About this tool
The Caffeine Half-Life Calculator is a free, in-browser tool that shows how the caffeine from a coffee, tea or energy drink decays in your body over the day. Caffeine is eliminated exponentially with a half-life of about 5 hours in healthy adults, so a 200 mg coffee still leaves ~100 mg five hours later.
Enter the dose and when you drank it, and the tool charts the remaining amount at two-hour steps plus the clock time when you fall under 100 mg and 50 mg — useful thresholds for planning a caffeine cutoff before bed. Typical doses: espresso ~65 mg, filter coffee ~95 mg, black tea ~45 mg, cola ~35 mg, energy drink ~80 mg per 250 ml.
Frequently asked questions
How is the remaining caffeine calculated?
With exponential decay: remaining = dose × 0.5^(hours ÷ half-life). With the default 5-hour half-life, half is gone after 5 h, three quarters after 10 h.
Why can I change the half-life?
Elimination varies widely between people — roughly 4–6 h for most healthy adults, faster in smokers and much slower during pregnancy or with some medications. Pick the value that matches your situation.
Why do 100 mg and 50 mg matter?
Sleep research often uses ~100 mg (one coffee) as the level that can still measurably delay or fragment sleep in sensitive people; under 50 mg the effect is small for most. Many clinicians simply suggest no caffeine within 6–8 hours of bedtime.
Does the tool account for multiple drinks?
It models a single dose. For several drinks, add doses that are close together into one total, or run the tool per drink — decay is linear, so the amounts simply add. Everything is computed locally in your browser.