What a cup actually holds

FDA's current caffeine figures are printed per 12 fluid ounces, which is worth knowing because most caffeine tables you will find elsewhere use 8 ounces and the numbers are not interchangeable. Per 12 fluid ounces, FDA gives regular brewed coffee 113 to 247 mg, black tea 71 mg, green tea 37 mg, a caffeinated soft drink 23 to 83 mg, and an energy drink 41 to 246 mg. The same page gives energy drinks a separate range of 54 to 328 mg per 16 fluid ounces. Decaffeinated coffee is not zero: FDA puts it at 2 to 15 mg per 8 fluid ounces.

MedlinePlus, from the National Library of Medicine, lists 95 to 200 mg for an 8 ounce cup of coffee and 14 to 60 mg for an 8 ounce cup of tea. Those numbers are not in conflict with FDA's. They are just measured against a smaller cup. This is exactly why the size column in your log matters more than the name of the drink.

Do the arithmetic once and it stays with you. A 16 ounce coffee is a third again as large as FDA's 12 ounce serving, which puts it somewhere around 151 to 329 mg. A 20 ounce coffee is two thirds again as large, roughly 188 to 412 mg. Two 12 ounce cups of ordinary home brewed coffee in a morning could plausibly land anywhere from 226 mg to almost 500 mg depending on how strong you make it.

Those ranges are wide because brewing strength, bean, grind, and machine all move the number. Your log will produce an estimate, not a laboratory value. An estimate is still far better than a guess, and it is what a health care professional can actually work with.

The caffeine you are not counting

Coffee and tea are easy to remember. The rest of it is where logs usually go wrong.

FDA notes that energy and protein bars, ice cream, chewing gum, dietary supplements, and over the counter medicines are among the products that may contain caffeine. MedlinePlus adds that synthetic caffeine is put into pain relievers, cold medicines, and nonprescription products sold for alertness. Chocolate carries caffeine naturally, and FDA points out that when the caffeine comes from an ingredient like chocolate, the word caffeine may not appear in the ingredient list at all. The chocolate is listed instead.

Nonprescription medicines deserve their own line in your notes for a second reason. The 2025 guideline lists decongestants such as phenylephrine and pseudoephedrine among substances that can raise blood pressure, alongside caffeine and several others. A combination cold product can therefore show up on that list twice. The guideline notes that roughly 20 percent of adults with high blood pressure regularly use over the counter medicines that may raise blood pressure or interfere with treatment, which makes this one of the more productive things to bring to an appointment.

Write down the product type and the label dose, not a brand. "Two headache tablets that list caffeine, around 2 p.m." is the useful entry.

A three day log that takes two minutes a day

You do not need a week. Three days captures the pattern if you choose them well: two ordinary weekdays and one weekend day, because weekend intake often looks different. If you work nights or rotating shifts, log one work day and one off day instead, and say so on the page.

Five columns cover everything that matters.

1. 1 What you drank, described plainly. Brewed coffee, espresso drink, black tea, green tea, cola, energy drink, hot chocolate. If it came from a coffee shop, note whether it was brewed coffee or an espresso based drink, because the two are not the same.
2. 2 Size in fluid ounces. Measure your usual mug once, at home, with water and a kitchen measuring cup, then write the number on a sticky note and stop guessing forever. Coffee shop sizes are usually printed on the menu or stamped on the cup.
3. 3 Time, to the nearest half hour. This is the column people skip and the one health care professionals find most useful.
4. 4 Anything else with caffeine in it that day. Chocolate, a headache tablet, a cold product, an alertness product, a caffeinated gum.
5. 5 Any home blood pressure readings you took that day, with their times.

If you share a household, a single sheet on the refrigerator works better than an app nobody opens. If you would rather use your phone, a note file with one line per drink is plenty. The format matters less than the times being real rather than reconstructed from memory hours later.

Two habits improve the record quickly. Write the entry when you pick up the drink, not after you finish it. And do not clean up the log before the appointment. A weekend with four drinks on it is information, not a confession.

Why the time column changes the conversation

Caffeine shortly before a reading is a measurement problem, and it is a large one. A 2017 systematic review of 328 studies on sources of blood pressure measurement error found that acute caffeine was associated with systolic readings 3 to 14 mm Hg higher and diastolic readings 2.1 to 13 mm Hg higher. That is why the standard measurement checklist, reflected on the American Heart Association's home monitoring page, calls for no caffeine, no smoking, and no exercise for at least 30 minutes before a reading. Mayo Clinic describes the short term rise as occurring in roughly the 30 to 120 minutes after a caffeinated drink.

Put those two facts next to a log and something practical falls out. If your morning home readings run higher than your afternoon ones, and your log shows coffee 20 minutes before every morning reading, you may be looking at a technique issue rather than a blood pressure pattern. Fixing the sequence costs nothing. Explaining the pattern to a health care professional without a log is nearly impossible.

