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Caffeine Cutoff Calculator

How a last-caffeine window is estimated from bedtime, intake, and self-reported sensitivity, and why the answer has to be a wide range rather than a time.

Open the tool
The tool

See the width of the answer

Tell the calculator what you drank, when, and when you go to bed, and it draws the decay curve at the published clearance rates. Those rates differ between people by more than six-fold, so what comes back is two times rather than one, and the gap between them is the honest part.

This produces a range, never a single time, and the range is wide on purpose. It cannot tell you that caffeine has cleared your system and it will never claim to, because no web form can measure that. Treat the output as the starting point for your own fortnight-long experiment rather than a personalised medical calculation.

The form below runs in your browser, so it needs JavaScript to work. The published method further down this page does not need one, and it is the part worth reading first.

The drink

Pick one to fill in a starting figure, or type the milligrams yourself. The ranges are the FDA’s own, and a cup of coffee spanning 113 to 247 milligrams is exactly why a cup is not a unit of anything.

The night

The bedtime you keep, rather than the one you would like to keep.

Optional, and used for one thing: to say whether the day has gone past the FDA’s general-health reference figure. It is not a sleep figure.

You

Two of these change what the calculator is willing to say at all. The third changes only which end of the range it reads out first.

How strongly caffeine affects you

Your own read. Nobody has published a way to turn this into a clearance rate, so it changes the emphasis and leaves the range exactly the same width.

Caffeine clears more slowly in pregnancy, so the slow end of every figure moves further out.

Tick this and the calculator produces no window. That instruction was given with your history in front of someone; this page has none of it.

Where what you type goes

Nowhere. Everything above runs in this browser, on this device. Restadian never receives what you enter, because nothing here sends it anywhere.

What this tool will not tell you, and why

  • A caffeine dose below which sleep is unaffected. The one controlled cutoff figure available tested a single dose at a single interval. Scaling it to other doses would be arithmetic dressed up as evidence.Drake et al., Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed
  • How self-reported caffeine sensitivity maps onto a clearance rate. Both sources say the spread between people is large and name some of the reasons: genetics, smoking, pregnancy, liver function, medication. Neither turns "I notice it" into a number of hours. So sensitivity changes which end of the published range the calculator talks about first, and changes nothing about the range itself: the window you are shown is the same width whichever answer you give. That is checked by an assertion in the code, not left to good intentions.Institute of Medicine, Caffeine for the Sustainment of Mental Task Performance, ch. 2 · Evans, Richards & Battisti, Caffeine, StatPearls

Every number above, and where it came from

Each figure this tool uses is registered in the code with the sentence it was taken from. Open one and you can check the number against the source without taking our word for it.

  1. Institute of Medicine, Caffeine for the Sustainment of Mental Task Performance, ch. 2National Academies Press · 2001
    The sentence the figure came from
    caffeine's elimination half-life may range between 1.5 and 9.5 hours … The mean half-life of caffeine in plasma of healthy individuals is about 5 hours … both innate individual variation, and a variety of physiological and environmental characteristics that influence caffeine metabolism (e.g., pregnancy, obesity, use of oral contraceptives, smoking, altitude).
  2. Evans, Richards & Battisti, Caffeine, StatPearlsNCBI Bookshelf · 2024
    The sentence the figure came from
    the half-life of caffeine in the average adult is approximately 5 hours … In smokers, the half-life is reduced by up to 50% compared to nonsmokers … pregnant patients, especially in the final trimester, may experience a prolonged half-life of up to 15 hours … patients with liver disease or those taking cytochrome inhibitors will experience prolonged half-lives due to reduced enzyme activity.
  3. Drake et al., Caffeine effects on sleep taken 0, 3, or 6 hours before going to bedJournal of Clinical Sleep Medicine · 2013
    The sentence the figure came from
    The magnitude of reduction in total sleep time suggests that caffeine taken 6 hours before bedtime has important disruptive effects on sleep and provides empirical support for sleep hygiene recommendations to refrain from substantial caffeine use for a minimum of 6 hours prior to bedtime.
  4. Spilling the Beans: How Much Caffeine is Too Much?U.S. Food and Drug Administration · 2024
    The sentence the figure came from
    For most adults, the FDA has cited 400 milligrams a day — that's about two to three 12-fluid-ounce cups of coffee — as an amount not generally associated with negative effects. However, there is wide variation in both how sensitive people are to the effects of caffeine and how fast they eliminate it from the body … Typical Caffeine Content in 12-fluid-ounce Drinks … caffeinated soft drink 23 to 83 milligrams … green tea 37 milligrams … black tea 71 milligrams … coffee, regular brewed non-specialty 113 to 247 milligrams … energy drink 41 to 246 milligrams … decaf coffee typically has 2 to 15 milligrams in an 8-fluid-ounce cup … If you are concerned about a condition or medication, or if you're pregnant, trying to become pregnant, or breastfeeding, we recommend talking to your health care provider about whether you need to limit caffeine consumption.
The method

