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While there can be significant variation, on a per-volume basis, espresso contains approximately three times the caffeine content of regular brewed coffee (1.700 g/l (50 mg per fluid ounce) of espresso versus 0.50–0.75 g/l (14–22 mg per ounce) for brewed coffee). Compared on the basis of usual serving sizes, a 30 ml (1 fluid ounce) shot of espresso has about half the caffeine of a standard 180 ml (6 fluid ounce) cup of American-style coffee, which varies from 80 to 130 mg