Mayo Clinic also describes a simple way to test your own response: check your blood pressure before a caffeinated drink, then again 30 to 120 minutes afterward. A rise of about 5 to 10 points may suggest you are sensitive to caffeine's effect on blood pressure. Treat that as one observation to bring in, not a verdict. Blood pressure moves on its own throughout the day, and a single before and after comparison is not a diagnosis.

Two daily numbers, two different questions

The FDA figure of 400 mg and the guideline figure of under 300 mg often get presented as a contradiction. They are answers to different questions.

FDA's 400 mg is a general population statement about healthy adults and the amount not generally linked to negative effects. The under 300 mg figure appears in the 2025 AHA and ACC guideline's table of substances that can raise blood pressure, which is written for people whose blood pressure is already the thing being watched or treated. A general safety ceiling and a suggestion aimed at blood pressure management are simply not the same instruction.

Neither number is a personal prescription. FDA states plainly that there is wide variation in both how sensitive people are to caffeine and how quickly they clear it from the body. Your log is what turns those general figures into a conversation about you specifically.

What research says about regular caffeine and blood pressure

Here is where honesty helps more than certainty. The short term effect of caffeine on a reading is well documented. The effect of drinking caffeine regularly, week after week, is real but small on average, and smaller still when the caffeine comes from coffee.

A 2005 systematic review in the Journal of Hypertension pooled 16 randomized controlled trials with more than 1,000 participants, each lasting at least a week. Combined, regular caffeine intake was associated with systolic blood pressure about 2.04 mm Hg higher and diastolic about 0.73 mm Hg higher. When the authors separated the trials, those that gave caffeine in a measured form on its own showed a systolic rise of about 4.16 mm Hg, while the coffee trials showed about 1.22 mm Hg. Their conclusion was that regular caffeine intake raises blood pressure, but that the effect when it arrives through coffee is small.

A 2023 dose response review of randomized trials reached a similar place, reporting systolic blood pressure about 1.94 mm Hg higher (95 percent confidence interval 0.52 to 3.35) and diastolic about 1.66 mm Hg higher (0.75 to 2.57). The increases were larger at doses above 400 mg a day and in trials running longer than nine weeks, and larger in men than in women.

Mayo Clinic notes that people who drink caffeine regularly develop tolerance to it, which is one plausible reason the long term averages are so much smaller than the jump you can measure 45 minutes after a cup.

A few honest limits belong with those numbers. These trials mostly enrolled generally healthy volunteers, the averages hide people who respond much more and much less, and an average of about 2 mm Hg is small next to what the same guideline estimates for changes like sodium reduction or weight loss. None of this tells you what caffeine does to your readings. It tells you what to expect from the group, and where your own log becomes the interesting document.

What to ask, and what to mention

Bring the log itself, not a summary of it. Then a short list of questions makes the appointment go faster.

1. 1 Looking at this total, is caffeine worth changing in my case, given my readings and my other conditions?
2. 2 Do any of my medicines interact with caffeine, or does caffeine affect how they work?
3. 3 Am I timing my home readings badly? Should I move them relative to my first drink of the day?
4. 4 Is there anything in my history, such as an irregular heartbeat, anxiety, reflux, poor sleep, or pregnancy, that would make a lower figure sensible for me?
5. 5 If I do cut back, how should I do it, and over how long?

That last question matters more than it sounds. MedlinePlus lists caffeine withdrawal symptoms including headaches, drowsiness, irritability, nausea, and trouble concentrating, and notes they usually pass within a couple of days. Someone who reads their own log, feels alarmed, and quits everything on a Monday can spend the week feeling terrible and blaming the wrong thing.

Mention the rest of what you take while you are there. Tell your health care professional about every medicine, vitamin, and supplement you use, including the ones you buy without a prescription, because several of them appear on the same guideline list as caffeine.

Timing is worth raising too. MedlinePlus notes that caffeine peaks in the blood within about an hour and that people can feel its effects for four to six hours. A cup at 4 p.m. can still be working at 9 p.m., and FDA lists insomnia among the effects of too much caffeine. If your log shows late afternoon caffeine and you sleep badly, that connection is worth saying out loud.

Reading your own log before anyone else does

Add up each day before the appointment, using FDA's per 12 ounce figures and the actual sizes you wrote down. Use the middle of each range rather than the top, and note that you did. You will end up with three daily totals and a rough sense of whether you are near 150 mg, 350 mg, or 600 mg.

Then look at the shape rather than the total. Is it all before 9 a.m., or spread until evening? Is one day wildly different from the other two? Did the days with more caffeine also have poorer sleep, more jitters, or a faster heartbeat? FDA lists increased heart rate, palpitations, higher blood pressure, insomnia, anxiety, jitters, upset stomach, nausea, and headaches among the effects of consuming too much.

None of that makes a diagnosis. A blood pressure diagnosis rests on averaged readings taken properly on more than one occasion and interpreted by a health care professional, not on a caffeine tally. What the log does is replace a vague exchange with a specific one, and specific is what gets you a useful answer.