Going diet doesn't change the caffeine here: a 12 fl oz can of Mountain Dew and the same size can of Diet Mountain Dew both carry 54 mg of caffeine, according to PepsiCo's own published data. The diet version cuts sugar and calories, not caffeine.
See the source link in the table below, or add either one to the caffeine calculator to see how it fits your personal daily limit.
Standard can, side by side
Sources
| Drink | Serving | Caffeine | Source |
|---|---|---|---|
| Mountain Dew | 12 fl oz (355 ml) | 54 mg | PepsiCo Product FactsSource |
| Diet Mountain Dew | 12 fl oz (355 ml) | 54 mg | PepsiCo Product FactsSource |