Hydration

Does Coffee Dehydrate You? What the Research Actually Says

Coffee is a mild diuretic, but for regular drinkers a normal cup hydrates about as well as water. Here's what the studies show, where the myth came from, and when caffeine does make you pee more.

By the Caffeine vs Water team · 26 September 2026 · 5 min read

"For every cup of coffee, drink an extra glass of water." You've probably heard it, maybe from a fitness app. It sounds sensible, because caffeine is a diuretic. But when researchers actually measured people's hydration, the story turned out to be much less dramatic.

Is caffeine a diuretic?

Yes, technically. Caffeine increases blood flow to the kidneys and the amount of sodium they excrete, which increases urine output. That's where the myth started: early studies, often in people who didn't normally drink coffee and given large doses of pure caffeine, saw more urine and concluded coffee dehydrates you.

Two things weaken that effect in real life:

• A cup of coffee is mostly water. The fluid you drink far outweighs the small extra amount you pass.
• Tolerance. Regular coffee drinkers become largely tolerant to caffeine's diuretic effect.

What the studies show

Coffee vs water, head to head

In a 2014 study at the University of Birmingham, 50 men who habitually drank 3–6 cups a day spent three days drinking four 200 ml cups of coffee daily, and three days drinking the same amount of water. Researchers measured total body water with isotope tracers, plus blood and urine markers of hydration. There were no differences between coffee days and water days, including 24-hour urine volume (2,409 ml vs 2,428 ml). The authors concluded that moderate coffee "provides similar hydrating qualities to water".

The Beverage Hydration Index

A 2016 study had 72 men drink a litre of 13 different drinks and measured how much they peed over the next four hours. Coffee, tea, cola and even lager were no different from still water. Milk and oral rehydration solution actually hydrated better than water, thanks to their electrolytes and nutrients.

Where the effect is real: big doses

A 2017 study gave regular coffee drinkers coffee with either about 269 mg or 537 mg of caffeine. The lower dose didn't change fluid balance. The higher dose (6 mg/kg, roughly five or six cups at once) nearly doubled urine output over the next three hours. A 2015 meta-analysis found caffeine raises urine volume by about 109 ml on average, mostly at rest, and the effect largely disappears during exercise.

So "coffee dehydrates you" is wrong for normal amounts, and partly right if you have a lot at once.

Does coffee count toward your daily water intake?

Yes. When the U.S. National Academies set its water guidance (about 2.7 litres a day for women and 3.7 litres for men, from all drinks and food), it said roughly 80% of our water comes from drinks, "including caffeinated beverages". The panel also said most healthy people meet their needs by drinking with meals and following their thirst.

For a full breakdown of how much you need, see how much water you should drink a day.

So why pair coffee with water at all?

Because it's a useful habit, not a correction:

• It spaces out your caffeine. A glass of water between coffees slows the next refill.
• It covers the big-dose case. After a very strong coffee, cold brew or energy drink, you may lose a little more fluid.
• It keeps water on your radar. Coffee drinkers often reach for another coffee when they're actually thirsty.
• Mild dehydration is noticeable. In a study of young women, losing just 1.4% of body weight in fluid worsened mood, concentration and headaches.

That's the approach we took in the app: a strong coffee adds a glass to your water goal as a reminder, capped for the day, while normal cups simply count as fluid.

What about energy drinks and tea?

The same logic applies. Tea hydrates about as well as water. Energy drinks usually have a similar or smaller amount of caffeine per can than a large coffee, but they're easy to stack, and high-caffeine cans (200–300 mg) get into the dose range where you'll pee more. Check the label, and see our caffeine content chart for common brands.

Sources

1. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake. PLoS One, 2014.
2. Maughan RJ, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr, 2016.
3. Seal AD, et al. Coffee with high but not low caffeine content augments fluid and electrolyte excretion at rest. Front Nutr, 2017.
4. Zhang Y, et al. Caffeine and diuresis during rest and exercise: a meta-analysis. J Sci Med Sport, 2015.
5. National Academies. Report sets dietary intake levels for water, salt, and potassium, 2004.
6. Armstrong LE, et al. Mild dehydration affects mood in healthy young women. J Nutr, 2012.

General wellness information, not medical advice. If you're pregnant, have a heart condition, or take medication that affects caffeine, check your limits with a doctor.