Where "coffee dehydrates you" comes from
Caffeine is a mild diuretic: it makes your kidneys produce a bit more urine for a few hours. Early studies that showed this often gave large doses to people who had stopped having caffeine beforehand, and that is where the advice to drink an extra glass of water for every coffee came from.
The effect is much smaller in people who drink coffee every day, because the body gets used to caffeine. A review of the research concluded that a noticeable increase in urine needs a large single dose of caffeine, at least 250 to 300 mg, about three cups at once, and mainly shows up in people who have not had caffeine for a while. In regular drinkers, normal servings of coffee and tea appear to have no diuretic effect.
What the studies found
In a 2014 study at the University of Birmingham, 50 men who regularly drank coffee spent three days drinking four mugs of coffee a day and, separately, three days drinking the same amount of water. Their body water and their blood and urine hydration markers were the same either way.
The Beverage Hydration Index study in 2016 had men drink a litre of different drinks and measured how much of it their bodies held on to. A litre of coffee, with 212 mg of caffeine, did no worse than a litre of still water.
How much coffee is fine?
For healthy adults, the European Food Safety Authority considers up to 400 mg of caffeine a day safe, and up to 200 mg in one go. That is roughly four cups of filter coffee a day. During pregnancy the advice is no more than 200 mg a day.
Caffeine varies a lot between drinks. A cup of filter coffee has around 90 mg, a single espresso around 80 mg, and decaf only a few milligrams. A large café drink with several shots can pass 200 mg on its own.
With many coffee drinks the bigger catch is sugar, not hydration. Iced coffee with condensed milk or syrup still counts as fluid, but it is not a like-for-like swap for water.
Why Waterie counts coffee at 95%, not 100%
The research says coffee hydrates about as well as water, so 95% is a slightly conservative choice, not a penalty. It keeps the count honest for people who drink a lot of strong coffee. Espresso counts at 90% because it carries the most caffeine per millilitre, and milky drinks such as lattes count at 97% because they are mostly milk.
How Waterie counts it
These are the values the app uses for coffee drinks. A 250 ml coffee adds about 240 ml to your daily goal.
| Drink | Hydration value | 250 ml counts as |
|---|---|---|
| Coffee | 95% | 238 ml |
| Americano | 95% | 238 ml |
| Espresso | 90% | 225 ml |
| Latte | 97% | 243 ml |
| Cappuccino | 95% | 238 ml |
| Iced Coffee | 95% | 238 ml |
| Cold Brew | 95% | 238 ml |
| Decaf Coffee | 95% | 238 ml |
More questions
Does decaf coffee count as water?
Yes. Decaf has very little caffeine, so it hydrates like water. Waterie counts it at 95%, the same as regular coffee, to keep the coffee values simple.
Does iced coffee count?
Yes. Iced coffee and cold brew count the same as hot coffee, at 95% in Waterie. Sweetened versions still count as fluid, but they add sugar and calories that water does not.
Do I need a glass of water with every coffee?
Not for hydration. At normal amounts coffee already adds to your fluid intake. A glass of water alongside is still a good habit if coffee makes up most of what you drink.
How much caffeine is too much?
For healthy adults the European Food Safety Authority considers up to 400 mg a day, and up to 200 mg at once, safe. During pregnancy the advice is no more than 200 mg a day. Children and teenagers should have much less.
Sources
- Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One, 2014. Funded by the Institute for Scientific Information on Coffee.
- Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. American Journal of Clinical Nutrition, 2016. Funded by the European Hydration Institute.
- Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: a review. Journal of Human Nutrition and Dietetics, 2003.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the safety of caffeine. EFSA Journal, 2015.
Last reviewed 4 October 2026
General information for healthy adults, not medical advice. If you are pregnant or have a condition that affects your fluid balance, follow your doctor's advice.