How to Write Down Blood Pressure Numbers Correctly

Blood pressure is written as two numbers separated by a forward slash, with the higher number (systolic) on the left and the lower number (diastolic) on the right, followed by the unit “mmHg.” A reading of 120/80 mmHg, for example, means the pressure in your arteries is 120 millimeters of mercury when your heart contracts and 80 when it relaxes. That notation looks simple, but the details around recording it correctly matter more than most people realize, and small mistakes in how numbers get written down can change medical decisions.

The Basic Format and What Each Number Represents

The standard way to write a blood pressure reading is systolic over diastolic. The systolic number always comes first, and it is always the larger of the two. You write them separated by a slash: 118/76 mmHg. The “mmHg” stands for millimeters of mercury, a unit that dates back to the mercury column manometers once used in every doctor’s office. Even though most modern devices are digital, the unit stuck.

When you take a reading at home or receive one from a clinician, the format stays the same. If your monitor displays 134 and 82, you write 134/82 mmHg. Do not reverse them. Do not round them. Do not add a zero to make a number look tidier. If the device says 117/79, that is exactly what goes in your log. The precision matters, and the rounding habit in particular is a well-documented problem worth understanding.

Why You Should Never Round Your Numbers

One of the most common mistakes in blood pressure recording is rounding to the nearest zero. Someone sees 118/82 and writes 120/80. It feels harmless, but this tendency has a name in research: terminal digit preference. And it has real consequences.

In a large New Zealand study, about a third of systolic blood pressure values recorded in clinical settings ended in zero, far more than the 10% you would expect by chance. That rounding led to measurable misclassification of cardiovascular risk, with roughly 3 to 4 percent of men incorrectly placed into higher risk categories and similar rates of misclassification in women.¹ A separate study from a primary care database found that zero end-digits appeared in about 64% of systolic readings and 62% of diastolic readings. Under certain treatment guidelines, this rounding misclassified as many as one in 12 patients, mostly pushing them toward being prescribed medication they may not have needed.²

Even in clinical screening programs, the problem runs deep. One industrial screening study found that nearly 79% of systolic and 74% of diastolic readings were digit-preferenced, and the doctors doing the screening were actually least likely to round near diagnostic cutoff points like 150 systolic or 90 diastolic, precisely where rounding would be most visible. Away from those thresholds, rounding was rampant.³ The takeaway for you at home is straightforward: write exactly what your device displays. If it says 137/84, record 137/84. Leave the rounding to nobody.

How Small Errors Change Medical Decisions

You might wonder whether a few millimeters of mercury really matter. They do, and the math is striking. An overestimate of diastolic blood pressure by just 5 mmHg can more than double the number of people whose readings cross a diagnostic threshold. Specifically, overestimating diastolic pressure by 5 mmHg increases the count of patients exceeding 90 mmHg diastolic by 132%. On the flip side, underestimating by the same amount causes about 62% of genuinely hypertensive patients to be missed entirely.⁴

For systolic pressure, the stakes are similar. Overestimating by just 3 mmHg increases the number classified as hypertensive by about a quarter. Bump that error to 5 mmHg and the increase reaches 43%. These are not exotic scenarios. A person who rounds 137 up to 140 each time, or who writes 128 as 130, is introducing exactly this kind of drift into their records. Over weeks or months of home monitoring, that systematic nudge can make the difference between a doctor recommending lifestyle changes alone versus starting a prescription.

Recording Multiple Readings the Right Way

If you have been told to monitor your blood pressure at home, you are almost certainly taking more than one reading per session. Guidelines generally recommend taking two or three consecutive measurements, and how you record them matters as much as how you take them.

Write down each individual reading, not just the average. Your log should show each separate measurement along with the date and time. If you take two morning readings and two evening readings, that is four entries for the day, each with its own systolic/diastolic pair. Your doctor or nurse will calculate the averages from the raw numbers. Giving them only the average hides useful information, like whether your first reading is consistently higher than your second, which is common and clinically meaningful.

