About a quarter of a coffee per ounce
USDA’s analysis of dark chocolate with 70 to 85% cacao solids finds 80 mg of caffeine per 100 g. A 1 oz (28 g) piece therefore carries about 23 mg, roughly a quarter of an 8 oz cup of drip coffee at USDA’s reference strength (95 mg). A whole 101 g bar carries about 81 mg, about the same as a standard 8.4 oz can of Red Bull (80 mg).
Percentage is only a rough guide
More cacao generally means more caffeine in USDA’s figures. Milk chocolate comes in at 20 mg per 100 g, dark chocolate with 45 to 59% cacao at 43 mg, and 70 to 85% at 80 mg. USDA records no caffeine at all in white chocolate.
The gradient is not strict, though. USDA’s figure for the 60 to 69% band is slightly higher than for 70 to 85%, at 86 mg per 100 g, so a bigger number on the wrapper does not guarantee more caffeine. Don’t expect the ingredient list to help either: the FDA notes that when a food contains an ingredient that naturally has caffeine, such as chocolate, caffeine itself will not appear on the list.
Where it becomes significant
For most adults, chocolate is a small part of the caffeine day. A whole 70 to 85% bar is about a fifth of the 400 mg the FDA cites for healthy adults, and an ounce is well under a tenth of it.
It matters more in three cases. Chocolate eaten in the evening adds to whatever coffee is still circulating; with caffeine’s mean half-life of about five hours (NCBI), a late-night bar is not neutral. Children, with lower limits, get proportionally more from the same bar. And chocolate-covered coffee beans are a different product altogether: USDA measures the dark-chocolate kind at 839 mg per 100 g, about 336 mg in a 40 g serving of 28 beans, more than a Grande Starbucks Cold Brew (205 mg).