Coffee Kick Calculator

Estimate how much caffeine is still in your bloodstream after a coffee, and when it drops low enough to sleep — using caffeine's ~5-hour elimination half-life.

Quick Facts

Method
Exponential decay: remaining = dose × (1/2)t / half-life
Caffeine's half-life averages about 5 hours in healthy adults; roughly half is cleared every 5 hours.

Your Results

Calculated
Caffeine remaining now
-
In your bloodstream
Dose taken
-
Total caffeine consumed
Percent still active
-
Share of the original dose
Down to sleep-safe (≤50 mg)
-
Time from intake

Ready

Enter your caffeine dose and how long ago you drank it, then calculate.

How the Coffee Kick Calculator works

The "kick" from coffee comes from caffeine, and your body clears it on a predictable schedule. Caffeine follows first-order (exponential) elimination: a roughly constant fraction is removed per unit time, not a constant amount. This calculator estimates how much caffeine is still circulating a given number of hours after you drank a coffee, and when that amount falls below a level unlikely to interfere with sleep.

The formula

The amount of caffeine remaining is:

remaining = dose × (1/2)(t / half-life)

where dose is the total milligrams consumed, t is the hours since intake, and half-life is the time for half of it to be eliminated. The default half-life is 5 hours, the average for healthy adults. Every time t equals one half-life, the remaining caffeine halves: after 5 hours half is left, after 10 hours a quarter, after 15 hours an eighth, and so on.

How much caffeine is in common drinks

  • Brewed coffee (8 oz / 240 ml): about 95 mg (typical range 70–140 mg).
  • Espresso (1 shot, ~30 ml): about 63 mg.
  • Instant coffee (8 oz): about 62 mg.
  • Black tea (8 oz): about 47 mg; green tea about 28 mg.
  • Cola (12 oz): about 34 mg; typical energy drink (8 oz): about 80 mg.
  • Decaf coffee (8 oz): about 2–5 mg.

Why it matters

Health authorities consider up to about 400 mg of caffeine per day safe for most healthy adults (roughly four 8 oz cups of coffee). Because caffeine lingers, an afternoon coffee can still be active at bedtime: a 95 mg cup at 3 p.m. leaves about 48 mg at 8 p.m. and about 24 mg at 1 a.m. with a 5-hour half-life. Research suggests even ~40 mg near bedtime can measurably delay sleep in sensitive people, which is why the calculator flags a 50 mg sleep-safe threshold.

What changes your half-life

Five hours is an average, not a rule. Smoking roughly halves caffeine's half-life; pregnancy can more than double it (up to 15 hours in the third trimester); oral contraceptives and some medications (e.g., certain antibiotics and antidepressants) lengthen it; and liver disease slows clearance. If you know your body clears caffeine faster or slower than average, adjust the half-life input.

Frequently Asked Questions

How much caffeine is in a cup of coffee?
A standard 8 oz (240 ml) cup of brewed coffee contains about 95 mg of caffeine, though it commonly ranges from 70 to 140 mg depending on the beans, roast, grind, and brew strength. A single espresso shot has about 63 mg, an 8 oz cup of black tea about 47 mg, and decaf about 2–5 mg. Enter the number that matches your drink for the most accurate estimate.
How long does caffeine stay in your system?
Caffeine's average elimination half-life is about 5 hours in healthy adults (commonly 3–7 hours). Half the dose clears every half-life, so after a 95 mg cup roughly 48 mg remains at 5 hours, 24 mg at 10 hours, and 12 mg at 15 hours. It is rarely fully gone — it just falls to negligible levels. Practically, most of the noticeable stimulant effect is gone within about two half-lives.
When should I stop drinking coffee before bed?
A common guideline is to have your last caffeine 6–8 hours before bedtime. This calculator lets you be more precise: it shows when your dose decays below 50 mg, a level unlikely to disrupt sleep for most people. With a 5-hour half-life, a single 95 mg cup drops below 50 mg after about 4.6 hours; a 200 mg dose takes about 10 hours.
Why does the calculator use a half-life instead of a fixed decay?
Caffeine is eliminated by first-order kinetics, meaning a constant percentage is cleared per unit time rather than a fixed number of milligrams. That produces exponential decay, so a larger dose takes proportionally longer to reach any given level. A fixed "so many mg per hour" model would be wrong at both high and low concentrations.

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