The lightest of the three Pepsis
A 12 fl oz serving of Diet Pepsi carries 35 mg of caffeine, by PepsiCo’s own product facts, taken from the label on a 24 fl oz bottle listed as two 12 oz servings. That’s a touch less than both regular Pepsi and Pepsi Zero Sugar, which carry 38 mg each. The 3 mg gap is small enough that it rarely matters day to day, but it does mean Diet Pepsi is the one member of the Pepsi line that doesn’t quite match the others.
That pattern is the opposite of Coca-Cola’s lineup, where the diet version carries more caffeine than the original rather than less: Diet Coke has 46 mg per can against 34 mg in regular Coca-Cola. There’s no industry rule linking a diet formula to more or less caffeine; each brand sets its own recipe.
Against the rest of the shelf
At 35 mg, Diet Pepsi sits below Diet Coke (46 mg) by 11 mg, and well below Mountain Dew (54 mg), which carries more caffeine than any of the mainstream colas. It’s close to USDA’s generic figure for diet cola, which the same data set reports separately from regular cola. Against coffee, a can of Diet Pepsi is roughly a third of an 8 oz cup of drip coffee at USDA’s reference strength (95 mg).
Diet soda and caffeine, together
Diet Pepsi removes the sugar, not the caffeine, and switching from a sugared soda to a diet one for calorie reasons does not meaningfully change caffeine intake. At 35 mg a can, eleven cans stay under the 400 mg daily figure the FDA cites for healthy adults, and in pregnancy, five cans (175 mg) fit under the 200 mg daily ceiling ACOG uses. For a caffeine-free diet cola, PepsiCo also sells Caffeine Free Diet Pepsi.