☕ How long does caffeine stay in your system?
← Health & FitnessA coffee is not a switch that goes off when you stop tasting it. Caffeine leaves the body by halves — half of what is left goes every few hours — so an afternoon one is still there at midnight. Put in what you drank and when, and see the curve: what is left now, when half of it has gone, and how much is still in the body at bedtime.
What is left
Hour by hour
| Time | Left in your body | Share of the dose |
|---|
If you had it at another time
| Time | When | Left at bedtime |
|---|
An estimate, and not medical advice. The peak is put at 45 minutes and the decay is counted from there, while in real life the level tops out somewhere between half an hour and two hours after the first sip. The dose is the average in a drink of that size rather than the one in your cup. The clock is read the shorter way round up to twelve hours either side, so a bedtime more than twelve hours after the drink is read as the evening before. Everything else follows from the half-life, which is the one figure here that belongs to you.
Why the curve matters more than the number
Most of what people believe about caffeine is about the first half hour: the lift, the alertness, the reason the coffee gets made at all. What decides whether you sleep tonight happens afterwards, and it is slower than it feels. Caffeine is cleared by first-order kinetics, which is a fussy way of saying the body removes a fixed share of whatever is left rather than a fixed amount — so every half-life, the level in the blood halves.
A single espresso leaves about 145 mg in the blood at 3:45 pm, a cup of coffee about half that. At a five-hour half-life there is still 72 mg at 8:45 pm and 36 mg at 1:45 am, which is the arithmetic that surprises people: not the number of coffees but the length of the tail behind them. Half of an afternoon coffee is still there when the washing-up is done, and a quarter of it is there as you are getting into bed.
This page puts that tail on the screen — the amount now, the hours to the half, the point where a quarter is left, and what is still in the body at the time you actually go to bed.
How it's worked out
One sentence of arithmetic does all of it:
what is left = the dose, halved once for every half-life since the peak
- The dose is the milligrams in a drink times how many of them — two espressos is 290 mg, not 145.
- The peak is put 45 minutes after the first sip, which is when caffeine has been absorbed. From there the level only falls, and the page counts down until 10 mg is left, which is about a bar of chocolate and the point where there is nothing useful left to say.
- Everything is in halves: half gone one half-life after the peak, a quarter at two, an eighth at three. That is why the same drink is never simply "still in the system" — it is always at some fraction of where it started.
- The half-life is the reader's to choose, because it belongs to the person rather than the drink: three to seven hours in healthy adults, five in the middle. The table below sets out what moves it.
- Both times are read as today's clock, and the gap between them the shorter way round, up to twelve hours. A bedtime after midnight is therefore seven hours after an 11 pm coffee rather than seventeen.
What is in the drinks
These are the average caffeine contents at the serving sizes the Australian figures are published in, which is why the cup is 250 mL and the cola can is 375 mL. A café flat white or latte is usually two shots of espresso, so count it as two.
| Drink | Serving | Caffeine |
|---|---|---|
| Espresso | 50 mL, one shot | 145 mg |
| Coffee or energy drink | 250 mL | 80 mg |
| Instant coffee | 1 teaspoon in 250 mL | 80 mg |
| Black tea | 220 mL cup | 50 mg |
| Cola | 375 mL can | 36 mg |
| Milk chocolate | 50 g bar | 10 mg |
Energy drinks vary enormously — the FDA puts them between 54 and 328 mg per 16 fluid ounces — so the figure printed on the can is always the one worth using. Decaffeinated coffee is not caffeine-free either, though it holds a small fraction of what the regular kind does.
The daily figure quoted most often is 400 mg for adults, which the FDA describes as about two to three 12-ounce cups of coffee and as an amount not generally associated with negative effects. The average Australian intake is around 3 mg per kilogram of body weight a day, about 210 mg for a 70 kg person, and in pregnancy the advice is to stay at or under 200 mg a day. None of that is a personal recommendation — sensitivity to caffeine varies widely, and the FDA's own line is that what counts as too much varies from person to person.
