“Coffee doesn’t count, it dehydrates you” is one of the most repeated tips about drinking water. If it were true, every cup would need a glass of water to make up for it. Here is what the research actually found, why the myth exists, and how I count coffee in Hydra.
What the studies found
The clearest test comes from a 2014 study. Fifty men who drink coffee every day spent three days drinking four mugs of coffee a day, and three days drinking four mugs of water instead. Everything else stayed the same. Their hydration was measured several ways: total body water, blood tests and urine. There was no difference between the coffee days and the water days.
A larger 2016 study compared 13 drinks. Men drank a litre of each drink on different days, and the researchers measured how much of it their bodies kept over the next four hours. The result is now called the beverage hydration index. Coffee, tea, cola and sports drinks were kept just as well as still water. Milk, orange juice and oral rehydration solution were kept even better, because of the protein, sugar and salts in them.
Together they answer the practical question: a cup of coffee adds to your water for the day, the same as a cup of water would.
Where the myth comes from
The idea isn’t made up. Caffeine is a mild diuretic: in large amounts it makes the kidneys pass a little more urine for a short while. The effect is strongest in people who rarely have caffeine and fades in people who drink coffee every day, because the body gets used to it.
What the myth leaves out is the cup itself. A coffee is almost entirely water. In normal amounts, the water you drink with the coffee is far more than the small extra amount the caffeine sends to the kidneys. So the net result is positive, and close to water.
How I count coffee in Hydra
When I built Hydra, I had to give every drink a number: how much of it counts as water. The app is built on net hydration, so a litre of beer can’t count the same as a litre of water. For coffee the studies left little room for doubt, so it counts in full:
| Drink | Counts as |
|---|---|
| Water | ×1 |
| Coffee, espresso, decaf | ×1 |
| Tea | ×1 |
| Soda, fresh juice, sports drink | ×1 |
| Milk | ×1.2 |
| Electrolytes | ×1.5 |
Milk is the one I thought about longest. The study measured it at about 1.5 times water, but a large latte would then count for much more than you drank. I settled on 1.2: milk still counts a little more than water, without inflating the day of someone who drinks milky coffee. A latte is mostly milk, so in Hydra you can log it as milk.
The coffee factor feeds into the same daily goal as everything else. If you want to know how the daily goal is built, it’s explained line by line, and you can work out how much water you need in a day with the free calculator.
What to take from it
- Count coffee and tea towards the water you drink in a day.
- You don’t need a glass of water “to make up” for each coffee.
- If you rarely drink coffee, a very large dose of caffeine may send you to the bathroom a little more for a few hours. It won’t undo the water in the cup.
- Caffeine has other effects, on sleep for example. That’s a different question from hydration.
Alcohol is where things really change: how much water you should drink after alcohol depends on how strong the drink is.
Questions
Does coffee count as water intake?
Yes. In normal amounts, coffee hydrates about as well as water, so you can count it towards your daily water.
How much coffee is too much for hydration?
In the 2014 study, four mugs a day hydrated people as well as four mugs of water. How much caffeine is right for you is a separate question, about sleep and health rather than water.
Does decaf coffee hydrate you?
Yes. Decaf has almost no caffeine, so it counts like water.
Does tea dehydrate you?
No. Tea was kept as well as water in the beverage hydration index study, and it counts in full in Hydra.
Do espresso and iced coffee count?
Yes, by their volume. A 30 ml espresso adds 30 ml; a 300 ml iced coffee adds 300 ml.
Does a latte count as coffee or milk?
A latte is mostly milk. Milk is kept a little better than water, so a latte counts at least as much as its volume.
Should I drink a glass of water with every coffee?
Not for hydration. Do it if you like; your coffee already counts.
Does coffee affect people who rarely drink it differently?
A little. The mild diuretic effect of caffeine is stronger in people who aren’t used to it, and fades in daily coffee drinkers.