What it measures, what it assumes, and where that breaks down

What it estimates

Two times rather than one: when a quarter and an eighth of what you drank would be left if you clear caffeine at the fast end of the published range, and when they would be left if you clear it at the slow end. The gap between those two times is the answer, and its width is the honest part.

Note the shape of that claim. It is arithmetic on a decay curve drawn at published clearance rates, not a statement about your bloodstream at a given hour.

What it asks you for

  • Your usual bedtime.
  • Roughly what you drink, how much, and when.
  • Your own read on how strongly caffeine affects you: whether an afternoon coffee is something you notice at night, or something you have never thought about.

That last input does less work than you might expect, and the calculator is explicit about it. Nobody has published a way to turn “I notice it” into a clearance rate, so your answer changes which end of the range the summary reads out first and changes nothing about the range itself. The window you are shown is the same width whichever answer you give.

What it gives back

A window rather than a time, with its width on the page rather than hidden, and the reason the width is what it is next to it rather than in a footnote.

It also shows what a smaller drink and an earlier one would do, so you can see whether the useful lever in your case is the amount, the hour, or neither.

The assumptions it rests on

  1. Caffeine clearance follows a broadly predictable decay, with a highly variable rate. The shape of the curve is well described. The rate for any individual is not, and that variability is the dominant source of uncertainty in the output.
  2. Nothing is fitted, scaled or interpolated. The only arithmetic is exponential decay at a published half-life. Where a figure would have to be invented to go further, the calculator stops.
  3. A wide honest range beats a narrow false one. Where the method is uncertain, the range widens rather than the tool picking a midpoint and presenting it confidently.

Where the numbers come from, and where they stop

The half-life range is the Institute of Medicine’s: one and a half to nine and a half hours in healthy adults, mean about five. That is a more than six-fold spread between one person and the next, and it is the entire reason this tool cannot answer with a time. The pregnancy figure of up to fifteen hours in the final trimester is StatPearls’. The typical amounts in drinks are the FDA’s, and their ranges are printed next to each one, because brewed coffee spans 113 to 247 milligrams per twelve ounces and a cup is not a unit of anything.

The one directly measured pre-bed interval is Drake’s: four hundred milligrams six hours before bed measurably shortened sleep. Six hours was the longest gap that study tested, so the calculator treats it as a floor rather than a safe distance, and says so.

Two figures are missing on purpose. Nobody has published a dose below which sleep is unaffected, and nobody has published a mapping from self-reported sensitivity to a clearance rate. Scaling the one measured figure to other doses would be arithmetic dressed up as evidence.

What it does not do

  • It does not guarantee, state, or imply that caffeine has cleared your system by any particular time. There is no field in the estimate that could hold such a claim, and the refusal leads every result.
  • It does not produce a single “cutoff time” presented as a fact.
  • It does not model your medications or your health conditions. Several medications meaningfully change caffeine metabolism, and a general calculator cannot see them. Tell it you are pregnant and the slow end moves out; tell it a clinician has restricted your caffeine and it draws no window at all.
  • It does not advise you to increase your intake.

How to use it honestly

Take the window, hold it for two weeks, and note what changes. If nothing does, caffeine timing was probably not your constraint, which is a useful result, because it means you can stop adjusting it and look somewhere else.

The calculator cannot distinguish between caffeine keeping you awake and something else keeping you awake while you happen to have had coffee. Only the experiment can do that.