A standard 8.4 fl oz can of Red Bull Energy Drink carries 80 mg of caffeine. A standard 16 fl oz can of Monster Energy Original carries 160 mg - 80 mg more, but the can is also 7.6 fl oz bigger. Per fluid ounce they're close: 9.52 mg/oz for Red Bull Energy Drink versus 10.0 mg/oz for Monster Energy Original.

Figures are each brand's own published nutrition data; see the source link in the table below. Add either one to the caffeine calculator to see how it fits your personal daily limit.

Standard can, side by side

Red Bull Energy Drink, 8.4 fl oz (250 ml) 80 mg
Monster Energy Original, 16 fl oz (473 ml) 160 mg

Sources

Drink Serving Caffeine Source
Monster Energy Original 16 fl oz (473 ml) 160 mg Monster Energy product pageSource
Red Bull Energy Drink 8.4 fl oz (250 ml) 80 mg Red Bull product Q&ASource

Only the standard can of each drink is shown above. Red Bull Energy Drink also comes in 12, 16 and 20 oz cans; see its own page for every size.