Research on home monitoring reliability suggests that taking two morning and two evening readings over at least three days provides a reliable estimate of your typical blood pressure and is sufficient for diagnosing out-of-clinic hypertension.⁵ The first reading in a series tends to run higher than subsequent ones. A pediatric study illustrates this pattern well: hypertension prevalence was highest when only the initial reading was used and lowest when all readings in a visit were averaged together.⁶ That drop from first to third reading is exactly why recording every number individually, rather than cherry-picking or averaging in your head, gives the clearest clinical picture.

Between readings, wait at least one minute. A study comparing intervals found that a one-minute gap between home blood pressure readings produced values that agreed more closely with daytime ambulatory blood pressure averages than readings taken only ten seconds apart.⁷

What Else to Record Besides the Numbers

The two blood pressure numbers alone do not tell the whole story. Context makes a reading interpretable, and a bare entry of “130/85” on a scrap of paper is less useful than it could be. At minimum, your log should include the date, the time of day, and which arm you used. If your doctor has asked you to use a specific arm, stick with that arm consistently. Switching arms without noting it introduces variability that muddies trends.

Some other details worth logging:

Body position: Were you sitting, standing, or lying down? Most home readings should be taken seated with feet flat on the floor, but if your doctor asks for standing readings too, note the position.
Recent activity: If you just climbed stairs, drank coffee, or exercised within 30 minutes, a quick note helps explain an unexpectedly high reading.
Medication timing: Whether you took the reading before or after your blood pressure medication can shift the numbers considerably. Note which it was.
Symptoms: If you felt dizzy, had a headache, or felt unusual at the time of the reading, write that down too.

This kind of contextual information is something researchers and clinicians increasingly recognize as critical. A review in digital health found that blood pressure data pulled from electronic health records is often complicated by variability in measurement methods and clinical contexts, producing readings that look similar on paper but are clinically distinct.⁸ Your home log is a miniature version of the same problem. A reading of 145/92 taken after rushing to sit down is different from 145/92 taken after five minutes of quiet rest. Without the context, both look the same.

Understanding the Categories Your Numbers Fall Into

Once you write down your numbers, it helps to know what ranges they fall into. The American Heart Association and American College of Cardiology updated their blood pressure guideline in 2025, which replaced the previous 2017 version.⁹ The general categories, which have been in use since the 2017 guideline, break down like this:

Normal: Systolic below 120 and diastolic below 80.
Elevated: Systolic 120–129 and diastolic below 80.
Stage 1 hypertension: Systolic 130–139 or diastolic 80–89.
Stage 2 hypertension: Systolic 140 or higher, or diastolic 90 or higher.
Hypertensive crisis: Systolic above 180 and/or diastolic above 120, requiring immediate medical attention.

Note the word “or” in the hypertension stages. If your systolic is 136 but your diastolic is 74, you are still in Stage 1 hypertension territory based on the systolic number alone. You classify based on whichever number falls in the higher category. This is a common point of confusion: people see one “normal” number and assume everything is fine, missing that the other number puts them in a higher bracket.

When recording your readings at home, you do not need to classify them yourself. Just write the numbers accurately and let your clinician assess the pattern over time. But understanding the categories helps you recognize when a reading warrants a phone call rather than a wait-until-the-next-appointment approach.

Home Logs Versus What Happens in a Clinic

There is a meaningful difference between blood pressure recorded at home and blood pressure recorded in a medical setting. Many people run higher in a clinical environment due to the stress of the visit, a phenomenon often called white-coat hypertension. Others actually run higher at home, a pattern called masked hypertension. Neither is detectable without comparing the two sets of numbers.

For home monitoring, the standard approach is a validated automated cuff. You sit quietly for five minutes, take two or three readings a minute apart, and record all of them. These monitors display exact numbers, so rounding should not be an issue as long as you write what the screen shows. If your device stores readings in memory, you can cross-check your handwritten log against the device at each visit. Some monitors sync to a phone app, which removes the handwriting step entirely, though it is still worth understanding the format so you can spot errors.

In a clinical setting, automated office blood pressure measurement is becoming more common. The device takes multiple readings while you sit alone in the room, which reduces the white-coat effect. But even automated clinical measurement has room for error. A review of automated office blood pressure procedures found persistent problems with clinicians skipping necessary steps, such as ensuring the patient is seated properly or waiting the recommended rest period before starting. The authors noted that there is still a need for retraining programs and better monitoring of assessment procedures to reduce these human errors.¹⁰ If you notice your clinician taking your blood pressure hurriedly, it is fair to ask whether you can sit quietly for a few minutes first.

