The same caffeine as regular
Diet Mountain Dew carries 54 mg of caffeine in a 12 fl oz can, exactly matching regular Mountain Dew at the same size. Caffeine and sugar are added separately, so removing the sugar for a diet formula doesn’t touch the caffeine; this is a pattern that holds across most diet sodas, not just Mountain Dew.
At 54 mg, it remains one of the more caffeinated mainstream sodas, well above a typical cola. It’s far below Mtn Dew Kickstart, the energy-drink-adjacent line built on the same brand, which carries considerably more per can.
Diet sodas aren’t uniformly lower
It’s a common assumption that “diet” means less caffeine, but that isn’t generally true: diet colas are often close to, or even above, their regular counterparts, the same pattern seen with generic diet cola running higher than regular Coca-Cola Classic. Checking the actual label, rather than assuming from the word “diet,” is the only reliable approach.
Mountain Dew’s citrus flavour tends to mask caffeine’s bitterness more than a cola does, which can make it easy to drink several cans without the taste signaling how much caffeine has been consumed.
Where it fits in a day
At 54 mg, a can is far under the 200 mg single dose EFSA assessed as safe for healthy adults. Seven cans, 378 mg, would come close to the 400 mg daily figure the FDA cites. In pregnancy, where ACOG uses a 200 mg daily ceiling, three cans still fit under the day’s allowance.