What the two numbers are measuring
The top number is the pressure in the arteries at the moment the heart contracts. The bottom number is the pressure that remains between beats, while the heart is filling. Both are quoted in millimetres of mercury, a unit inherited from the mercury column that the first cuffs used, and both are read from the same inflation of the same cuff. Pulse pressure is simply the difference between them, and mean arterial pressure is a weighted average that leans towards the diastolic number because the heart spends more of each cycle filling than contracting.
None of those four numbers is a property of a person. They are properties of a moment. Blood pressure varies through the day by 20 mmHg or more in someone with nothing wrong, drops during sleep, rises on waking, and responds within seconds to posture, conversation and a stressful thought. That is why a chart placement from a single reading is an arithmetic result rather than a finding.
Why the chart differs depending on where you read it
The band edges used on this page — 120, 130, 140, 180 systolic — are the ones on the chart most commonly printed in the United States since 2017. Before that, the same organisations drew the first line at 140/90, and many national guidelines elsewhere still do, treating the 130-139 range as a monitoring zone rather than a labelled stage. Neither version is a measurement. They are decisions about where the evidence on risk and on the benefit of treatment justifies drawing a line, and reasonable bodies have drawn them in different places.
The practical consequence is that the same reading can carry a label in one country and none in another. That is a reason to treat the label as a rough locator rather than a status. What actually matters for any individual is their whole picture: age, other conditions, kidney function, cholesterol, whether they smoke, and what their readings look like averaged across days rather than taken once. Only a clinician who has that picture can say what range is being aimed for in a particular case.
Pulse pressure and the wide gap
Pulse pressure sits near 40 mmHg in most younger adults. As arteries stiffen with age the top number tends to rise while the bottom number flattens or falls, so the gap widens, and a reading such as 155/72 is a common pattern in later life. A narrow gap, under about 25 mmHg, more often reflects a measurement problem than anything else — a cuff that does not fit, or a reading taken through clothing — and is worth repeating before it is worth worrying about.
Mean arterial pressure appears here because it is the number that shows up in hospital notes and on monitors, and people who have seen it there sometimes want to reconcile it with a home reading. The formula used is the usual bedside approximation, diastolic plus a third of the pulse pressure. It is an estimate, not a measured quantity.
Getting a reading worth placing on a chart
Cuff technique moves readings more than most people expect. A cuff that is too small for the arm circumference reads high, sometimes by 10 mmHg or more, and undersized cuffs are the single most common source of a spuriously alarming home number. An arm resting below heart level reads high; an arm held up reads low. Talking during the measurement adds several points. So does a full bladder, which is easy to fix and rarely thought about.
Upper-arm monitors are generally preferred over wrist units, and a monitor that has never been checked against a clinic device is a monitor of unknown accuracy. If the numbers a home device produces are going to inform any conversation, take it to an appointment once and read against the clinic cuff. Beyond that, the useful output of home monitoring is not any one number but a log of them, which is exactly the thing a single-reading calculator cannot give you.
Questions people ask
I got 145/92 once. Does that mean I have high blood pressure?
It means that reading landed above the line the chart draws at 140/90. A single measurement is not how the condition is identified. Readings move with stress, caffeine, posture, cuff fit and time of day, and a number taken in a waiting room after rushing to an appointment is not the same as an average across a week at home. What clinicians work from is repeated readings on different occasions, often supplemented by home or 24-hour monitoring, interpreted alongside everything else they know about you. If a reading like that shows up, the useful next step is to take more of them under decent conditions and bring the log to a clinician, not to draw a conclusion from one.
Why does my home monitor read lower than the clinic?
For most people it does, typically by around five points on each number, and the charts aimed at home monitors shift their equivalent lines down to match. Part of that is the setting: clinic readings capture a moment that includes being in a clinic. There are also people whose readings are high only in a clinic, and people whose readings are high only outside one, and telling those apart is precisely why home and ambulatory monitoring exist. It is not a case of one device lying. It is two different measurements that need different reference lines, which is why this page mentions the shift rather than converting between them.
What is a normal pulse pressure?
Around 40 mmHg is the figure usually quoted for younger adults, with a gradual widening through middle age as arteries lose elasticity. A gap of 60 or more is common in older people and is what produces readings with a high top number and an unremarkable bottom one. On its own it is a description of arterial stiffness, not a diagnosis, and it is interpreted in context rather than against a threshold. A very narrow gap is more often a technique or equipment issue than a physiological one, so repeat the reading with a correctly sized cuff on a bare arm before treating it as meaningful.
Should I stop my medication if the reading is normal?
No, and that decision is not one this page can contribute to. Medication working is the most likely explanation for a normal reading in someone taking it, and stopping on that basis removes the thing producing the number. Changes to dose or to whether a medication continues are made by the clinician who prescribed it, with the readings in front of them. Bring the log; it is genuinely useful to them. Do not act on it alone.