Caffeine Half-Life Calculator
See how much caffeine is still in your system over time.
An average estimate, not personal medical advice — actual half-life ranges roughly 1.5–9.5 hours and is affected by genetics, pregnancy, liver function, and other medications. Pregnant individuals metabolize caffeine much more slowly.
Remaining after 8 hours
31 mg
Time until under 25 mg
9.6 hr
Caffeine remaining over 24 hours
For informational and educational purposes only — not medical advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Why half-life matters for sleep
Caffeine clears from your system exponentially — roughly half of it is gone every "half-life" period, around 5 hours on average, though this varies significantly between individuals. That means an afternoon coffee at 3pm can still leave a meaningful amount of caffeine in your system at bedtime, even if you don't feel obviously wired — caffeine can disrupt sleep quality well before you'd consciously notice its stimulant effect.
Frequently asked questions
What's caffeine's average half-life?
Roughly 5–6 hours in a typical healthy adult, meaning about half the caffeine you consumed is still in your system after that time — though this varies significantly based on genetics, liver function, pregnancy, and other factors.
Why does caffeine late in the day affect sleep even if I don't feel wired?
Because a meaningful amount can still be in your system 8+ hours later even after the initial alertness effect fades, and caffeine can disrupt sleep quality (particularly deep sleep) even when you don't consciously feel stimulated.