Caffeine Calculator
Remaining caffeine from dose, hours passed, and half life.
What it does
Caffeine fades exponentially with a half life near five hours in average adults: half remains after one half life, a quarter after two, an eighth after three. That long tail explains why a 2 PM coffee still guards the midnight gate.
The tool above takes the dose, hours passed, and personal half life. It returns milligrams left, percent remaining, and cups of coffee equivalent.
Metabolism varies widely. Smokers clear caffeine near twice as fast, oral contraceptives slow it, pregnancy stretches half life past ten hours, and gene variants split fast and slow metabolizers.
Dose stacking sneaks up through the day. Two morning coffees plus a pre workout scoop can total 500 milligrams with half still active at dinner, which is a sleep problem wearing a productivity costume.
How to use
- Enter total milligrams from all sources.
- Enter hours since the last dose.
- Set half life to 5 unless you know yours.
- Plan the last dose so little remains at bedtime.
Formula
- 0.5^...
- Exponential decay: each half life halves the rest.
- half life
- Hours to clear half. Average 5, range 3 to 10 plus.
- 95 mg
- Typical brewed cup used for the equivalent.
Zero order illusions aside, liver enzymes follow first order decay here, which is why equal time slices remove equal shares, not equal milligrams.
Worked example
Example: 200 mg, 5 hours later, 5 hour half life
Large coffee of 200 mg at noon, checked at 5 PM.
- Halvings: 5 / 5 = 1.
- Left: 200 x 0.5^1 = 100 mg.
- Share: 50% left.
- Cups: 100 / 95 = 1.1 cups still working at dinner approach.
Reference table
| Hours | Left | Share |
|---|---|---|
| 0 | 200 mg | 100% |
| 5 | 100 mg | 50% |
| 10 | 50 mg | 25% |
| 15 | 25 mg | 12% |
| 20 | 12 mg | 6% |
Limits
- Average kinetics. Individuals vary widely.
- One compartment model. Real distribution is messier.
- Tolerance is not clearance. Feeling fine differs from clean.
- Pregnancy, liver issues, and drugs shift half life a lot.
Common mistakes
- Counting only coffee while energy drinks and pills add up.
- Assuming 2 PM coffee sleeps off by ten.
- Chasing focus with rising doses instead of sleep.
- Ignoring half life changes in pregnancy.
- Quitting cold turkey and meeting the headache.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 mg, 10 hours, half life 5?
Two halvings: 100 x 0.25 = 25 mg left.
400 mg at once, 5 hours later?
One halving: 200 mg left, over two cups.
Questions
How long does caffeine last?
Half clears in about 5 hours, most in 10 to 15. Traces linger a full day.
How much caffeine is in coffee?
About 95 mg brewed, 65 espresso, 30 to 50 tea, 80 to 200 energy drinks.
When should I stop coffee?
Eight to ten hours before bed for sensitive sleepers. Noon cutoff suits many.
Is 400 mg a day safe?
The commonly cited adult ceiling, but pregnancy and heart conditions go lower.
Why am I tired after coffee?
Tolerance plus adenosine rebound as it wears off. Sleep debt is the usual root.
Does smoking change clearance?
Yes, smokers clear roughly twice as fast. Quitting slows it back.