"Drink eight glasses of water a day" is probably the most repeated health rule in the world. Here's what it actually equals in litres and ounces, where it came from, and how it compares to what NASEM, the NHS and EFSA actually recommend.
Eight standard 8-fluid-ounce glasses of water equals 64 US fl oz, or about 1.9 litres (1,893 mL) — roughly 8 US cups. This is where the shorthand "8x8" comes from: eight glasses of eight ounces each.
| Measurement | Amount |
|---|---|
| US fl oz | 64 fl oz |
| Millilitres | ≈1,893 mL |
| Litres | ≈1.9 L |
| US cups | 8 cups |
| Metric cups (250 mL each) | ≈7.6 cups |
The 8x8 rule doesn't adjust for your body size or activity. Get a figure tailored to you in seconds.
Open the Water Intake Calculator →The clearest account of this comes from a 2002 peer-reviewed review by Dartmouth physiologist Heinz Valtin, published in the American Journal of Physiology. Valtin traced the rule to a 1945 U.S. Food and Nutrition Board recommendation of roughly "1 millilitre of water for each calorie of food" — about 2 to 2.5 litres a day — which explicitly noted in its next sentence that most of this amount is already contained in prepared food. Valtin's review concludes that this qualifying detail was likely overlooked as the recommendation was repeated informally over the following decades, leaving the simpler — and less accurate — instruction to drink eight full glasses of water. After searching the available evidence, Valtin also found no scientific study establishing 8x8 as a requirement for healthy adults.
There has never been strong evidence for exactly "eight glasses" as a precise, universal target. It endures mainly because it's memorable and easy to follow, not because it was derived from a rigorous individual requirement study.
Compared with the reference values from major health authorities, 1.9 litres sits closest to fluid-focused guidance and noticeably below total-water guidance:
| Source | What it recommends | vs 8 glasses (1.9 L) |
|---|---|---|
| NHS (UK) | 6–8 cups fluid (~1.9 L) | Matches closely |
| NASEM (US) — men | 3.7 L total water | 8 glasses is well below this |
| NASEM (US) — women | 2.7 L total water | 8 glasses is somewhat below this |
| EFSA (Europe) — men | 2.5 L total water | 8 glasses is somewhat below this |
| EFSA (Europe) — women | 2.0 L total water | Close, slightly below |
The apparent "disagreement" mostly comes down to definitions rather than genuinely conflicting science: NASEM and EFSA measure total water (food plus all drinks), while the 8x8 rule and the NHS guidance are essentially fluids-only targets. Since food typically supplies about a fifth of total water, a fluids-only target of around 1.9–2.1 L combined with normal eating broadly lines up with a total water figure in the 2.4–2.7 L range — closer to the picture than it first appears, though still on the lower end for larger or more active adults.
Eight glasses works reasonably well as a rough average for a moderately sized, lightly active adult in a mild climate who eats a typical, varied diet. It is too low for:
It may be roughly right or even slightly high for smaller-bodied, sedentary adults who eat a lot of water-rich food (fruit, vegetables, soups) and drink other fluids like tea, coffee or milk throughout the day.
How many ounces is 8 glasses of water?
Eight 8-fl-oz glasses equals 64 US fl oz, which is about 1.9 litres (1,893 mL), or roughly 8 US cups.
How many litres is 8 glasses of water a day?
8 glasses of water, using the standard 8 fl oz glass size, equals approximately 1.9 litres per day.
Where did the 8 glasses of water a day rule come from?
A 2002 peer-reviewed review by physiologist Heinz Valtin traced it to a 1945 U.S. Food and Nutrition Board recommendation of about 2 to 2.5 litres of water a day, which noted that most of this amount is already contained in prepared food — a detail that appears to have been dropped as the recommendation was repeated over the following decades. Valtin found no scientific study establishing 8x8 as a strict requirement.
Is 8 glasses of water a day the same for everyone?
No. It does not adjust for body size, sex, activity level, or climate, all of which genuinely change fluid needs. It works reasonably well as a rough average for a moderately sized, moderately active adult in a mild climate, but is too low for larger or more active people and more than some smaller, less active people need.
Eight glasses of water a day works out to about 1.9 litres, 64 fl oz, or 8 US cups. It traces back to a 1945 recommendation that originally included food-water in its total, and it's a reasonable — if imprecise — starting point for many adults. It is not a scientifically fixed daily target, and your own needs likely differ based on your body size, activity level and climate.