Caffeine Half-Life Calculator
Estimate caffeine remaining over time.
Your caffeine estimate
Caffeine remaining
After 5 hours • assumed half-life 5 hours
Estimated caffeine over time
Single-dose exponential model.
How is this calculated?
1. Remaining amount (first-order elimination)
Remaining = Initial × (½)Hours / Half-life
200 × (½)5 / 5 = 100.0 mg
2. Eliminated amount
Eliminated = 200 − 100 = 100.0 mg
Half-life is not the same as effect duration
A modeled amount does not tell you when sleep is guaranteed.
Model & assumptions
This calculator uses a single-dose, first-order elimination model: caffeine decreases by half each assumed half-life. Absorption (how quickly caffeine enters the body) is not modeled. Individual clearance varies and there is no zero-clearance promise.
Worked example (current values)
200 × (½)5 / 5 = 100.0 mg
Input limits & interpretation
Enter a finite amount of 0–10,000 mg and elapsed time of 0–10,000 hours. Half-life must be greater than zero and no more than 1,000 hours. These are numerical software bounds, not safe intake recommendations. The amount and effects of caffeine vary; this is not a medical measurement or personal dosing recommendation.
Frequently asked questions
What does the Caffeine Half-Life calculate?
It estimates the amount of a single caffeine dose remaining after a chosen time, using an assumed half-life. It is not a measured blood concentration.
Which values should I enter?
Enter the initial caffeine amount in milligrams, hours since that intake, and an assumed half-life in hours. Use product-label caffeine content when available. This tool does not combine multiple doses.
What formula does this calculator use?
Remaining = initial amount × (1/2)^(elapsed hours / half-life). Eliminated amount = initial − remaining. The same full-precision result drives the chart, steps and exports.
Why might my result differ from another source?
Half-life and caffeine content vary. Medicines, pregnancy, smoking and health conditions may affect clearance. Other tools may assume a different half-life or include repeated doses and absorption.
Does the result update automatically?
Yes. Valid changes immediately update the amount, chart and worked solution. Invalid or missing inputs clear the estimate and disable copying and downloading.
More questions (5)
Can this predict when I will sleep?
No. An estimated amount is not an individual sleep-safety threshold. Sensitivity varies; this tool cannot guarantee alertness, impairment, or when sleep will be possible.
Why is the default half-life 5 hours?
It is an illustrative adult assumption consistent with the approximate 4–5 hour half-life described by NIH ODS. It is not a measurement of your personal clearance.
Does caffeine ever reach exactly zero in this model?
For a positive initial dose, exponential decay approaches zero but never reaches it at a finite time. Very small values are shown with scientific notation or a numerical-limit label rather than an exact-zero claim.
What are the input limits?
The initial amount and elapsed time must be finite and nonnegative; the half-life must be finite and positive. Software limits are 10,000 mg, 10,000 elapsed hours and 1,000 half-life hours. These broad numerical limits are not safe intake recommendations.
Is remaining caffeine the same as a daily intake limit?
No. Daily intake guidance concerns total caffeine consumed, not the modeled amount remaining at one moment. This calculator does not set a personal daily limit or recommend a dose.
Sources & References
National Institutes of Health (NIH)Caffeine absorption and elimination. Approximate adult half-life context.
U.S. Food and Drug Administration (FDA)Caffeine intake and individual sensitivity. General consumer information.
European Food Safety Authority (EFSA)Caffeine and sleep context. Effects and individual considerations.References provide context, not endorsement. No institutional certification or review is claimed.
Page information
Updated: October 27, 2026