Caffeine Half-Life Calculator
Adjust the inputs below. Results update as you type.
How it works
Caffeine's half-life averages 5–6 hours but ranges from 2–12 hours depending on genetics, liver function, smoking status, and pregnancy. After one half-life, 50% remains; after two, 25%; after three, 12.5%. To have less than 10% remaining at bedtime, your last cup should be 3 half-lives before sleep. Oral contraceptives roughly double the half-life.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Caffeine remaining = dose × (1/2)^(hours ÷ half-life). Caffeine's half-life averages about 5 hours, so it clears slowly over the day.
Worked example
A 200 mg coffee at noon leaves about 100 mg by 5 PM and 50 mg by 10 PM, which can still disrupt sleep.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
How long does caffeine stay in your system?+
With a ~5-hour half-life, about half remains after 5 hours and a quarter after 10. It can affect sleep many hours after consumption.
What changes caffeine's half-life?+
Pregnancy, some medications, liver function, and genetics can lengthen it; smoking can shorten it, so individual clearance varies widely.
When should I stop drinking coffee?+
Because a meaningful amount lingers for hours, many people avoid caffeine within about 6–8 hours of bedtime to protect sleep.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.