Caffeine Calculator: Half-Life, Daily Budget and Bedtime

The same shop sold a coffee with 259 mg and one with 564 mg, so this tool answers in ranges

Every other caffeine calculator hands you one confident number. That number cannot be right, because neither of the two things it is built from is a number. Your dose is a range, and so is your half-life. McCusker, Goldberger and Cone bought a 16 oz brewed coffee from the same outlet on six consecutive days and measured 564, 498, 259, 303, 300 and 307 mg (Journal of Analytical Toxicology, 2003; the authors’ own university identified the product as Starbucks Breakfast Blend from one Gainesville store, though the paper itself says only “the same coffee beverage”). Same shop, same drink, same size, and a spread of more than twofold. A calculator that takes “one coffee” and returns “95 mg” is not being precise, it is discarding the only honest part of the answer. So every drink here is a range, every half-life here is a range, and every output is a range.

What this tool is doing

All three tabs run one piece of arithmetic: caffeine leaves by first-order elimination, so a fixed fraction goes every hour rather than a fixed number of milligrams. What is left is the dose times one half, raised to hours divided by half-life. What separates a good calculator from a bad one is not that equation, it is what you feed it.

How half-life actually works

It is a half-life, not a lifetime. Each one removes half of what is left rather than half of the original dose, so the amount never reaches zero and the tail runs longer than people expect: a quarter left after two, 12.5 percent after three, and still 6.25 percent after four. A 200 mg coffee at 8am leaves roughly 13 mg in you at midnight on a four hour half-life and roughly 32 mg on a six hour one.

That is also why a bigger dose costs less extra waiting than you would guess: doubling it adds one half-life, not double the wait. In Gardiner’s own figures, 107 mg to 217.5 mg cost 4.4 extra hours, not another 8.8.

Where the 4 to 6 hours comes from

Two bodies have put a figure on it and they disagree. The Institute of Medicine’s 2001 review gives a mean of about 5 hours, range 1.5 to 9.5. The European Food Safety Authority’s 2015 opinion gives about 4 hours, range about 2 to 8. Blanchard and Sawers dosed 10 healthy men and measured 2.7 to 9.9 hours inside that one small group. The 4 to 6 hour default sits between the two authorities, and the wider 2 to 10 setting is near the outer edge both report. “3 to 7 hours” and “2 to 12 hours”, quoted all over this category, trace to no scientific source we could find.

Why your number may be off

Things known to move caffeine clearance, so you can read the estimate with the right amount of doubt. None of it is a diagnosis.

• Ordinary enzyme variation. About 95 percent of caffeine metabolism runs through one liver enzyme, CYP1A2, and people differ a lot. The consumer gene test does not capture it: 786 people showed no relevant CYP1A2 polymorphism once covariates were accounted for, 131 human livers showed no genotype effect on activity, and a 2018 meta-analysis of 3,570 people concluded the reported variant affects inducibility, mostly in smokers.
• Oral contraceptives. You will read that they double the half-life. The two studies behind that claim do not. Patwardhan and colleagues measured 10.7 hours in 9 women taking them against 6.2 in 9 who were not, and Abernethy and Todd measured 7.9 against 5.4. Call it 1.5 to 1.7 times.
• Pregnancy.Knutti and colleagues measured a mean of 8.3 hours in 57 pregnant women against 3.4 in 25 who were not, and EFSA summarizes late pregnancy as three to four times baseline. The “15 hours in the third trimester” figure traces to no study that measured a half-life.
• Liver disease. Renner and colleagues measured 13.7 hours in 15 people with cirrhosis against 3.8 in 10 healthy volunteers, with a standard deviation of 13 hours: an enormous spread.
• Smoking, the other way. Parsons and Neims measured 3.5 hours in 13 smokers against 6.0 in 13 non-smokers, and quitting reverses it. Caffeine that felt fine while you smoked can feel like too much later.
• The size of the dose. The model assumes constant clearance. In a crossover of 12 people, clearance fell and half-life lengthened at 500 mg against 250 mg, so a big single dose lingers longer than this predicts.

Tolerance changes how a dose feels, not how fast you clear it

Evans and Griffiths put 32 people on 900 mg a day for 18 consecutive days and found complete tolerance to a central nervous system effect. A later study of 16 people on 400 mg a day found tolerance on subjective but not physiological measures. On the clearance side, 10 healthy men given a week of high-dose caffeine showed no change in caffeine pharmacokinetics, and caffeine does not induce the enzyme that clears it. A heavy user feeling nothing from 200 mg is not clearing it faster, they are feeling it less while carrying the same amount, which matters most for the bedtime tab: those trials measured sleep, not how awake anyone felt. The receptor story usually told alongside tolerance is a rodent hypothesis, and the 2021 International Society of Sports Nutrition position stand says adenosine receptor changes have not been studied in humans.

What it cannot tell you

It cannot tell you how alert you will feel: blood caffeine and felt alertness are related but they are not the same curve. It cannot account for absorption, because the model assumes the whole dose lands the second you swallow it, when oral caffeine takes roughly 30 to 120 minutes to peak. And it cannot tell you about you, because it is population arithmetic applied to numbers you typed in.

On dose size: the 400 mg line is a general-population marker, not a performance target. That same 2021 position stand puts the ergogenic range at 3 to 6 mg per kg of body mass, roughly 210 to 420 mg for a 70 kg adult. An 80 mg pouch or a mug of coffee is not a performance dose.

The 80 mg row in the budget tab is Roon’s own pouch, a single number because it is a dosed label figure rather than something brewed. Its ingredient list is on the product page. The 8.8 hour pre-bed finding applies to a pouch exactly as it does to coffee.

None of this is medical advice and none of it is a diagnosis. If your sleep or your energy has been wrong for more than two weeks, that is worth raising with a clinician rather than a calculator.