Does coffee dehydrate you?
Even though coffee is 98% water?
Maybe in high doses and maybe if you’ve been caffeine-deprived (maybe not), but the answer is pretty much no.
Meta-analyses and reviews:
Here is a systematic review that looked at 27 studies and found:
Caffeine ingestion did not lead to excessive fluid loss in healthy adults.
This is a literature overview of hydration and health:
Caffeine has a mild diuretic effect, but moderate caffeine consumption (up to about 500 mg per day – the amount in about five mugs of instant coffee) does not appear to cause dehydration (Armstrong 2002; Armstrong et al. 2005). With a few exceptions (e.g. strong Turkish coffee), the fluid in a caffeinated beverage compensates for the short-term diuretic effect it produces.
Stookey (1999) suggested that each milligram of caffeine consumed stimulates the formation of 1.1 ml of urine. However, the evidence suggests that the relationship is not linear, but rather there is a threshold level below which little or no effect on urine production is observed (Maughan & Griffin 2003). The review by Maughan & Griffin suggested the level of caffeine consumption below which, hydration status will not be compromised is 300 mg per day, although more recent studies indicate this may be 500 mg per day (Armstrong 2002; Armstrong et al. 2005). A review by Ruxton (2008b) found that there was some increase in urinary output when caffeine intakes were in the region of 600 mg per day, but no effect was observed below this level of consumption (Ruxton 2008b).
Here is a meta-analysis that looked at 36 years worth of studies that specifically at the impacts of coffee on hydration. In large doses, they did find short-term stimulation of urine particularly if the individual had been deprived of coffee for days or weeks. But for normal coffee intake for normal coffee drinkers, which is the vast majority of coffee consumption, they found no effect.
Here is another meta-analysis that pretty much found the same thing. It looked at 8 double-blind placebo-controlled studies on the hydration effect of coffee. They found that up to 400 mg a day, coffee had no impact on hydration. At more than 400mg a day they found some evidence of dehydration, particularly if the participants had obtained from drinking coffee for days prior.
For reference, here is the caffeine content of various coffees. A medium Starbucks coffee is 310 mg of caffeine. A medium McDonalds coffee is 145 mg of caffeine.
I could not find any research that found the opposite conclusion.
What do institutions say?
Drinking caffeine-containing beverages as part of a normal lifestyle doesn't cause fluid loss in excess of the volume ingested. While caffeinated drinks may have a mild diuretic effect — meaning that they may cause the need to urinate — they don't appear to increase the risk of dehydration.
Coffee is high in caffeine, a substance that may have diuretic properties. This means that it may cause you to urinate more frequently, which may affect your hydration status.
Coffee and tea also count in your tally. Many used to believe that they were dehydrating, but that myth has been debunked. The diuretic effect does not offset hydration.
The Running Clinic says,
your morning coffee is believed to be as effective as a glass of water when it comes to hydration!
As far as caffeine is concerned, ingestion of moderate to low levels (less than 300mg) in one shot did not contribute to dehydration either at rest or during exercise2. Similar conclusions can be drawn for the long-term ingestion of caffeine.
the evidence shows that coffee's diuretic properties aren't enough to make java dehydrating.
All of our experts agree that coffee is extremely unlikely to cause dehydration.
Moderate coffee drinking can be as hydrating as drinking water.
Health Digest says,
Not only does coffee not make you dehydrated, but there's actually a lot of water in that mug o' joe.
With the exception of Healthline, every institution I could find said coffee doesn’t dehydrate you. Even Healthline's claims were slippery, basically saying, “caffeine is a diarrhetic so who knows? It’s possible”, which does not inspire much confidence.