Caffeine Cutoff Calculator
Enter your bedtime and what you’ve had. See your live caffeine level, your latest safe time for one more, and exactly how much is left when you sleep — tuned to your body.
This maps to your CYP1A2 liver enzyme — it clears caffeine 3–9× differently than the next person.
How this calculator works
Caffeine clears your body by first-order kinetics — a fixed fraction leaves every hour. We model each drink separately and add them up, so a second cup correctly stacks on whatever’s left of the first:
C(t) = Σ doseᵢ × 0.5(t − tᵢ) / half-life
Your half-life is the only personal number that matters, and it varies a lot — roughly 3 to 9 hours between people. We start from a 5-hour baseline and adjust it for your sensitivity, metabolism speed, body weight, and factors like pregnancy, smoking and oral contraceptives (all of which change how your CYP1A2 enzyme clears caffeine).
The “ok to sleep” line sits at ~50 mg — below that, caffeine is unlikely to disrupt sleep for most people. Your last call is the latest clock time you could have one more standard cup and still land under that line by bedtime.