Let's say I know how much caffeine I drank and when. Just knowing my sex, weight (and maybe height), what is the best estimate for how much caffeine is in my system now?

For example: Suppose I drank 180 mg of caffeine 5 hours ago, another 180 mg of caffeine 2 hours ago, I'm female and weigh 140 pounds.

I'm guessing food intake may also have a lot to do with it, but let's ignore that. Is a good estimate available?

Motivation: I am logging my blood pressure and want to know if it's correlated with the amount of caffeine in my blood. Rather than just write down how much caffeine I had that day, I wondered if there might be a better measure of how much caffeine I have at the time of measurement.

  • I am new to the site; I hope the question is appropriate and let me know if there are things I can do to make it more so. Jun 26, 2017 at 16:54
  • It's not just about the absorption & half life, because tolerance develops so that blood pressure is unaffected after about 4 days: health.stackexchange.com/a/16996/809
    – Jonathan
    Jul 27, 2018 at 18:28

2 Answers 2


Caffeine is readily absorbed, with up to 99% absorbed within 45 minutes of ingestion.[1]

The mean half-life in healthy individuals is 5 hours, although this varies widely between individuals, ranging from 1.5 to 9.5 hours. For our purposes, we'll use the mean of 5 hours.[1]

Knowing the above plus how much caffeine you've ingested will allow you to plot caffeine levels over time. Given the wide variation in half-life, I'm going to ignore the 99% absorption and treat it as 100%. Using your example we get:

  • The 180 mg you ingested 5 hours ago is now down to 90 mg.
  • The 180 mg you ingested 2 hours ago is now down to 136 mg.
  • Therefore, there are now 90 + 136 = 226 mg in your system.

Clearly that's only a ballpark figure given the wide variance of half-life times, but it's probably as close as you can get without more rigorous measuring techniques.

  • Thank you -- directly answered the quesiton, and I learned a lot of things from reading some of the referenced pdf. Jun 27, 2017 at 0:58
  • 1
    But I calculate the amount after 2 hours would be 136mg, not 144mg (180 * 2^(-2/5) = 136). Am I right? Jun 27, 2017 at 1:03
  • 2
    You're the one with 31K rep on Mathematics, so yeah, you're right. :-) Edited accordingly.
    – Carey Gregory
    Jun 27, 2017 at 3:55

This study correlated blood pressure and caffeine, and it showed that the body regulated after 4 days of consumption, therefore if you goal is to measure blood pressure, keep in mind tolerance

Acute caffeine in subjects who do not normally ingest methylxanthines leads to increases in blood pressure, heart rate, plasma epinephrine, plasma norepinephrine, plasma renin activity, and urinary catecholamines. Using a double-blind design, the effects of chronic caffeine administration on these same variables were assessed. Near complete tolerance, in terms of both humoral and hemodynamic variables, developed over the first 1-4 d of caffeine. No long-term effects of caffeine on blood pressure, heart rate, plasma renin activity, plasma catecholamines, or urinary catecholamines could be demonstrated. Discontinuation of caffeine ingestion after 7 d of administration did not result in a detectable withdrawal phenomenon relating to any of the variables assessed.

  • Great info, thanks! What I'm wondering now is if there are then health problems related to taking caffeine occasionally, instead of daily, to avoid any dependence. Maybe that does raise blood pressure. Jul 27, 2018 at 18:48
  • From my understanding the biggest is just potential withdrawal, but I'm not sure. There's probably many health benefits as well.
    – Jonathan
    Jul 27, 2018 at 18:50
  • See here: health.stackexchange.com/questions/1452/…
    – Jonathan
    Jul 27, 2018 at 18:50

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