Caff ConsciousHow long does caffeine stay in your system?
Caffeine fades gradually. Half-life describes when half of an amount remains; it does not mark the moment all the caffeine is gone.
CDC/NIOSH gives a typical half-life of about five to six hours and notes that caffeine can persist much longer. That is a useful starting point for an illustration, but it is not a personal countdown.
What a five-hour half-life means
Start with an imaginary drink containing 100 mg of caffeine. Assume the entire amount is available immediately and halves every five hours. No other caffeine is included.
| Time since the drink | Estimated caffeine remaining |
|---|---|
| At the starting time | 100 mg |
| 5 hours | 50 mg |
| 10 hours | 25 mg |
| 15 hours | 12.5 mg |
| 20 hours | 6.25 mg |
Each step halves what is left, rather than removing another 50 mg. This equation approaches zero gradually, so it cannot supply an exact “all gone” time. A small calculated amount also does not prove that you will sleep well.
Use the amount and time of your actual drink
In this example, 100 mg at 2 PM leaves about 28.7 mg at an 11 PM bedtime. If the starting amount doubles, the estimate doubles to 57.4 mg under the same assumptions. The 100 mg value is an example, not a claim about every coffee.
When a label gives caffeine per serving, check how many servings you drank. If the amount is unknown, treat the input as an estimate and keep that uncertainty attached to the result.
Try your timing in the calculator
Why one estimate will not fit everyone
The FDA describes wide variation in caffeine sensitivity and elimination speed. It also recommends discussing caffeine with a health professional when conditions, medications, or pregnancy may affect what is appropriate for you.
A calculator lets you change an assumed half-life and see how the arithmetic changes. It does not measure your metabolism. For the same 100 mg drink and nine-hour interval, a five-hour assumption gives 28.7 mg, while a six-hour assumption gives 35.4 mg.
What if you have more than one drink?
Within this simplified model, calculate the remainder of each drink at the same target time, then add them. A 100 mg drink at 2 PM and a 60 mg drink at 4 PM would contribute about 28.7 mg and 22.7 mg at 11 PM with a five-hour half-life: about 51.5 mg in total before rounding the individual results.
The web calculator handles one drink at a time. Caff Conscious uses the drinks you log to build a bedtime estimate, including when a drink is logged as sipped over time.
From half-life to bedtime planning
Compare amounts and times without treating any calculated number as a sleep guarantee. Our guide to caffeine before bed explains the difference between a timing estimate and an individual cutoff.
Sources and method
This guide’s numerical examples use the calculator’s published formula and assumptions. General caffeine information comes from CDC/NIOSH and the FDA. This is educational information, not medical advice or a personalized sleep recommendation.