While coffee is a mild diuretic when consumed in large amounts, regular users build up a tremendous tolerance to its diuretic effect. The caffeine in coffee produces the diuretic effect.
The amount of caffeine in a standard serving of coffee is not enough to produce a diuretic effect. Instead, a minimum of 2 to 3 cups of coffee must be consumed to create a short-term stimulation of urine output. The person must be deprived of caffeine for a few days before consuming this amount for it to have the desired effect. Because of the unique conditions that must be met before coffee serves as a diuretic, there is no connection between regular, daily coffee consumption and fluid loss or hydration.