A flat 40 mg, whatever the size
Starbucks lists 40 mg of caffeine for hot Earl Grey Tea at every size it sells, Short through Venti, matching its English Breakfast Tea exactly, both being black tea bases. Like the rest of Starbucks’ brewed hot teas, the dose comes from a single bag, so a bigger cup means more hot water around the same bag rather than more caffeine.
The tea behind the London Fog
This is the tea Starbucks builds its London Fog Latte from, steamed with milk and vanilla syrup, and that drink carries the same 40 mg at every size as the plain tea it starts from.
Against home brewing, 40 mg runs a little below USDA’s figure for black tea steeped at home, 47 mg for an 8 oz cup, likely because a café bag steeps for less time than a fully brewed pot.
Bergamot oil, the ingredient that distinguishes Earl Grey from a plain black tea, doesn’t add or remove caffeine; the leaf underneath is the same type of black tea Starbucks uses for its English Breakfast blend, just flavoured differently. Choosing between the two at the counter is entirely a matter of taste, not caffeine, since both publish the identical 40 mg figure.
Where it fits in a day
At 40 mg, even a Venti is far under the 200 mg single dose EFSA assessed as safe for healthy adults. Ten Ventis, 400 mg, would be needed to reach the daily figure the FDA cites for healthy adults. In pregnancy, where ACOG uses a 200 mg daily ceiling, five cups still fit under the day’s allowance.