Does Coffee Cause Dehydration?

By Erin Riley, PA-C, MS, RD

Probably not, if you drink it regularly and in moderation. In one coffee-industry-funded trial, researchers followed 50 regular male coffee drinkers for three days and could not detect a hydration difference between coffee and water [1].

Every summer, the same warning makes the rounds: your iced coffee is secretly drying you out, so you had better chase every cup with a glass of water. It is one of those beliefs that sounds responsible, gets repeated confidently, and rarely gets checked. So let us check it. The honest answer is more reassuring than the warning, and more specific than either side usually admits. (If you are still sorting out the difference between cold brew and iced coffee, start there, then come back.)

Where did the "coffee dehydrates you" idea come from?

The belief is not invented out of nowhere. Caffeine is a mild diuretic, meaning it can increase urine production. That much is real. What the warning leaves out is tolerance and dose. The authors of the trial discussed in the next section point to work from 1928 showing that regular caffeine consumers develop a tolerance to its diuretic effect, and they note that roughly four days off caffeine is enough for that tolerance to fade [1]. Mayo Clinic reads the current evidence this way: at the caffeine levels people typically drink, the liquid in the cup offsets the diuretic effect, and a pronounced response is likelier in someone who does not drink caffeine regularly [2]. Much of the alarming older work used large doses in people who had abstained beforehand, which is not how habitual drinkers actually drink coffee.

What does the research actually show?

The most direct test of the question comes from the University of Birmingham, published in PLOS ONE in 2014. Its stated aim was to compare coffee against water head to head across a range of validated hydration measures [1]. Here is what it did, and what it did not do.

Researchers recruited 50 male habitual coffee drinkers who normally had three to six cups a day. In a counterbalanced crossover design, each completed two three-day trials: one drinking four 200 mL servings of coffee daily providing 4 mg/kg of caffeine, the other the same volume of water. Physical activity, food, and fluid intake were controlled. The team measured total body water using deuterium oxide dilution, plus urinary and blood markers and body mass [1].

Across that range of markers, researchers found no statistically significant differences between the coffee condition and the water condition. The authors concluded that coffee consumed in moderation by caffeine-habituated males "provides similar hydrating qualities to water" [1].

Two things deserve emphasis, and this is where most coverage of this study goes quiet.

First, the trial was funded by the Institute for Scientific Information on Coffee, a coffee-industry organization, and one author was employed by PepsiCo at the time. The paper states the funder had no role in design, data collection, analysis, or the decision to publish [1]. That is the appropriate safeguard, and you should still know who paid for the work.

Second, and more important for how you read it: finding no statistically significant difference is not the same as proving two drinks are equivalent. This study was not designed or powered as an equivalence trial. It tested 50 men, for three days, at a single moderate dose, with activity controlled. That is meaningful evidence against the claim that habitual moderate coffee leaves you dehydrated. It is not proof that coffee and water are interchangeable for everyone.

“Finding no statistically significant difference is not the same as proving two drinks are equivalent.”

Dose matters more than the myth admits

A smaller study helps explain why the old warnings took hold. Researchers gave ten habitual coffee drinkers, eight men and two women, a single serving of water, coffee at 3 mg/kg of caffeine, or coffee at 6 mg/kg, then collected urine hourly for three hours at rest 3.

The higher dose, averaging about 537 mg of caffeine at once, increased fluid and electrolyte excretion compared with water. The lower dose, averaging about 269 mg, did not [3].

So both things are true. A moderate daily habit does not appear to compromise hydration in people used to caffeine. A large amount of caffeine at once can measurably increase urine output, at least short-term. That was a small, brief, at-rest study, so treat it as a signal about dose, not a rule about your day.

Does iced coffee count toward my daily hydration?

Yes. Your daily fluid target is total water intake, which comes from plain water, other beverages, and food, not plain water alone. The CDC describes daily water intake as coming mostly from water and other beverages [4]. There is no magic number of glasses everyone must hit; needs vary by age, sex, activity, and climate [4].

Coffee is mostly water. Harvard's Nutrition Source is direct about it: because research does not support the idea that normal caffeine intake dehydrates you, caffeinated drinks like coffee and tea contribute to your total daily water intake [5].

Keep that in proportion, though. Mayo Clinic still calls plain water probably your best bet for staying hydrated, since it is calorie-free, caffeine-free, and inexpensive [2]. Coffee counting toward your fluids is not the same as coffee being the best way to get them.

So how much is "moderate"?

For most healthy adults, the FDA cites about 400 mg of caffeine a day, roughly two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects [6]. Read that carefully: it is a reference point, not a recommendation, and not a target. The FDA also notes wide variation in sensitivity and clearance, and that some conditions and medications increase sensitivity [6].

