It’s one of the most repeated bits of health advice. Coffee is a diuretic, so for every cup you drink, you need an extra glass of water.
So does coffee dehydrate you? The research says no, not in normal amounts.
Caffeine does make you pee a little more, especially if you don’t usually drink it. But the water in the coffee more than makes up for it.
- In a trial of 50 regular coffee drinkers, four mugs a day hydrated them as well as the same amount of water.
- A meta-analysis found caffeine raised urine by about 109 mL on average, mostly at rest and less in regular users.
- The NHS and Harvard both say coffee counts towards your daily fluid intake.
Does coffee dehydrate you?
No. For most people, moderate coffee drinking doesn’t cause dehydration.
The NHS says that water, lower-fat milk and lower-sugar drinks, “including tea and coffee, all count as part of your daily intake”.
Harvard’s Nutrition Source makes the same point. It says more than 180 mg of caffeine a day, about two cups of brewed coffee, “may increase urination in the short-term in some people”.
But it adds that this “will not necessarily lead to dehydration”.
The myth has a real root. Caffeine is a mild diuretic, which means it nudges your kidneys to make more urine.
The question is whether that outweighs the fluid you drink with it. For a normal cup of coffee, it doesn’t.
What the research shows
The best trial of real coffee
The clearest test comes from a 2014 trial in the journal PLOS ONE.
They recruited 50 men who already drank three to six cups of coffee a day. Each man did two three-day trials, in random order.
In one, they drank four 200 mL mugs of black coffee a day, with a caffeine dose of 4 mg per kg of body weight. In the other, they drank the same amount of water instead.
The researchers measured total body water with a precise tracer method, using a special form of water called deuterium oxide. They also collected all urine over 24 hours.
What it found
There was no difference in hydration between coffee and water.
| Measure | Coffee | Water |
|---|---|---|
| Total body water | 51.5 kg | 51.4 kg |
| 24-hour urine | 2,409 mL | 2,428 mL |
| Blood markers of hydration | No difference | No difference |
The authors concluded that, in moderation, coffee “provides similar hydrating qualities to water” for men used to caffeine.
The catch in this trial
It only included men, and only regular coffee drinkers.
It was funded by the Institute for Scientific Information on Coffee, a non-profit focused on coffee and health. The paper says the funder had no role in the study, and one author was employed by PepsiCo.
In normal amounts, coffee hydrates you about as well as water. Caffeine has a mild effect on urine, but the fluid in the coffee more than covers it.
How much does caffeine make you pee?
The meta-analysis
A 2015 meta-analysis in the Journal of Science and Medicine in Sport pooled 16 studies of caffeine and urine output. The median dose was 300 mg.
On average, caffeine increased urine by 109 mL, or about 16%, compared with no caffeine.
| Situation | Effect of caffeine on urine |
|---|---|
| At rest | Moderate increase |
| During exercise | No significant effect |
| In women | Larger effect |
| In men | No significant effect |
The authors called it “a minor diuretic effect which was negated by exercise”. They said concerns about fluid loss from caffeine are “unwarranted”.
Most of the studies tested caffeine, often in capsules, rather than coffee. And most participants were young adults.
Do regular drinkers get used to it?
A 2024 review by the International Society of Sports Nutrition looked at moderate daily doses, around 250 to 300 mg. These “do not appear to increase urine volume in habitual caffeine consumers”, it said.
It added that much higher doses, over 500 mg, may cause a short-term rise in urine output.
How many cups is that? The same review puts a typical 8-ounce cup of coffee at about 80 to 100 mg of caffeine.
So 250 to 300 mg is roughly three cups a day, and 500 mg is five or more.
That review had no funding, but the society takes grants from companies that sell caffeine products.
The newest research
Strong energy drinks are different
A 2025 trial in Nutrients tested caffeinated energy drinks in 28 adults, half of them women.
A drink with 280 mg of caffeine was less hydrating than water over four hours. A drink with 106 mg of caffeine, plus carbohydrates and electrolytes, hydrated as well as water.
That trial was funded by Liquid I.V., a hydration drink brand owned by Unilever. It tested energy drinks, not coffee.
What coffee does to a water hormone
A 2025 study in Endocrine Connections looked at copeptin, a marker of the body’s water-holding hormone. In 26 adults, a cup of coffee lowered it by up to 27% on average within 150 minutes.
That helps explain why coffee makes you pee a little more. But it didn’t measure whether people became dehydrated.
What earlier research found
Coffee vs other drinks
A 2016 trial in the American Journal of Clinical Nutrition gave 72 men a litre of different drinks. Urine output after coffee, tea, cola and several other drinks was no different from water.
Milk and oral rehydration solution were actually more hydrating than water.
Caffeine over 11 days
A 2005 trial gave 59 people caffeine capsules, at up to 6 mg per kg of body weight a day. It found “no evidence of hypohydration”.
Caffeine and fluid retention
A 2019 study by the same group found caffeine up to 400 mg per litre didn’t change how much fluid people kept.
How it fits together
Across studies, the pattern is clear. Normal coffee drinking doesn’t dehydrate you, though large single doses of caffeine can increase urine for a few hours.
How much should you trust this?
Promising. Several controlled studies agree, but they’re small and mostly in young men.
What makes it convincing
- The 2014 trial compared real coffee with water, using a precise measure of body water.
- A meta-analysis of 16 studies points the same way.
- Other trials with different methods reach similar results.
- The NHS and Harvard both count coffee towards daily fluids.
What makes me cautious
- Most trials were small, and many included only men.
- The main coffee trial was funded by a coffee research body.
- Women seemed more sensitive to caffeine’s effect on urine.
- Very high doses weren’t well tested in real coffee.
| The evidence shows | The evidence does not show |
|---|---|
| Moderate coffee hydrates about as well as water | That coffee is better than water |
| Caffeine raises urine a little, mostly at rest | That large doses are fine for everyone |
| Regular drinkers seem less affected | That coffee can replace water entirely |
| Strong caffeine doses can be less hydrating | Exact effects in women and older adults |
What this means for you
- Count your coffee. It’s part of your daily fluid intake, according to the NHS and Harvard.
- Stay within safe limits. The FDA says up to 400 mg a day, about two to three 12-ounce cups, is generally fine for adults.
- Take extra care if pregnant. The NHS advises no more than 200 mg of caffeine a day during pregnancy.
- Watch big single doses. Strong energy drinks or several espressos at once can make you pee more for a few hours.
- Mind your sleep. The European Food Safety Authority says 100 mg of caffeine may affect sleep in some adults.
The NHS guide to water and drinks explains how much to drink and which drinks count.
In this short video, Cleveland Clinic answers whether coffee dehydrates you:
What we still don’t know
- Does coffee affect women’s hydration differently? The main coffee trial only included men.
- What about people who rarely drink coffee? They may feel the diuretic effect more.
- What about older adults? Most studies tested young people.
- What about very large amounts? High-dose coffee hasn’t been well studied for hydration.
- What about hot weather? Most trials took place in normal conditions.
My take: drink your coffee, guilt-free
What I like about this research is how directly it tests the myth. Real coffee, real people, measured against plain water.
I’m less convinced by claims that coffee is a perfect substitute for water. The studies are small, and caffeine has its own limits.
But the “extra glass of water for every cup” rule isn’t backed by the evidence. A few cups a day count towards your fluids.
Last reviewed: 2026-10-04
Covers: Whether coffee dehydrates you, how much caffeine increases urine, energy drinks, and safe caffeine limits
Evidence: Promising — consistent controlled studies, but small and mostly in men
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