How this instrument works
Caffeine leaves the body through first-order kinetics, meaning a roughly constant fraction of what's left gets cleared in each equal stretch of time — not a constant fixed amount. That's the same decay pattern used to describe radioactive decay or medication clearance, and it's why caffeine level is described by a half-life: the time it takes for exactly half of the current amount to clear, regardless of how much or little is left when that clock starts.
In healthy adults, caffeine's half-life averages around 5 hours, though the real figure varies meaningfully by individual — pregnancy, certain medications, smoking status, and liver function can all shift it notably faster or slower. This calculator uses 5 hours as a default starting point but lets you adjust it, since a single fixed number can't capture that individual variation.
- Enter the caffeine dose you consumed, in milligrams.
- Set the half-life to use, or keep the ~5-hour average default.
- Enter how many hours have passed since you consumed it.
- Read off how much caffeine remains, both in milligrams and as a percentage of the original dose.
Three hours after a 95mg cup of coffee
A typical 95 mg cup of drip coffee, 3 hours later, at the standard ~5-hour average half-life: remaining = 95 x 0.5^(3/5) = 95 x 0.5^0.6 ~ 62.7 mg still active in the body, or about 66.0% of the original dose.
That's a useful reference point for why an afternoon coffee can still affect evening sleep — even 3 hours after drinking it, roughly two-thirds of the caffeine hasn't cleared yet, and it takes a full additional 5-hour half-life beyond that point to knock the remaining amount down to half of what it currently is.
Questions
How long does caffeine actually stay in your system?
There's no single cutoff where caffeine simply disappears — it decays exponentially, so technically a small trace remains for a long time, but the practically noticeable effects fade over roughly 2 to 3 half-lives, or about 10 to 15 hours at the average ~5-hour half-life. That's why a coffee at noon can still measurably affect a bedtime hours later, even though its most noticeable stimulant effect has usually faded well before then.
Why does caffeine's half-life vary so much between sources?
Caffeine metabolism depends on liver enzyme activity, which varies by individual and shifts with factors like pregnancy (which can roughly double the half-life), oral contraceptive use, smoking status (which speeds clearance), and certain medications. Published half-life figures for healthy adults commonly range from about 3 to 7 hours, with 5 hours as a frequently cited average — this calculator defaults to that average but lets you adjust it if you have reason to think yours runs faster or slower.
Does this account for how caffeine actually makes you feel, not just blood levels?
No — this calculator tracks the amount of caffeine mathematically remaining in the body based on pharmacokinetic decay, not the subjective feeling of alertness or a 'crash,' which depends on additional factors like individual tolerance, sleep debt, and how quickly the caffeine was absorbed in the first place. Two people with identical remaining-caffeine percentages can feel quite differently, especially with regular caffeine use building tolerance over time.
Is it true that exactly 50% is gone after 5 hours?
At the default 5-hour half-life, yes — that's the definition of a half-life, the time for exactly half the current amount to clear. What's easy to misjudge is that the next 5 hours don't clear the rest; they clear half of what remains, leaving 25% at 10 hours, 12.5% at 15 hours, and so on, which is why caffeine lingers longer than a simple 'gone in 10 hours' intuition would suggest.