Caffeine Half-Life Calculator
Estimate how much caffeine remains in your body over time and when to stop drinking coffee before bed.
Fill in the fields and press Calculate. Nothing is sent to a server.
What this calculator does
A caffeine half-life calculator estimates how much caffeine remains in your body over time. With a typical adult half-life of about 5 hours, half of a dose remains after 5 hours, a quarter after 10 hours, and so on.
Formula
Remaining = Dose × 0.5^(hours since intake / half-life)
Worked example
100 mg coffee at 3 PM, half-life 5 h, bedtime 10:45 PM (7.75 h later): 100 × 0.5^(7.75/5) ≈ 34 mg remaining.
Assumptions
- Average adult caffeine half-life of about 5 hours.
- First-order elimination (simple exponential decay).
- No additional doses.
Limitations
- Pregnancy, some medications, smoking and genetics change caffeine half-life substantially.
- This tool does not replace medical advice.
Frequently asked questions
How long does caffeine stay in your system?▾
It can take 8–10 hours or more for caffeine levels to drop meaningfully in most adults.
When should I stop drinking coffee?▾
Many sleep experts suggest avoiding caffeine at least 6–8 hours before bedtime, or earlier if you are sensitive.
Related calculators
Learn more
Wellness information, not medical advice. This calculator is for general education and planning. It is not a diagnosis and does not replace advice from a qualified health professional.