What moves the half-life
Five hours is the middle of the range for a healthy adult, and the figure to use when there is no reason to think otherwise. These are the things that shift it:
| What | What it does |
|---|---|
| Smoking | Shortens the half-life by 30 to 50 per cent — a smoker clears caffeine faster |
| The combined contraceptive pill | Roughly doubles it |
| The last trimester of pregnancy | Prolongs it |
| Liver enzyme function, partly genetic | Sets the baseline, and some medicines block the enzyme |
| The antidepressant fluvoxamine | Sends it from about 4.9 hours to about 56 hours |
| Newborns | 80 hours or more, falling towards the adult figure within about six months |
That is the whole reason this page asks instead of deciding for you. The difference between the two ends of the range is not a rounding error: a 145 mg espresso is down to 10 mg in about twelve hours at a three-hour half-life, and in about twenty-eight hours at seven — the difference between an afternoon coffee that is gone by midnight and one that is still measurable at breakfast the next day.
Where these numbers come from
- The caffeine contents and serving sizes are the figures Food Standards Australia and New Zealand publishes, as set out by healthdirect and the Alcohol and Drug Foundation, which is also where the 3 mg per kilogram average intake comes from.
- The three-to-seven-hour half-life, the 45-minute absorption, the fact that caffeine is cleared by first-order kinetics, and the effects of smoking, the contraceptive pill, pregnancy, fluvoxamine and newborn livers all sit together in the pharmacokinetics of this reference on caffeine.
- The 400 mg a day figure for adults, the range for energy drinks, and the warning that sensitivity varies from person to person are the FDA's.
- The 200 mg a day figure in pregnancy comes from the Better Health Channel, which is published by the Victorian Department of Health.
None of it is medical advice, and the arithmetic is this site's own. If caffeine is affecting your sleep, your heart or your mood, that is a conversation with a doctor rather than with a calculator.
Frequently asked questions
How accurate is this?
The shape is right and the last milligram is not. Caffeine really does leave the body by halves, and the amounts in the drinks are the published averages — but the dose in your cup, the time you finish it and the speed of your own liver are all estimates. The curve is worth reading for the shape: a coffee at 3 pm is half gone by about 8:45 pm, and that is true whatever the exact milligrams were.
Which half-life should I pick?
Five hours, unless you have a reason to think otherwise. It is the middle of the three-to-seven-hour range for healthy adults. A reason to think otherwise would be smoking (faster), the combined contraceptive pill or the later stages of pregnancy (slower), or a medicine that interferes with the liver enzyme that clears caffeine. The difference is large: at three hours a 145 mg espresso is down to 10 mg in twelve hours, and at seven hours it takes twenty-eight.
Will this keep me awake?
Only your own nights can answer that, and the page does not pretend otherwise. What it can say is what is still in the body: half of a 3 pm espresso is there at 8:45 pm and a quarter of it at 1:45 am. Argue with those two figures rather than with the cup. If the answer matters to you, the sleep calculator works out what a night is meant to add up to in the first place.
How many shots is a café coffee?
Usually two, which is why the milk-based coffees are the ones that catch people out — a flat white or latte is commonly a double shot, so put how many at 2 with espresso selected and the page counts 290 mg rather than 145. Change the milligrams box instead if you know your café pulls its shots differently.
Does tea count? Chocolate?
Both, and both are on the list: a black tea is about 50 mg and a 50 g bar of milk chocolate about 10 mg. A chocolate bar will not keep you awake, which is why the curve stops at 10 mg, but a strong tea at 9 pm is a third of a coffee at 9 pm and the page will show you that.
Why does it ask for my bedtime?
Because "how long does caffeine last" is really the question "will it still be there when I want to sleep". Everything before that — the peak, the half, the quarter — is the same curve drawn at different points; bedtime is the one point on it that is about you. The table of times either side is there because moving the drink by an hour changes the answer more than most people expect.
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