Caffeine Half-Life Calculator
Estimate how much caffeine remains at bedtime using a range, because half-life varies widely between people.
- 01Add each caffeinated drink with roughly the time you had it.
- 02Set your bedtime — everything is measured against that moment.
- 03Flag anything that changes your clearance, since these shift the estimate more than the drink amounts do.
- 04Read the range, not just the middle number.
What the terms mean
- Half-life
- The time for the caffeine in your system to fall by half. Around five hours in a typical healthy adult, but the usual range is three to seven.
- First-order elimination
- Clearance proportional to how much is present — so it halves repeatedly rather than draining at a fixed rate.
- CYP1A2
- The liver enzyme that does most caffeine metabolism. Genetic variation in it is the main reason people differ so much.
These shift the estimate substantially — smoking roughly halves half-life, pregnancy can more than double it.
There is no established safe number — 50 mg is roughly half a cup of coffee, offered as a reference point only.
Range 3–22 mg depending on how fast you clear it · 95 mg consumed today
The shaded band is the honest part of this chart. Caffeine half-life varies roughly three to seven hours between healthy adults, so the same coffee can leave very different amounts on board at midnight. Treat the line as a middle estimate, not a measurement.
Method & sources
Caffeine clears by first-order elimination, so the amount remaining halves every half-life: C(t) = C₀ × ½^(t ÷ t½). Multiple drinks are summed, each decaying from its own time.
A half-life of five hours is used as the middle estimate, with three and seven hours as the range — the usual span reported in healthy non-smoking adults. Modifiers scale that range: smoking shortens it markedly, while oral contraceptives, pregnancy and impaired liver function extend it.
Serving amounts are typical values. Real caffeine content varies a great deal with bean, grind, brew time and cup size — a strong filter coffee can be double the figure used here.
No residual amount is established as safe for sleep.Sensitivity differs severalfold between individuals, and genetics (particularly CYP1A2) account for much of it. The threshold line is a reference point for comparing your own nights, not a clinical cutoff.
What these numbers
actually tell you.
The range matters more than the number
Half-life varies roughly threefold between healthy adults, so the same afternoon coffee can leave anywhere from a trace to most of a cup on board at midnight. The shaded band on the chart is the honest answer; the line through it is a convenience.
There is no safe threshold for sleep
No amount of residual caffeine is established as the point where sleep stops being affected. Some people sleep through a double espresso at nine and others lie awake after tea at four. Use the threshold line to compare your own nights against each other, not against a standard.
Serving sizes are the loosest input here
A cup of filter coffee is listed at 95 mg, but real cups range from about 70 to 200 depending on beans, grind, brew time and how big the cup actually is. If your coffee is strong, the estimate is low.
What the modifiers do
Smoking induces the enzyme that clears caffeine and roughly halves half-life. Oral contraceptives inhibit it and commonly double it. Pregnancy extends it substantially, especially later on, which is why intake guidance drops to around 200 mg a day. If a clinician has told you something different about your own clearance, follow them.
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