Caffeine Half-Life Calculator
Enter one or multiple caffeine doses in milligrams and the time each was consumed. Adjust the half-life to estimate how much caffeine remains, view 24- or 48-hour decay, and calculate when the modeled amount reaches an optional target.
This is a mathematical planning model, not a measurement of your blood caffeine level and not a recommendation about safe intake or sleep timing.
Caffeine doses
Add 1–8 caffeine intakes from the previous 72 hours. Amounts are entered in milligrams.
1 of 8 doses
Half-life assumption
The default is 5 hours. Change it only when you intentionally want to model a different half-life.
Chart horizon
Choose how far the estimated caffeine decay curve should extend after the calculation time.
Custom reference amount (optional)
Estimate when the model reaches an amount you choose. This is not a safety or sleep threshold.
⚡ How does a caffeine half-life calculator work?
A caffeine half-life calculator estimates how much of a recorded caffeine dose remains over time using exponential decay. With a 5-hour half-life, 50% remains after 5 hours, 25% after 10 hours, 12.5% after 15 hours, and 6.25% after 20 hours.
For multiple coffees, drinks, or other logged doses, the calculator models each intake separately from its recorded time and adds the estimated remaining amounts together.
Single-dose decay
Remaining = Dose × 0.5^(Elapsed hours ÷ Half-life)
After one half-life, 50% remains; after two, 25%; after three, 12.5%.
Multiple logged doses
Total remaining = Sum of each dose remaining
Each dose is decayed from its own intake time, then the remaining amounts are added together.
Time to a custom target
Hours to target = Half-life × log2(Current amount ÷ Target amount)
If the target is equal to or above the current estimate, the modeled time is now. A target is not a safety threshold.
NCBI references describe caffeine elimination using first-order kinetics and report a mean half-life of about 5 hours in healthy individuals, with substantial variation. This calculator uses 5 hours only as an editable planning default. It assumes each dose is immediately available at its recorded time, although real oral caffeine can take roughly 15–120 minutes to reach peak plasma concentration.
📊 Caffeine Remaining After Each Half-Life
This mathematical reference starts with 200 mg and applies a 5-hour half-life without rounding intermediate values.
| Time elapsed | Half-lives elapsed | Estimated caffeine remaining | Percent remaining |
|---|---|---|---|
| 0 hours | 0 | 200 mg | 100% |
| 5 hours | 1 | 100 mg | 50% |
| 10 hours | 2 | 50 mg | 25% |
| 15 hours | 3 | 25 mg | 12.5% |
| 20 hours | 4 | 12.5 mg | 6.25% |
| 25 hours | 5 | 6.25 mg | 3.125% |
These values describe the selected exponential model only; they do not measure blood concentration or establish a safe remaining amount.
⏳ How long does caffeine stay in your system?
In a mathematical half-life model, the estimated amount keeps being divided by two and approaches zero without reaching exact zero in a finite amount of time. After one half-life, 50% remains; after two, 25%; after three, 12.5%; after four, 6.25%; and after five, 3.125%.
Actual caffeine elimination can differ substantially between people and circumstances. Use the result as a planning estimate, not as a clearance guarantee or a measured caffeine concentration.
☕ Multiple-Dose Worked Example
Suppose Dose 1 is 100 mg at 8:00 AM and Dose 2 is 100 mg at 1:00 PM, using a 5-hour half-life. Each dose follows its own decay calculation; the estimated combined remaining amount is their sum.
| Reference time | Dose 1 elapsed / remaining | Dose 2 elapsed / remaining | Estimated combined remaining |
|---|---|---|---|
| 6:00 PM | 10 hours / 25.00 mg | 5 hours / 50.00 mg | 75.00 mg |
| 11:00 PM | 15 hours / 12.50 mg | 10 hours / 25.00 mg | 37.50 mg |
The worked values come from the same production calculation as the interactive tool. The 11:00 PM amount is not labeled safe or unsafe for sleep.
📉 Understanding the Caffeine Decay Curve
The caffeine half-life chart plots estimated caffeine remaining over time. A logged dose can create an upward jump at its intake time; between dose events, the modeled amount declines exponentially. After the calculation time, with no unlogged future intake, the combined total follows the selected half-life.
