Last Coffee of the Day
Caffeine has a half-life of roughly 5.7 hours. Divide what you drank by what you want left, take the base-2 logarithm to get the number of half-lives, and multiply by 5.7 for the hours.
Your numbers
Hours later
9h
Formula
Hours later = log₂(Caffeine (mg) ÷ Remaining) × 5.7
That half-life ranges from about four to nine hours between people, and stretches much longer during pregnancy or on certain medicines.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Caffeine (mg) (mg)Caffeine taken in; a single Americano runs about 120 mg. | 150 | 0 and up |
| Remaining (mg)How much is still in you; it halves about every five hours. | 50 | 0 and up |
Step by step
Quick reference table
Results when only Caffeine (mg) (mg) changes and everything else stays put.
| Caffeine (mg) (mg) | Hours later (h) |
|---|---|
| 75 | 3.3 |
| 113 | 6.7 |
| 150 | 9 |
| 225 | 12.4 |
| 300 | 14.7 |
What each result means
| Result | At default values |
|---|---|
| Hours later (h)How long it has been since you drank it. | 9 |
Common mistakes
That half-life ranges from about four to nine hours between people, and stretches much longer during pregnancy or on certain medicines.
Glossary
- Caffeine (mg)
- Caffeine taken in; a single Americano runs about 120 mg.
- Remaining
- How much is still in you; it halves about every five hours.
- Hours later
- How long it has been since you drank it.
Frequently asked questions
Q. How is Last Coffee of the Day calculated?
Hours later = log₂(Caffeine (mg) ÷ Remaining) × 5.7 — Caffeine has a half-life of roughly 5.7 hours. Divide what you drank by what you want left, take the base-2 logarithm to get the number of half-lives, and multiply by 5.7 for the hours.
Q. Can you walk through an example?
With Caffeine (mg) 150mg, Remaining 50mg, the answer is Hours later 9h.
Q. What do I need to enter?
Enter Caffeine (mg), Remaining. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Caffeine (mg) (mg) moves the answer to Caffeine (mg) (mg) 75 → Hours later (h) 3.3 and Caffeine (mg) (mg) 300 → Hours later (h) 14.7. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
That half-life ranges from about four to nine hours between people, and stretches much longer during pregnancy or on certain medicines.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.