How many litres of water should I drink per day?
There's no single right number. The honest answer is a range — roughly 2.7 litres a day for women and 3.7 for men in total (food included), per the US National Academies — and the figures that actually matter sit at the edges: a floor where you're dehydrated, and a ceiling where you've drunk more than your kidneys can clear.
Your hydration range
a band, not a single numberAn estimate for healthy adults — with the upper limit your kidneys can actually handle.
The ceiling: your kidneys can only clear about 0.8–1 litre per hour. Don't push much past a litre an hour, and avoid downing 3–4 litres in a short burst — that's how over-hydration (hyponatremia) happens. For most people, drinking to thirst stays safely inside the band.
Dehydrated or water-logged? Tap your shade
The quickest real-world gauge is the colour in the bowl.
Pale yellow is the sweet spot — well hydrated, not over-doing it.
Vitamins (especially B2), beetroot, food dyes and some medicines can tint urine and throw this off.
General guidance for healthy adults — not medical advice. Needs change with pregnancy or chestfeeding, medications, illness, and conditions such as kidney or heart disease (which may require limiting fluid). Follow your clinician over any calculator.
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Where "2 litres / 8 glasses" comes from — and what it leaves out
The "8×8" rule — eight 8-ounce glasses, about 1.9 litres — is a memorable baseline, not a scientific mandate. The US National Academies of Sciences set adequate total fluid intake at about 3.7 litres a day for men and 2.7 for women, but that figure already counts everything: tea, coffee, milk, and the roughly 20% of fluid you get from food. The CDC, for its part, publishes no official daily target, because the right amount genuinely depends on your size, activity, climate and health. So the single headline number is the one thing the science won't give you.
The ceiling nobody mentions: you can drink too much
Hydration advice almost always points one way — drink more — but there's a hard upper limit. Healthy kidneys can excrete only about 0.8 to 1 litre of water per hour. Drink faster than that for long, or sink 3–4 litres in a short stretch, and the excess water dilutes the sodium in your blood. Below about 135 mmol/L of sodium, that's hyponatremia — "water intoxication" — and its symptoms run from headache and nausea to confusion and, in severe cases, seizures. It's rare, and it mostly strikes marathon runners, soldiers and people with particular medical conditions, but it's real: more isn't always better.
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Sodium is half the equation
Here's the part that surprises people: to get rid of water, your kidneys have to get rid of solute — mostly sodium and the urea from protein. On a very low-solute diet there isn't enough to carry the water out, so the safe ceiling drops sharply. The textbook example is "beer potomania," where heavy beer drinking with little food provides almost no solute and hyponatremia sets in at much lower volumes. It's also why endurance events hand out electrolyte drinks, not just water — plain water alone, in large amounts, is exactly the wrong move.
The best gauge isn't a number — it's your body
For healthy adults, thirst is a reliable signal — it lags slightly, but "drink to thirst" beats any fixed schedule and steers you clear of both extremes. The simplest visual check is the one in the widget above: urine colour. Pale yellow is the target; dark yellow means top up; water-clear, hour after hour, can mean you're over-doing it. It's a gauge, not a lab test — vitamins, beetroot and some medicines tint urine regardless of how hydrated you are — but for everyday life it's more useful than counting glasses.
Frequently asked
How many litres of water should I drink per day?
There is no single correct number. The US National Academies put adequate total fluid intake at about 3.7 litres a day for men and 2.7 for women, including the roughly 20% that comes from food. The CDC says there is no official target — it depends on your weight, activity, climate and health. For most healthy adults, drinking to thirst keeps you in the right range.
Can you drink too much water?
Yes. Your kidneys can only clear about 0.8 to 1 litre per hour. Consistently drinking faster than that, or downing 3–4 litres in a short time — especially when low on sodium — can dilute your blood sodium and cause hyponatremia (water intoxication). It is rare and mostly affects endurance athletes, soldiers, or people with certain conditions, but in severe cases it is life-threatening.
Does coffee or tea count toward my water intake?
Yes. Caffeine is only a mild diuretic, so coffee, tea and other drinks are still net hydrating and count toward your daily total.
Is urine colour a reliable hydration check?
It is a good, simple gauge. Pale yellow is the target; dark yellow means drink more; persistently clear can mean you are over-doing it. Note that vitamins, beetroot, food dyes and some medicines can change the colour independently of hydration.
Do I need more water when exercising?
Yes — you can lose 0.5 to over 1 litre of sweat per hour, so drink before, during and after. For long endurance events, replace electrolytes too, not just water, since plain water alone can push you toward hyponatremia.
Sources
- US National Academies of Sciences, Engineering, and Medicine (Institute of Medicine, 2004) — adequate total water intake ≈ 3.7 L/day (men), 2.7 L/day (women), ~20% from food.
- US Centers for Disease Control and Prevention (CDC) — no official daily water target; needs vary by weight, activity, climate and health.
- Renal physiology / hyponatremia reviews — maximal water excretion ≈ 0.8–1.0 L/hour (≈778–1043 mL/hr); risk rises drinking 3–4 L rapidly or low on sodium.
- Exercise-associated hyponatremia literature — endurance athletes and soldiers as the main at-risk groups; "drink to thirst" as the preferred strategy.
- Urine-colour hydration assessment (Armstrong urine colour scale) — pale yellow as the practical target.
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Related
The right answer isn't a number to hit — it's a range to stay inside, read off your own thirst and the colour in the bowl.