How it works
A single cup is easy to estimate, but a real day is several cups that overlap. Each coffee clears at the same half-life while the next one adds on top, so your level climbs during the morning and falls slowly after the last cup.
This calculator simulates that: it adds up every coffee you enter, applies the half-life hour by hour, and draws the whole day as a curve. The two numbers that matter most are the peak and — if you are here for sleep — how much is still around at your bedtime.
If you want a personal estimate, adjust the half-life input. Most adults sit near 5-6 hours; if you are very sensitive or on certain medications, a longer half-life (6-7 hours) is closer to your body.
The rule to remember: caffeine drops to about half every 5-6 hours, and below about 25 mg (one espresso) it rarely affects sleep. Use this with the when-to-stop tool to plan your cutoff.
Questions people actually ask
How long does caffeine stay in your system?
For most adults it takes 5-6 hours to drop to half, and 10-12 hours to be mostly gone. This calculator shows the hour-by-hour picture for your own coffees.
How much caffeine is in my body right now?
It depends on how much you drank, when, and your half-life. Add your coffees above to see an estimate for the whole day.
Does caffeine build up during the day?
Yes. Because each cup clears slowly, a coffee at 14:00 still overlaps with one from 08:00. The bars show how the amounts add together.
What is a normal caffeine half-life?
For a healthy adult, usually 5-6 hours. It varies with genetics, weight, some medications, and regular caffeine use.
When is caffeine low enough not to affect sleep?
A useful rule of thumb is below roughly 25 mg, about one espresso. The calculator marks when your level drops under that.
Why does caffeine affect me more than others?
People clear caffeine at different speeds. A longer half-life means more stays in your system at bedtime.