Coffee, tea, alcohol: which ones really dehydrate you?
by AUA Hydrate
«Coffee dehydrates you.» You hear it at the office, at the table, and usually just as someone sets a glass of water down next to the espresso. The idea is so widespread it has become self-evident. Yet of the three drinks most often blamed, only one truly deserves its reputation — and it is not coffee.
⏱️ The essentials
- Coffee does have a diuretic effect, but a modest one: the water it delivers more than makes up for it, and the balance stays positive.
- Tea, with two to three times less caffeine, hydrates you almost exactly like water.
- Alcohol is the only one that genuinely dehydrates: it blocks the hormone that tells your kidneys to hold on to water.
Does coffee really dehydrate you?
Caffeine is indeed a mild diuretic: it slightly raises the kidneys' filtration rate and reduces sodium reabsorption. So yes, you pass a little more urine. So far the rumour is right.
What it forgets is the rest of the cup. Coffee is more than 98 % water. The diuretic effect of the caffeine it contains is never enough to cancel out that volume: you end up with a net gain in fluid, not a loss.
Two things reinforce this. First, tolerance: in someone who drinks coffee daily, the diuretic effect of caffeine fades markedly within a few days. Second, dose: it takes a substantial amount at once — on the order of 300 mg of caffeine, or three to four cups back to back in someone unaccustomed to it — for the difference to become measurable.
In other words: your coffees count towards your daily fluid intake. They do not cancel it out.
And tea?
With tea the question barely arises. A cup of black tea contains roughly 30 to 50 mg of caffeine, green tea slightly less — against about 90 mg for filter coffee and 60 to 80 mg for an espresso.
At those doses the diuretic effect is negligible. As far as hydration goes, a cup of tea is roughly equivalent to a cup of water. Caffeine-free herbal infusions are quite simply flavoured water.
| Coffee | Tea | Alcohol | |
|---|---|---|---|
| Active substance | 60 to 95 mg caffeine | 30 to 50 mg caffeine | 10 to 12 g alcohol per drink |
| Diuretic effect | Mild, fades with habit | Negligible | Marked and immediate |
| Fluid balance | Positive | Positive | Negative |
| Verdict | Counts towards intake | Counts towards intake | Costs you more than it gives |
Alcohol, the only real culprit
Here the mechanism is anything but marginal. Ethanol inhibits vasopressin, the antidiuretic hormone that tells your kidneys to conserve water. Deprived of that signal, they let it go.
The order of magnitude is clear: 10 g of alcohol causes roughly 100 ml of additional urine, on top of the volume you drank. A glass of wine, a beer or a measure of spirits each contain 10 to 12 g of alcohol. Four drinks in an evening, and you have passed close to half a litre more than you took in.
It is exactly the same hormone described in Water retention: why drinking less makes it worse — except that here it is switched off rather than overactivated.
Add to that an indirect effect that is often decisive: when you drink alcohol, you stop drinking water. The water glass disappears from the table.
The morning after: what hydration explains, and what it does not
Dehydration accounts for part of the morning-after symptoms: the dull headache, the dry mouth, the fatigue. They are the same signs as the dehydration headache described in Migraine or dehydration headache.
But only part. The acetaldehyde produced as alcohol breaks down, the disrupted sleep and the inflammatory response weigh at least as heavily. No drink cures a hangover. The only thing that genuinely works is drinking less alcohol.
Rehydrating helps restore what was lost — water and minerals. It does not neutralise alcohol, and it would be dishonest to claim otherwise.
What it changes day to day
- Stop trying to offset your coffees. The glass of water next to the espresso is a good habit, but not for the reason people think: it is extra water, not a repair.
- Count tea and coffee in your intake. They are part of the daily total, as explained in How much water should you really drink a day.
- Genuinely alternate on drinking nights. A glass of water between two drinks is not token moderation: it roughly makes up for what vasopressin is no longer doing.
- Drink before bed, not only on waking. The night after a party is a seven- or eight-hour fluid fast that already starts in deficit.
- Think minerals, not just water. Heavy urine output carries away sodium and potassium, not only fluid — the subject of Why water alone is no longer enough.
💡 The AUA tip: the most useful moment is not the next morning, it is before you go to sleep. One stick in 500 ml of water when you get home, sugar-free and 2 kcal, makes the glass pleasant enough that you actually drink it — and puts minerals back where the night will keep taking them. It erases nothing. It simply stops the day starting with one more deficit.
Water you actually want to drink
Sugar-free, 2 kcal, one stick in 500 ml of water: discover our three flavours.
Frequently asked questions
Does coffee count towards my daily fluid intake?
Yes. A cup is more than 98 % water, and the diuretic effect of caffeine is not enough to cancel that volume. The balance stays positive, even for someone drinking several cups a day.
How many coffees before it becomes a problem?
For hydration, the question barely arises at habitual intakes. European guidance places the safe caffeine limit at around 400 mg a day for an adult, or roughly four to five cups — a limit that concerns sleep and heart rhythm, not your fluid balance.
Do you really need a glass of water between each alcoholic drink?
It is the single most effective measure, for a precise reason: roughly 100 ml of extra urine per 10 g of alcohol corresponds almost exactly to one glass of water per drink consumed. You are not just slowing down, you are compensating.
Do iced tea and iced coffee hydrate just as well?
In terms of volume, yes. The difference lies elsewhere: many commercial versions contain substantial amounts of sugar, which is a separate matter from hydration.
