A 12 oz can of Diet Coke has 46 mg of caffeine. That's more than a can of regular Coca-Cola or Coke Zero, which both have 34 mg (Coca-Cola).
I checked Coca-Cola's US pages on November 2, 2026. CSPI lists a 20 oz bottle at 76 mg (CSPI).
Diet Coke next to the other Cokes
| Drink | Size | Caffeine | Source |
|---|---|---|---|
| Diet Coke | 12 fl oz can | 46 mg | Coca-Cola |
| Diet Coke | 20 fl oz bottle | 76 mg | CSPI |
| Diet Cherry Coke | 12 fl oz | 34 mg | Coca-Cola |
| Coca-Cola | 12 fl oz can | 34 mg | Coca-Cola |
| Coca-Cola Zero Sugar | 12 fl oz | 34 mg | Coca-Cola |
| Caffeine Free Diet Coke | 12 fl oz | 0 mg | Coca-Cola |
Diet Coke has about a third more caffeine than Coke or Coke Zero per can. Diet Cherry Coke is listed at 34 mg, matching regular Coke. A 20 oz bottle is 76 mg, close to an 8.4 oz Red Bull at 80 mg (Red Bull).
Coca-Cola lists a 12 oz coffee at 140 mg or more. A grande Starbucks Cold Brew has 205 mg (Starbucks), about four and a half cans of Diet Coke.
The FDA cites 400 mg a day for most healthy adults, and says people differ.
What's left at bedtime
These numbers use the same model as the Onset caffeine calculator: caffeine drops by half every five hours, and anything under 5 mg counts as cleared.
mg left = mg in the drink x 0.5 ^ (hours since you finished it / 5)
I ran one drink at a time, with a 10 pm bedtime. Values are rounded to the nearest mg.
| Drink | Caffeine | Finished noon | 3 pm | 6 pm | 8 pm |
|---|---|---|---|---|---|
| Coke, Coke Zero, or Diet Cherry Coke, 12 oz | 34 mg | 9 | 13 | 20 | 26 |
| Diet Coke, 12 oz can | 46 mg | 12 | 17 | 26 | 35 |
| Diet Coke, 20 oz bottle | 76 mg | 19 | 29 | 44 | 58 |
| Red Bull, 8.4 oz | 80 mg | 20 | 30 | 46 | 61 |
| Starbucks Cold Brew, grande | 205 mg | 51 | 78 | 118 | 155 |
A can finished at 8 pm still has about 35 mg left at 10 pm. A 20 oz bottle finished at 6 pm leaves more than that (44 mg). A grande cold brew finished at noon leaves about 51 mg.
If you go to bed at 11, the same can finished at 8 pm is down to about 30 mg.
Onset treats a drink as cleared once it drops under 5 mg. For a 12 oz Diet Coke that takes about 16 hours, so a can finished at 3 pm is still around until about 7 am. A 20 oz bottle takes about 19.6 hours.
A few cans in one day
If you have more than one, each can starts on top of what is left from the last. Three 12 oz cans are 138 mg. I added them up at 10 pm and 11 pm, plus a four-can day.
| Cans finished at | Total caffeine | Left at 10 pm | Left at 11 pm |
|---|---|---|---|
| 9 am, 1 pm, 5 pm | 138 mg | 44 mg | 38 mg |
| Noon, 3 pm, 6 pm | 138 mg | 55 mg | 48 mg |
| Noon, 3 pm, 6 pm, 8 pm (four cans) | 184 mg | 90 mg | 79 mg |
Three cans ending at 6 pm leave 55 mg at 10 pm, more than a single 20 oz bottle at 6 pm (44 mg). Four cans ending at 8 pm leave 90 mg, more than a grande cold brew finished at noon (51 mg).
Five hours is an adult average. The FDA-approved label for caffeine citrate gives an adult half-life of about 5 hours, and the FDA says sensitivity varies with weight, medications, and health. If that noon, 3 pm, and 6 pm day ran on a 7-hour half-life instead, about 71 mg would be left at 10 pm, not 55. The 7-hour figure is an example for comparison, not a measured rate for any group.
What studies found
I didn't find a study that tested Diet Coke itself. These two used caffeine doses close to a few cans.
A 2025 randomized crossover trial in SLEEP gave 23 men 100 mg or 400 mg at 12, 8, or 4 hours before bed. 100 mg had no significant effect at those times. 400 mg delayed sleep onset and changed sleep stages when taken within 12 hours of bed (SLEEP, zsae230). They gave one dose, not several cans spread across a day.
Gardiner et al. 2023 pooled 24 studies. Its model put the cutoff at 8.8 hours before bed for a 107 mg coffee to avoid a reduction in total sleep time (Sleep Med Rev 2023). Two Diet Coke cans are 92 mg.
Those studies used fixed doses at fixed times. The tables estimate how much is left. They aren't a cutoff. For coffee and energy drinks, see how many hours before bed to stop caffeine and how much caffeine is in Celsius. For how half-life works, see how long caffeine stays in your system.
If you are pregnant, breastfeeding, or take medication, ask your doctor what applies to you.
The caffeine calculator can add a morning coffee to the cans and show the day hour by hour. Onset is free on the App Store and uses the same model.
This post is for general information and is not medical advice. Estimates use a 5-hour half-life, and your own response varies.
Sources
- Coca-Cola US, Diet Coke products (Diet Coke 46 mg / 12 fl oz; Diet Cherry Coke 34 mg / 12 fl oz; Caffeine Free Diet Coke has no caffeine; checked 2026-11-02)
- Coca-Cola US FAQ, What is caffeine? (Coke 34 mg and Diet Coke 46 mg per 12 oz can; 12 oz coffee 140 mg or more; checked 2026-11-02)
- Coca-Cola US, Coca-Cola Zero Sugar products (34 mg per 12 oz serving; checked 2026-11-02)
- CSPI, How much caffeine is in coffee, tea, soda, and other foods? (20 oz bottle of Diet Coke has 76 mg; checked 2026-11-02)
- Red Bull, How much caffeine is in a can of Red Bull Energy Drink? (8.4 fl oz 80 mg; checked 2026-11-02)
- Starbucks, Cold Brew nutrition (Grande 16 fl oz 205 mg; checked 2026-11-02)
- FDA, Spilling the Beans: How Much Caffeine is Too Much? (400 mg a day for most healthy adults; varies by person)
- FDA-approved CAFCIT label (2020), adult caffeine half-life about 5 hours
- 2025 randomized crossover trial, SLEEP zsae230 (100 mg vs 400 mg at 12, 8, 4 h before bed; PubMed abstract)
- Gardiner et al. 2023, Sleep Medicine Reviews 69:101764 (meta-analysis; 8.8 h for 107 mg coffee)
- Onset caffeine model (5-hour half-life, under 5 mg counts as cleared)
Keep reading
See how much caffeine is in Starbucks cold brew, or learn how caffeine half-life works.