Although not zero, the amount of calories in a single, typical cup of brewed black tea, 2.4, amounts to only about 0.12% of the recommended daily intake of calories for a typical adult.
Where do the trace calories come from?
The calorie content of tea comes mostly from a small amount of carbohydrate in the leaf, which is extracted into the brewed cup after steeping in hot water. In contrast with coffee, which tends to have around 1 gram of protein per cup, tea has no measurable protein.
The fat content of tea is negligible, with measurable, but trace amounts of both omega 3 and omega 6 fats, typical for anything produced from the leaves of a plant.
Read the full report:
The full report also discusses the calorie content of common units of sugar or similar sweeteners added to tea, as well as the effects of adding milk or cream: