Does Coffee Dehydrate You? What It Means at Your Desk

Quick answer: No, not in the amounts most people drink at work. In a study of 50 regular coffee drinkers, four 200 mL cups of coffee a day for three days kept them just as hydrated as the same amount of water (Killer 2014). Caffeine does make you pee a little more, but in regular drinkers the fluid in the cup more than makes up for it.

Your coffee counts toward your fluids. The better questions for a desk worker: is it replacing water, and what does the 3pm refill do to tonight's sleep?

Is coffee a diuretic?

Yes, mildly. A diuretic is anything that makes your kidneys release more water as urine, and caffeine does this a little.

A meta-analysis pooling 16 studies found that caffeine (median dose 300 mg) raised urine output by about 109 mL on average compared with no caffeine (Zhang 2015). That is about half of one of the 200 mL cups used in the Killer study.

Two details from that meta-analysis matter if you sit all day:

The effect showed up at rest, not during exercise. Exercise cancelled it out. A desk worker is at rest for most of the day, so this is the one group where the small effect is real.
Women showed a larger effect than men in the pooled data.

Neither of those turns coffee into a dehydrating drink. They just mean the "coffee is exactly as good as water" line is slightly too neat.

Why your daily coffee barely moves the needle

Tolerance. An older review of the research found that a large dose of caffeine (at least 250 to 300 mg, roughly 2 to 3 cups of coffee) briefly increases urine output mainly in people who have gone without caffeine for days or weeks (Maughan 2003). Regular drinkers build a strong tolerance, and the review found that standard servings of tea, coffee and soft drinks showed no diuretic action.

That fits the bigger trials:

| Study | Who | What they drank | What happened | |---|---|---|---| | Killer 2014 | 50 men who drank 3 to 6 cups a day | 4 × 200 mL coffee daily for 3 days, vs water | No difference in total body water or 24-hour urine volume | | Maughan 2016 | 72 men | 1 L of one of 13 drinks | Urine over 4 hours after coffee was no different from water |

Both studies used men only, worth knowing given the finding on women. Still, nothing here suggests a woman drinking her usual coffee ends the day behind on fluids.

The exception is someone who rarely has caffeine and then takes a large dose, like an occasional drinker grabbing a triple-shot latte before a presentation. Expect an extra bathroom trip.

Where coffee actually costs you at work

The diuretic question is mostly a distraction. These are the real costs.

It replaces water instead of adding to it

This is the real problem. If your morning is coffee, a meeting, another coffee and then lunch, you may have had nothing but coffee by noon. The coffee hydrated you fine. It just pushed out every other drink.

A simple fix: drink a glass of water before your first coffee, and refill your water bottle every time you refill your mug. Coffee becomes a fluid you add, not the only one you have.

"One cup" is not a unit

When researchers measured 104 espressos bought from coffee shops in Scotland, Italy and Spain, caffeine ranged from 48 to 317 mg per serving (Ludwig 2014). The same order can carry several times the caffeine depending on where you buy it. If you count cups to stay under a limit, look up your usual café's drink instead.

The 3pm cup reaches into tonight

This is the trade-off most guides skip. In a sleep study, 400 mg of caffeine taken six hours before bed still disrupted sleep compared with a placebo (Drake 2013). The authors concluded that people should avoid substantial caffeine for at least six hours before bedtime.

If you sleep at 11pm, six hours back is 5pm, and a big 4:30 coffee sits right on the line. That is a high dose (often more than one coffee shop drink), but the direction is clear, and a poor night makes the next afternoon's slump worse, which sends you back to the coffee machine. If your afternoons feel heavy, our post on the 3pm energy slump covers the other usual causes, and hydration and sleep looks at the night side.

What we'd actually do: keep the big coffee in the morning, and if you want an afternoon cup, make it early and small, or switch to decaf.

How much caffeine is too much?

For healthy adults, a large systematic review concluded that up to 400 mg of caffeine a day is not linked to overt adverse effects on the heart, behaviour, bones or acute health (Wikoff 2017). Given the espresso range above, two or three strong coffee shop drinks can get you there.

That limit is for healthy adults. If you're pregnant, take medication, or have a heart condition, ask your doctor what makes sense for you.

The early signs of overdoing it are the ones caffeine is known for: caffeine speeds up the heart and can have anxiety-like effects (Cappelletti 2015), and it disrupts sleep (Drake 2013). A racing heart, feeling on edge in a meeting, or lying awake at midnight are your cues to cut back. If your heart races, or you feel dizzy or faint, stop the caffeine and talk to a doctor rather than waiting it out.

To add up your own daily total, the caffeine hydration calculator tallies coffee, tea and soda in one place.

