The notion that tea and coffee are diuretic and don't count in your daily waterbalance, is an urban myth. At most, both are mildly diuretic (if any).
The assumption is based from an old study from 1928 with only three participants. While the small number of participants alone is a good reason to reject the study (it would never get published today) because the findings would lack any statistical power, the design is also questionable. You can find this story in reference 1 if you want to read it.
It is known that large amounts of caffeine indeed have a diuretic effect by increasing the blood flow to the kidneys and inhibiting the reabsorption of sodium. This leads to the outflow of water and an increased production of urine. See reference 2.
However, reference 3 found that this diuretic effect (which is only present at high concentrations of caffeine) diminishes due to tolerance when people regulary drink tea or coffee. They do not see any diuretic effect from normal tea, coffee or caffeinated beverages.
The explanation for this effect is most likely a misinterpretation of our observation. We seem to produce more urine when we drank tea or coffee, but we wrongly compare it to a time when we drunk nothing. If we would compare it with a time, when we drank the same amount of water, the effect (you have to pee more when you drank something) would be the same.
Drinking tea and coffee will therefore help to keep our waterbalance, as would water, juice or softdrinks.
- TOLERANCE AND CROSS-TOLERANCE IN THE HUMAN SUBJECT TO THE DIURETIC
EFFECT OF CAFFEINE, THEOBROMINE AND THEOPHYLLINE
- Caffeine and the Kidney: What Evidence Right Now?
- Caffeine ingestion and fluid balance: a review.