comes from drinks (including caffeinated ones) and about 20% from food. So:
Worked example: for a woman, 2.0 L × 0.8 = 1.6 L from drinks — about 54 fl oz, or roughly seven 8-oz glasses. The other 0.4 L or so comes from food such as fruit, vegetables, soup and yogurt.
Reference intakes at a glance
| Reference (adults) | Women | Men |
|---|---|---|
| EFSA — total water a day | 2.0 L (68 fl oz) | 2.5 L (85 fl oz) |
| US National Academies — total water a day | 2.7 L (91 fl oz) | 3.7 L (125 fl oz) |
| Approx. from drinks (EFSA × 0.8) | 1.6 L (54 fl oz) | 2.0 L (68 fl oz) |
The US figures describe the average total intake of people who appear adequately hydrated, which is one reason they’re higher than EFSA’s. Both include water from food.
About the 30–35 ml per kg rule
Multiplying body weight by 30–35 ml is a popular shortcut, but it isn’t an official reference value. At higher body weights it gives numbers well above the references — 120 kg (265 lb) works out to 3.6–4.2 L — which doesn’t mean you need that much. Treat it as a rough cross-check only.
When you might need more — or less
- More in hot weather, with exercise, and when you’re ill with fever, vomiting or diarrhea.
- More during pregnancy and breastfeeding — ask your midwife or clinician.
- Some heart and kidney conditions call for a fluid limit. If you’ve been given one, follow it.
Some people find they drink less when their appetite is smaller — for example while taking an appetite-reducing medication — and like a simple reminder: a bottle within sight, a glass with every meal, or ticking off glasses as they go.
