Caffeine calculator
Pick a drink and a count. Limits come from Health Canada, not from a general rule of thumb.
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How it clears
Working with a different rate? Edit the rates this calculator uses
These are the figures we verified on 2026-08-01. Change them to work with a historic period, or with a new rate that has been announced but which we have not published yet.
Five hours is an average across a very wide range. Fast metabolisers clear it in about two hours; pregnancy and some oral contraceptives can push it past ten. If you know yours, use it.
How much caffeine is safe per day?
Health Canada puts the limit for healthy adults at 400 mg a day, about three 237 mL cups of brewed coffee. For anyone pregnant or who may become pregnant it is 300 mg. For pre-adolescent children it is 2.5 mg per kilogram of body weight, which works out to 45 mg for ages 4 to 6, 62 mg for 7 to 9 and 85 mg for 10 to 12.
The limits here are Canadian
Most caffeine calculators quote an American or generic figure. Health Canada publishes its own guidance, and it is more specific than a single number.
The adult figure of 400 mg a day is expressed as 6.5 mg per kilogram of body weight, which matters if you are much lighter or heavier than average. The pregnancy figure of 300 mg applies to anyone who may become pregnant, not only those already pregnant. And the children's figures are per kilogram rather than fixed, with published equivalents for each age band.
The tool applies whichever of those matches your situation rather than defaulting to the adult number.
Half-life is why afternoon coffee shows up at midnight
Caffeine has a half-life of roughly five hours in a healthy adult. That means half of a dose is still circulating five hours later, a quarter after ten hours and an eighth after fifteen.
A 200 mg coffee at 3 pm leaves about 100 mg at 8 pm and 50 mg at 1 am — comparable to drinking half a cup of coffee as you go to bed. That is the arithmetic behind almost every complaint about caffeine and sleep.
Five hours is a midpoint. The real range is roughly three to seven hours, and pregnancy can stretch it past fifteen.
How it is calculated
A sum against a limit, then exponential decay.
daily total = Σ (servings × mg per serving)per kg = daily total ÷ body weightremaining after t hours:amount = dose × 0.5^(t ÷ half-life)half-life ≈ 5 hours in a healthy adultHealth Canada limits:adults 400 mg/day (6.5 mg/kg)pregnancy 300 mg/day (4.6 mg/kg)ages 4–6 45 mg/dayages 7–9 62 mg/dayages 10–12 85 mg/day
Serving figures are typical averages and vary widely in practice. A 237 mL brewed coffee averages about 135 mg, but a large takeaway cup can hold two to three times that, and the same bean brewed differently varies by half again.
Espresso trips people up in the opposite direction: a single shot carries roughly 64 mg, less than a mug of brewed coffee, because the serving is so much smaller despite being far more concentrated.
Worked example: three coffees and an afternoon cola
Three 237 mL brewed coffees through the day plus one cola, for a 70 kg adult finishing at 3 pm.
- 3 × brewed coffee at 135 mg405 mg
- 1 × cola at 36 mg36 mg
- Daily total441 mg
- Against the 400 mg adult limit110%
- Per kilogram6.3 mg/kg
- Still present at 11 pm, 8 hours later146 mg
441 mg — slightly above the Health Canada adult limit, with roughly 146 mg still circulating at bedtime.
That 146 mg is about the same as drinking a full cup of coffee as you get into bed. Dropping the last coffee, rather than the first, is what changes the sleep arithmetic.
Things that change how fast you clear it
- Pregnancy can extend the half-life to fifteen hours or more
- Oral contraceptives roughly double it
- Smoking shortens it substantially
- Liver conditions slow clearance
- Some antibiotics and antidepressants interact with caffeine metabolism
- Genetics account for a large part of the person-to-person variation
This is why two people can drink the same coffee at the same time and only one of them lies awake.
Assumptions and limits
- Uses a 5-hour half-life; the real range is about 3 to 7 hours
- Serving sizes are typical averages, not measurements of your actual drink
- Assumes a single point in time for the whole day's intake when modelling decay
- Does not account for medication interactions or medical conditions
- Energy drinks vary enormously — read the label rather than trusting an average
Terms on this page
- Half-life
- The time for half a dose to clear. About five hours for caffeine in a healthy adult, which is why an afternoon coffee affects sleep.
- mg per kilogram
- How Health Canada expresses its limits underneath the headline numbers — 6.5 mg/kg for adults, 2.5 mg/kg for pre-adolescent children.
- Tolerance
- Reduced response after regular intake. It affects how caffeine feels but not how fast your body clears it.
What has changed, and when
Rates on this page have not always been what they are today. If you are working on an earlier period, use the figure that applied at the time rather than the current one.
- Current Health Canada guidance: 400 mg a day for healthy adults, 300 mg during pregnancy, 2.5 mg per kilogram for pre-adolescent children.
- Ongoing These limits have been stable for some years. Energy drink labelling rules have changed more recently than the intake guidance itself.
Common questions
How much caffeine is too much in a day?
Health Canada sets 400 mg a day as the limit not associated with adverse effects in healthy adults, which is about three 237 mL cups of brewed coffee. The limit is 300 mg for anyone pregnant or who may become pregnant, and 2.5 mg per kilogram for pre-adolescent children.
How long does caffeine stay in your system?
It has a half-life of roughly five hours in a healthy adult, so half a dose remains after five hours and a quarter after ten. Traces persist for around a day. Pregnancy, oral contraceptives and liver conditions all lengthen that considerably.
When should I stop drinking coffee before bed?
Working backwards from the half-life, a coffee at 3 pm still leaves roughly a quarter of its caffeine in you at 1 am. Many people find cutting off eight to ten hours before bed makes a noticeable difference, though sensitivity varies a great deal.
Does espresso have more caffeine than coffee?
More per millilitre, less per serving. A single espresso shot carries around 64 mg against roughly 135 mg in a 237 mL brewed coffee, because the brewed serving is so much larger despite being weaker.
Is decaf caffeine free?
No. Decaffeinated coffee typically retains a small amount, around 3 mg per 237 mL cup against 135 mg for regular. That is negligible for most people but not literally zero, which matters if you are highly sensitive.
Where these numbers come from
- Health Canada — Caffeine in Foods The 400 mg adult limit, 300 mg pregnancy limit and 2.5 mg/kg children's limit used here.
- Government of Canada — Caffeine and pregnancy Guidance for those pregnant or who may become pregnant.
Last verified 2026-08-01 Rates on this page are checked against the sources above at least once a year, and whenever the governing authority announces a change. Spotted something out of date? Tell us and we will fix it.
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