Numbers Don't Lie
To have a significant diuretic effect, caffeine needs to be consumed in amounts greater than 500 mg.
Given that an 8 ounce cup of green tea contains 35 mg of caffeine, it would take at least 14 cups of green tea to produce a significant diuretic effect.
Researchers report that when consumed in moderate amounts, caffeinated drinks, including tea, are as hydrating as water.
It is commonly known that tea acts as a diuretic due to its caffeine content. This has led many to assume that tea causes dehydration.
In one study, 50 heavy coffee drinkers consumed either 26.5 ounces (800 ml) of coffee or the same quantity of water each day for 3 consecutive days. Comparatively, that’s the approximate caffeine equivalent of 36.5–80 ounces (1,100–2,400 ml) of tea.
Scientists observed no difference in markers of hydration between the days where coffee and water was drunk. (source)
In another small study, 21 healthy men drank either 4 or 6 cups (960 or 1,440 ml) of black tea or identical amounts of boiled water over 12 hours.
Again, the researchers noticed no difference in urine production or hydration levels between the two drinks. They concluded that black tea seems to be as hydrating as water when consumed in amounts smaller or equal to 6 cups (1,440 ml) per day (source)
The Bottom Line
The caffeine content of most teas is very low and drinking normal amounts — less than 3.5–8 cups (840–1,920 ml) of tea at once — is unlikely to have any dehydrating effects.
Tea can provide a pleasant alternative to plain water in helping you reach your daily fluid requirements.