Caffeine is a diuretic and for that reason, coffee and caffeinated beverages have been thought for many years to compromise hydration status and cause dehydration.
There is now plenty of evidence demonstrating that people who consume moderate amounts of coffee (3-6 cups of coffee per day) do not have an increased risk of dehydration. However, if you – and you won’t be alone – drink more than 6 cups a day, you are at risk of many other health issues such as disturbed sleep and nervousness.
Just be aware that if you purchase your coffee from a coffee shop, the caffeine dose per espresso serving varies greatly. Research found a dosage varied between 51 mg all the way up to a staggering 322 mg. To put this into perspective, most articles that discuss the caffeine content of coffee will suggest that the average cup of coffee provides between 50-100 mg of caffeine per cup.
If you are in good health then a moderate amount of coffee is not going to cause you any problems. But it is not recommended as your only source of hydration as caffeine, unlike water, can cause side effects.