Caffeine Half-Life Calculator
How much caffeine is left after a coffee or energy drink — first-order half-life sketch. Click any i for detail.
Dose & time
Caffeine dose (mg)
i
Dose
About 95 mg per 8 oz coffee is a common ballpark; check your label. |
|
|---|---|
Half-life (hours)
i
Half-life
Often ~5 hours in healthy adults; pregnancy, smoking, and genetics shift it. |
|
Hours since dose
i
Hours
Elapsed time since you finished the drink. |
How to use this calculator
- Enter milligrams of caffeine.
- Set half-life (default 5 h) and hours elapsed.
- Read remaining mg and percent left.
- Do not use this to override medical caffeine limits.
Results explained
Caffeine clearance is often modeled as exponential decay: remaining = dose × 0.5^(t / half-life). Average adult half-life is roughly 3–7 hours; pregnancy, liver disease, oral contraceptives, smoking, and genetics move it. Multiple doses stack — this page models a single bolus.
Quick reference: caffeine decay
remaining = dose × ½^(hours / half-life).
| Item | Detail |
|---|---|
| Typical half-life | ~5 hours (wide range) |
| 8 oz coffee | ~80–100 mg caffeine |
| Energy drink | Check the can — often 150–300 mg |
| Stacking doses | Not modeled here |
Not medical advice — ask a clinician about caffeine in pregnancy or heart conditions.
How the estimate is built
remaining = dose × 0.5^(hours / half_life).
Example scenario
200 mg, 5 h half-life, 8 hours later → 200 × 0.5^(8/5) ≈ 66 mg.
FAQ
Why change half-life?
If you know you clear caffeine slowly (jittery late at night), try 6–8 hours. Fast clearers may use ~3–4.
Does food change this?
Absorption timing shifts a bit; the exponential model is still a rough sketch.
What about naps / sleep?
Remaining caffeine can still affect sleep architecture even if you feel okay.
Is decaf zero?
Usually a few mg remain — check the product.