If you are pregnant, trying to become pregnant, breastfeeding, taking medication, or managing a health condition, the FDA advises talking with your healthcare provider about whether you need to limit caffeine [6]. That is a conversation for your clinician, not a blog. Timing is a separate question from hydration, and it is worth its own read: here is when to stop drinking caffeine before bed. 

What about summer heat?

This is where I want to be careful, because it is where the evidence runs out.

Heat and activity genuinely raise your fluid needs. The CDC notes your body needs more water in hot climates, when you are more physically active, or when you have a fever [4]. That is a reason to drink more of everything.

But the hydration research above was not conducted in the heat, and the Birmingham trial specifically controlled physical activity [1]. The dose study measured people at rest [3]. So the honest statement is narrow: there is no good evidence that a moderate coffee habit undermines hydration in habituated drinkers under ordinary conditions, and we lack strong evidence on how that plays out during heavy sweating in high heat. If you are training or working outside in real heat, water and electrolytes are the priority. Your cold brew is a pleasure, not a hydration strategy.

The bottom line

Is coffee dehydrating? For habitual drinkers at moderate intake, the available evidence does not show that it is, and health authorities count it toward your daily fluids. The central trial found no statistically significant difference against water, in 50 men, over three days, with industry funding worth knowing about. A large dose at once can still increase urine output. So enjoy the cold brew, keep your intake moderate, drink water when you are thirsty and more when it is hot, and take your own circumstances to your own clinician.

Frequently Asked Questions

Does iced coffee count toward my daily water intake?

Yes. Total daily fluid comes from water, other beverages, and food, not from plain water alone [4]. Harvard's Nutrition Source states that caffeinated drinks like coffee and tea contribute to total daily water intake [5]. Mayo Clinic still describes plain water as probably your best bet for hydration [2].

How much coffee is too much?

For most healthy adults, the FDA cites about 400 mg of caffeine a day, roughly two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects [6]. That is a reference point, not a recommendation or a target. Sensitivity varies widely, and pregnancy, medications, and some conditions change it, so ask your clinician [6].

Is cold brew more or less hydrating than hot coffee?

This has not been directly tested. The hydration trials discussed here used hot coffee, not cold brew [1] [3]. Physiologically, the factors that matter most are fluid volume and caffeine dose rather than temperature, so drinks of similar size and caffeine content would be expected to behave similarly. That expectation has not been confirmed in a study comparing the two. 

Should I drink a glass of water with every coffee?

There is no evidence that you have to. That habit comes from the idea that coffee actively dehydrates habitual drinkers at moderate intake, and the available evidence does not support it [1]. Drinking when you are thirsty, and drinking more in the heat, is better guidance than a fixed rule [4].

Does decaf hydrate differently?

Decaf contains much less caffeine, typically 2 to 15 mg in an 8-ounce cup according to the FDA [6]. Because the diuretic effect appears to depend on dose [3], decaf would be expected to behave much like water. This has not been tested directly.

This article is for general education and is not medical advice. It describes what research has and has not shown in the populations studied, which may not reflect your situation. For guidance specific to your health, caffeine sensitivity, pregnancy, or medications, talk with your own healthcare provider.

References

1. Killer, Sophie C., Andrew K. Blannin, and Asker E. Jeukendrup. "No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population." PLOS ONE, vol. 9, no. 1, 9 Jan. 2014, e84154. doi:10.1371/journal.pone.0084154. Funded by the Institute for Scientific Information on Coffee; author A.E. Jeukendrup employed by PepsiCo Inc.

2. "Caffeine: Is It Dehydrating or Not?" Mayo Clinic, 19 Aug. 2025, www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/caffeinated-drinks/faq-20057965. Accessed 9 July 2026.

3. Seal, Adam D., et al. "Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest." Frontiers in Nutrition, vol. 4, article 40, 18 Aug. 2017. doi:10.3389/fnut.2017.00040. PMID 28868290.

4. "About Water and Healthier Drinks." Centers for Disease Control and Prevention, Healthy Weight and Growth, reviewed 5 Mar. 2026, www.cdc.gov/healthy-weight-growth/water-healthy-drinks/index.html. Accessed 9 July 2026.

5. "Water." The Nutrition Source, Harvard T.H. Chan School of Public Health, nutritionsource.hsph.harvard.edu/water/. Accessed 9 July 2026.

6. "Spilling the Beans: How Much Caffeine is Too Much?" U.S. Food and Drug Administration, content current as of 28 Aug. 2024, www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much. Accessed 9 July 2026.