Why “a cup of coffee” is not one number
The 95 mg above is USDA’s reference value for drip coffee brewed with tap water: 40 mg of caffeine per 100 g of coffee, applied to an 8 fl oz cup. It is a laboratory average, and the cup in your hand can sit well away from it. The FDA’s own table puts a 12 fl oz cup of regular, non-specialty brewed coffee anywhere from 113 to 247 mg. USDA’s figure scales to 142 mg for the same 12 oz, near the bottom of that span.
What moves it is how much ground coffee goes into the brewer, how long the water stays in contact with it, and the size of the cup. Roast is a weaker lever than most people assume: Starbucks lists identical caffeine ranges for its blonde, medium and dark brewed roasts at every size.
Coffee-shop coffee runs much stronger
Chain coffee is where the reference value stops being useful. A Grande (16 oz) Starbucks Pike Place Roast carries 315 to 390 mg by Starbucks’ own figures, against 189 mg for 16 oz at USDA’s reference strength. If most of your coffee is bought rather than brewed at home, use the chain’s figure rather than this one.
How many cups fit in a day
The FDA describes 400 mg a day, the level it cites as not generally associated with negative effects in healthy adults, as about two to three 12 oz cups of coffee. At USDA’s reference strength it is four 8 oz cups (380 mg). At a coffee shop it can be a single cup: the top of Starbucks’ Venti Pike Place range, 490 mg, is over the line on its own. In pregnancy the ceiling ACOG uses is 200 mg, two 8 oz cups at this strength.
Caffeine also outlasts the cup. With a mean half-life of about five hours (NCBI), half of a 3 pm coffee is still circulating at 8 pm, which is why the last cup of the day matters more for sleep than the first.