Caffeine Half-Life Calculator
See how much caffeine stays in your system over the next 12 hours.
Runs entirely in your browser. Nothing you type is uploaded anywhere.
How to use this tool
Enter the amount of caffeine you drank in milligrams (a standard 8 oz cup of brewed coffee is roughly 95 mg). Pick the time you had it, then leave the half-life at 5 hours or adjust it if you metabolize caffeine faster or slower. Press Calculate and the table shows how much caffeine is left in your body at each hour going forward, so you can see when the amount drops low enough for sleep.
The caffeine decay model
Caffeine clears from the body through first-order elimination, which means a fixed fraction leaves during each fixed window of time. That window is the half-life, around 5 hours for a typical healthy adult. After one half-life you have half the caffeine, after two half-lives a quarter, after three an eighth, and so on. This calculator applies that same formula at each hour to track caffeine dropoff like a coffee metabolism timer.
A real example
Say you drink a 16 oz coffee with about 190 mg of caffeine at 3:00 PM and use the default 5 hour half-life. At 8:00 PM (5 hours later) you still have about 95 mg, half the starting dose. By 1:00 AM (10 hours later) you are down to roughly 48 mg, and by 3:00 AM (12 hours later) about 38 mg. That lingering amount is why an afternoon coffee can quietly disrupt sleep even when you no longer feel wired.
Common questions
What is the half-life of caffeine?
For most healthy adults it is about 5 hours, though estimates range from 3 to 7 hours. Pregnancy, some medications, and liver conditions can stretch it much longer, while smoking tends to shorten it.
How much caffeine is in common drinks?
A rough guide: 8 oz brewed coffee around 95 mg, a single espresso shot around 63 mg, 12 oz cola around 35 mg, 8 oz black tea around 47 mg, and a typical energy drink 80 to 150 mg.
When should I stop drinking coffee before bed?
Because caffeine decays slowly, many people aim to stop 8 to 10 hours before sleep. Use the table to find the hour when your remaining caffeine drops to a level that does not bother you.
Why can I change the half-life value?
People process caffeine at different speeds. Lowering the half-life models a fast metabolizer and raising it models a slow one, so you can see how your own caffeine dropoff might differ from the average.
Is this calculator medically accurate?
It is a simplified estimate for educational use, not medical advice. Real caffeine levels depend on body weight, genetics, and other factors. Talk to a healthcare professional for guidance specific to you.