A flat 40 mg, whatever the size
Starbucks lists 40 mg of caffeine for its Royal English Breakfast Tea at every size: Short, Tall, Grande and Venti. As with its other brewed hot teas, the caffeine comes from a single tea bag steeped in hot water, and the size you order changes how much water it’s steeped in, not how many bags go in, so the dose doesn’t scale with cup size the way espresso drinks do.
Slightly less than a home-brewed cup
That 40 mg figure is a bit below USDA’s figure for home-brewed black tea, 47 mg for an 8 oz cup, likely down to shorter steep times behind the counter. It matches Starbucks’ Earl Grey Tea exactly, both being black teas, and runs well above its green Emperor’s Clouds & Mist tea (16 to 25 mg in a Grande).
A classic breakfast blend
English Breakfast is traditionally a blend of black teas, often Assam, Ceylon and Kenyan leaves, chosen for a robust flavour that holds up to milk. Starbucks’ Royal English Breakfast follows that tradition, and its caffeine sits in the same range as other straightforward black tea blends rather than anything unusually strong or light for the category.
Where it fits in a day
Every size of English Breakfast carries the same 40 mg, a fifth of the 200 mg single dose EFSA assessed as safe for healthy adults. It would take ten cups to reach the 400 mg a day the FDA cites for healthy adults, and five to reach the 200 mg daily ceiling ACOG uses in pregnancy.