Caffeine Calculator
Caffeine has a five-hour half-life — see what is still in your system.
Your dose
Still in your system
The half-life is why afternoon coffee follows you to bed
Caffeine has a half-life of roughly five hours in healthy adults, meaning half of what you drink is still circulating five hours later. A 200 mg dose — about two cups of brewed coffee — leaves 100 mg at five hours, 50 mg at ten, and 25 mg at fifteen. A 4pm coffee still has meaningful caffeine in your system at midnight.
It interferes with sleep by blocking adenosine, the molecule that accumulates through the day and produces sleep pressure. The effect is measurable even when people report falling asleep normally: caffeine taken six hours before bed has been shown to reduce total sleep time by around an hour, with subjects unaware of the disruption. Reduced deep sleep is the usual cost, which is why a full night can still leave you unrefreshed.
Individual variation is large. The CYP1A2 gene substantially affects metabolism — fast metabolisers clear caffeine in half the time slow ones do — and pregnancy, oral contraceptives and some medications extend the half-life considerably, sometimes doubling it. The commonly cited ceiling for healthy adults is 400 mg a day, with lower limits advised in pregnancy.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Caffeine limits are lower in pregnancy, and it interacts with several medications and with cardiac arrhythmias and anxiety disorders.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
How long does caffeine stay in your system?
The half-life is about five hours, so half remains after five hours, a quarter after ten, and an eighth after fifteen. Traces persist far longer — reaching a negligible level typically takes around 24 hours.
How much caffeine is in a cup of coffee?
Roughly 95 mg in a standard brewed cup, 63 mg in an espresso shot, 47 mg in black tea and 35 mg in a can of cola. Energy drinks vary enormously, from about 80 mg to well over 300 mg per can.
When should I stop drinking coffee before bed?
At least six to eight hours before, and earlier if you are sensitive. Caffeine taken six hours before bed has been shown to reduce total sleep by around an hour even when people report no difficulty falling asleep.
Why does caffeine affect some people more?
Largely genetics. Variants of the CYP1A2 gene mean fast metabolisers clear caffeine in roughly half the time slow metabolisers do. Pregnancy, oral contraceptives and some medications also extend the half-life substantially.
How much caffeine is too much?
400 mg a day is the commonly cited ceiling for healthy adults, roughly four cups of brewed coffee. Pregnancy guidance is usually 200 mg or lower, and single doses above about 500 mg cause jitteriness and palpitations in most people.