Guide
What is a caffeine half-life calculator?
A caffeine half-life calculator estimates how much caffeine is still in your body at any moment. You enter what you drank and when, and the tool applies the half-life of caffeine, the time your body needs to clear half of it, to every drink. Instead of guessing how much of your afternoon coffee is left at bedtime, you get a number, a decay chart and an hour-by-hour timeline.
CaffeineLeft puts the whole calculation on one page, with a metabolism setting you can adjust to fit yourself. It runs entirely in your browser, needs no account, and never sends your entries anywhere.
How to use this caffeine calculator
- Enter your body weight in pounds or kilograms. The unit you choose is applied exactly as typed.
- Set your usual bedtime. The tool estimates your caffeine level at that time.
- Pick your metabolism: fast (about 3 hours), normal (about 5 hours) or slow (about 8 hours). Tick Pregnant or Smoker if either applies, because both change the half-life used.
- Add each drink. Search the list of common coffees, teas, sodas, energy drinks and supplements, or choose Custom amount and type the milligrams. Then set the servings and the time you had it.
- Press Calculate. You will see caffeine in your body now, caffeine at bedtime, your total for the day, a decay chart and a timeline.
The half life of caffeine, explained
Half-life is the time it takes for the amount of a substance in your body to fall by half. If you drink 200 mg of caffeine and your half-life is 5 hours, roughly 100 mg remains after 5 hours, about 50 mg after 10 hours and about 25 mg after 15 hours. Each half-life removes the same fraction, which is why the curve drops quickly at first and then flattens.
Many sources quote an average half life of caffeine of about 5 hours in healthy adults, but individual values range widely. You will also see it written without the hyphen, as "half life". It is the same idea, and the calculator uses the same maths either way.
This tool also models absorption. Caffeine does not arrive all at once, so the estimate rises for roughly the first hour after a drink and then declines. That makes the "in your body now" number more realistic soon after you drink.
Caffeine calculator by weight and age
Body weight does not change how fast you clear caffeine, but it changes how a given dose compares with your size. Here your weight is used only to flag when your estimated peak approaches 6 mg per kilogram of body weight. It does not change how fast caffeine leaves your body.
Age matters in a different way. Newborns clear caffeine very slowly, and some studies suggest clearance can change in later life, but the evidence is mixed. Age alone is a weak predictor compared with genetics, smoking, pregnancy and medication, so this calculator has no age box. If you think you clear caffeine more slowly, choose the slow profile instead.
Caffeine metabolism calculator: what changes your half-life
Caffeine is broken down mainly in the liver by the enzyme CYP1A2, and several things shift how quickly that happens.
- Genetics: variants in the CYP1A2 gene make some people fast metabolizers and others slow.
- Smoking: regular smoking is linked with faster clearance, so the tool halves the half-life.
- Pregnancy: clearance slows markedly, so the tool uses 11.5 hours.
- Other factors: hormonal contraceptives, liver conditions and some medicines can slow clearance too.
Because you cannot measure this at home, the half-life is something you choose, not something the tool can know. Read what changes your caffeine half-life.
How long does 75 mg of caffeine last?
The answer depends on what you mean by "last". Using a 5-hour half-life, a single 75 mg drink peaks at roughly 67 mg after about 50 minutes, falls to about 39 mg after 5 hours, and drops below 10 mg after roughly 15 hours. How long you feel it varies from person to person, so these figures describe how much remains in the body, not how alert you will be.
With a fast 3-hour half-life, 75 mg falls below 10 mg in about 9 hours, and with a slow 8-hour half-life it takes about 24 hours. Try your own drinks in the calculator, or see the hour-by-hour table for 75 mg and other doses in our guide to how long caffeine lasts.
Using the caffeine intake calculator across your day
Add every drink, not just the biggest one. If you are unsure of an amount, check the caffeine in common drinks list. The tool adds each drink's contribution to give a combined level, then shows your total for the day as a share of general daily guidance. Health agencies publish reference points such as about 400 mg a day for most healthy adults and about 200 mg during pregnancy. These are population-level guidance, not a personal limit or a promise of safety.
The bedtime section shows your estimated level at the time you entered, the drink that contributes most, and the latest time a typical 95 mg cup would still keep your estimate under 50 mg through the night. The 50 mg marker is a round reference line chosen for this tool, not a clinical threshold, and sensitivity varies.
How accurate is a half life of caffeine calculator?
Any caffeine half-life calculator is an estimate. It assumes steady, first-order clearance and ignores tolerance, food, hydration and differences in the real caffeine content of your drink. Treat the results as a way to plan and compare, not as medical advice. If you are pregnant, take medication or have a health condition, ask a doctor or pharmacist. See the formula and the disclaimer for details.