Choose 24 hours for shorter-term and overnight decay or 48 hours to inspect later and next-day modeled amounts. Checkpoints show Now, +2h, +4h, +6h, +8h, +12h, +18h, and +24h, with +36h and +48h added in 48-hour mode. If your planned bedtime falls within that window, the curve or checkpoints can estimate the modeled amount at that time without turning the result into sleep guidance.
📌 Planning Assumptions and Limits
One half-life for all doses
Every logged dose uses the same half-life setting. The calculator does not change metabolism over the day or by beverage type.
Instant availability assumption
Each entered dose is treated as available immediately at its recorded time. Real oral caffeine absorption is not instantaneous, and the model does not represent peak plasma concentration.
Editable 5-hour planning default
A mean half-life of about 5 hours is commonly reported for healthy individuals, but actual half-life can vary substantially. The calculator does not decide which value applies to you.
No personalized metabolism model
The tool does not infer half-life from pregnancy, smoking, medications, age, genetics, health conditions, or other individual factors.
No future unlogged intake
The future curve and checkpoints assume no additional caffeine doses are consumed after the calculation time.
Technical limits are not safety limits
The supported input ranges prevent invalid or impractically large calculations. They do not define safe caffeine intake or a safe remaining amount.
📚 Model References
NCBI Bookshelf — Pharmacology of Caffeine
Institute of Medicine reference describing first-order caffeine elimination, a mean half-life of about 5 hours in healthy individuals, substantial half-life variation, and oral absorption timing.
View source ↗NCBI Bookshelf — Caffeine pharmacokinetics
Reference describing apparent first-order elimination and variation in peak plasma timing after oral ingestion. FigureNorth intentionally simplifies intake as immediately available.
View source ↗❓ Caffeine Half-Life Calculator FAQ
What is caffeine half-life?
Caffeine half-life is the time used in an exponential decay model for the estimated caffeine amount to fall by half. After one half-life, 50% remains; after two, 25%; after three, 12.5%.
How does a caffeine half-life calculator work?
It models each recorded dose as dose × 0.5^(elapsed hours ÷ half-life hours). Multiple doses are calculated separately and their estimated remaining amounts are added together using one selected half-life.
How much caffeine is left after 5 hours?
With a 5-hour half-life, 50% of the modeled amount remains after 5 hours. For example, an initial 200 mg dose is estimated at 100 mg after 5 hours and 50 mg after 10 hours.
How long does caffeine stay in your system?
The mathematical half-life model approaches zero but does not reach exact zero in a finite time. After five half-lives, 3.125% remains. Actual elimination can differ substantially, so this is an estimate rather than a clearance guarantee.
Can I calculate multiple coffees or caffeine doses?
Yes. You can log up to eight coffees, drinks, or other caffeine doses from the previous 72 hours. Each dose follows its own decay curve, and the calculator adds their estimated remaining amounts without implying an interaction between doses.
What does a 5-hour caffeine half-life mean?
It means the mathematical estimate is divided by two every 5 hours: 50% remains after 5 hours, 25% after 10 hours, and 12.5% after 15 hours. Five hours is an editable planning default, not a personal estimate.
Why can caffeine half-life vary between people?
Reported caffeine half-life varies because elimination differs between individuals and circumstances. FigureNorth does not infer a personal half-life from pregnancy, smoking, medications, age, genetics, health conditions, or other biological factors.
Can I estimate caffeine remaining at bedtime?
Yes. If your planned bedtime falls within the displayed 24- or 48-hour window, use the curve or future checkpoints to estimate the modeled amount at that time. The result does not determine whether that amount is appropriate for sleep and does not recommend a bedtime cutoff.
When will caffeine fall below a chosen amount?
Enter an optional target amount in milligrams. If it is below the current estimate, the calculator uses the selected half-life to estimate when the modeled amount reaches it; if it is equal to or above the current estimate, the time is now. The target is not a safety threshold.
Does this calculator measure the actual caffeine level in my blood?
No. It estimates the remaining amount from recorded intake times using first-order exponential decay. It does not model absorption delay, measure blood or plasma concentration, or provide a personal pharmacokinetic profile.