What to drink if you want to stay hydrated

Water is still the easy default. But coffee, tea, sparkling water and most everyday drinks count too.

In the beverage hydration study, 4-hour urine output after coffee, tea (hot and iced), cola, diet cola, orange juice, sparkling water, a sports drink and lager was no different from water (Maughan 2016). Full-fat milk, skimmed milk and an oral rehydration solution did better: people kept more of the fluid two hours later.

You don't need milk at your desk. Drinking regularly matters more than what's in the glass. Our sparkling water guide goes through the common office drinks one by one, and the beverage hydration tool compares them side by side.

A workday coffee plan

Water first, then coffee.
Refill your water whenever you refill your mug.
Know your usual drink's caffeine, not a generic cup.
Set a coffee cutoff around six hours before bed.
Afternoon slump? Try water and a short walk before a second coffee.

If your urine stays pale yellow through the day, you're fine. If it's consistently very dark, or you're dizzy or getting persistent headaches, that's worth a conversation with a doctor.

Frequently asked questions

Does coffee count toward your daily water intake?

Yes. In a three-day study of regular coffee drinkers, four 200 mL cups of coffee a day hydrated people as well as the same amount of water, with no difference in body water or urine volume (Killer 2014). A larger trial found coffee's effect on urine output over four hours matched water's (Maughan 2016). Count it, but don't let it be your only drink.

Will espresso dehydrate you?

Not in normal amounts if you drink caffeine regularly. Regular drinkers develop tolerance to caffeine's mild diuretic effect (Maughan 2003). The catch with espresso is its caffeine, not its water: coffee shop espressos in one study ranged from 48 to 317 mg per serving (Ludwig 2014), so a double can be a much bigger dose than you think.

Does decaf coffee dehydrate you?

No. Decaf has very little caffeine, so even the mild diuretic effect is mostly gone, and you get the fluid. "Decaf" is not zero, though: one lab analysis found 0 to 13.9 mg of caffeine in 16 oz servings of decaf from different outlets (McCusker 2006). For most people that's trivial. It's a good afternoon swap if you want the ritual without the sleep cost.

What are two signs you've had too much caffeine?

A faster heartbeat and feeling anxious or on edge are two common ones: caffeine speeds up the heart and has anxiety-like effects (Cappelletti 2015). Trouble falling or staying asleep is another, even from caffeine taken six hours before bed (Drake 2013). If you notice a racing heart, dizziness or chest discomfort, stop the caffeine and speak to a doctor.

Is tea more hydrating than coffee?

In the beverage hydration study, hot tea, iced tea and coffee all produced the same urine output as water over four hours (Maughan 2016). Tea usually carries less caffeine per cup: an older review put 250 to 300 mg at 2 to 3 cups of coffee or 5 to 8 cups of tea (Maughan 2003). For hydration they're equal; for sleep, tea is easier on you.

Coffee is rarely the problem. Forgetting the water around it is. DeskSip is an iPhone app, available now, built for that gap: gentle hydration reminders that follow the workday you set, a quick tap to log a drink, and short Desk Reset movement breaks for the stretch of the afternoon when you'd otherwise reach for a third coffee. Download on the App Store.

References

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Zhang Y, Coca A, Casa DJ, Antonio J, Green JM, Bishop PA. "Caffeine and diuresis during rest and exercise: A meta-analysis." Journal of Science and Medicine in Sport. 2015. PubMed 25154702
Maughan RJ, Griffin J. "Caffeine ingestion and fluid balance: a review." Journal of Human Nutrition and Dietetics. 2003. PubMed 19774754
Maughan RJ, Watson P, Cordery PA, Walsh NP, Oliver SJ, Dolci A, Rodriguez-Sanchez N, Galloway SD. "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. PubMed 26702122
Ludwig IA, Mena P, Calani L, Cid C, Del Rio D, Lean ME, Crozier A. "Variations in caffeine and chlorogenic acid contents of coffees: what are we drinking?" Food & Function. 2014. PubMed 25014672
Drake C, Roehrs T, Shambroom J, Roth T. "Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed." Journal of Clinical Sleep Medicine. 2013. PubMed 24235903
Wikoff D, Welsh BT, Henderson R, et al. "Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children." Food and Chemical Toxicology. 2017. PubMed 28438661
Cappelletti S, Piacentino D, Sani G, Aromatario M. "Caffeine: cognitive and physical performance enhancer or psychoactive drug?" Current Neuropharmacology. 2015. PubMed 26074744
McCusker RR, Fuehrlein B, Goldberger BA, Gold MS, Cone EJ. "Caffeine content of decaffeinated coffee." Journal of Analytical Toxicology. 2006. PubMed 17132260