Derived Numbers You Might See

Beyond the standard systolic/diastolic pair, you may encounter two related numbers in medical records or health apps: pulse pressure and mean arterial pressure.

Pulse pressure is simply systolic minus diastolic. If your reading is 130/80, your pulse pressure is 50. A wide pulse pressure, generally above 60, can be a sign of arterial stiffness and is sometimes tracked separately, especially in older adults. You can calculate it yourself, but it is not something you typically need to record at home unless your doctor asks for it.

Mean arterial pressure (MAP) is a weighted average that estimates the overall pressure in your arteries during a full cardiac cycle. The most commonly used formula, developed by Gauer in 1960, estimates it as the diastolic pressure plus one-third of the pulse pressure.¹¹ For a reading of 120/80, that gives you 80 + (1/3 × 40) = roughly 93 mmHg. Researchers have developed more precise formulas that factor in heart rate, and newer versions outperform the classic calculation in accuracy.¹² MAP is used mainly in hospitals and intensive care, not in routine home monitoring. But if you see it on a discharge summary or in a health app, now you know what it is and where it comes from.

Blood Pressure Recording for Children

Recording blood pressure for children follows the same slash notation, but interpreting the numbers is completely different. Unlike adults, where fixed thresholds define hypertension, children’s blood pressure is evaluated against percentile tables that account for age, sex, and height. A reading of 110/70 might be perfectly normal for a 12-year-old but elevated for a 5-year-old.

Clinical decision support tools have been developed to streamline this process, drawing on blood pressure distribution data from hundreds of thousands of children across different age groups to establish appropriate percentiles for each subgroup.¹³ If you are recording blood pressure for a child at home, the format stays the same: write down the exact systolic/diastolic pair, the date, the time, and which arm was used. Leave the percentile interpretation to the pediatrician, who will plot the number against the right reference table.

Ambulatory Monitoring and the 24-Hour Record

If your doctor orders 24-hour ambulatory blood pressure monitoring, you will wear a cuff that inflates automatically throughout the day and night, typically every 15 to 30 minutes during waking hours and every 30 to 60 minutes while you sleep. The device stores all readings automatically, so your main job is keeping a diary of activities, sleep and wake times, and any symptoms you notice when the cuff inflates.

The Italian Society of Hypertension has published detailed standards for implementing and interpreting ambulatory monitoring, emphasizing that the technique is a reference tool for accurately diagnosing hypertension and that standardized processes are essential for its proper management.¹⁴ The diary you keep alongside the automated readings serves the same purpose as the contextual notes in your home log: it helps your doctor distinguish a genuine spike from a reading that happened while you were climbing stairs or arguing on the phone.

One practical tip during ambulatory monitoring: when the cuff inflates, stop moving and let your arm hang loosely at your side if you are standing, or rest it on a surface if you are sitting. Movement during inflation can produce inaccurate readings that the device may flag as artifacts. If a reading seems wildly off, note the time and what you were doing. Your doctor will decide whether to include or exclude flagged readings when interpreting the overall pattern.

Keeping a Log That Actually Helps Your Doctor

A common frustration in clinical practice is patients who bring in blood pressure logs that are difficult to use. Scattered sticky notes, a single averaged number for a whole week, or entries that omit dates and times all reduce the value of home monitoring. The format does not need to be fancy, but it does need to be consistent.

A simple table works well, whether on paper or in a spreadsheet. Columns for date, time, systolic, diastolic, heart rate (if your monitor shows it), arm used, and a notes column for anything unusual. Each row is one reading. If you take two readings a minute apart, that is two rows. This format lets your doctor scan for trends at a glance: morning versus evening patterns, day-to-day variability, and whether your first reading is consistently higher than your second.

Some people prefer apps that connect directly to their monitor via Bluetooth, which eliminates transcription errors entirely. If you go that route, make sure the app exports data in a format your clinic can use, or at least brings up a readable screen you can show at your appointment. The best log is whatever format you will actually fill in consistently. A beautifully designed paper chart that sits empty in a drawer is worth less than a messy notebook that gets used every morning.