Health

Blood pressure: the two numbers explained

A blood pressure reading such as 120/80 mmHg is two measurements. The first, systolic, is the pressure in your arteries while the heart contracts; the second, diastolic, is the pressure between beats. Below 120/80 counts as normal in the categories used by the US National Institutes of Health, 140/90 or higher in a clinic (135/85 at home) is hypertension for WHO, the NHS and NICE, and the risk does not start at a threshold: in a pooled analysis of 1 million adults, every 20 mmHg of systolic pressure roughly doubled the rate of death from stroke between the ages of 40 and 69.

What the two numbers measure

Blood pressure is the force of blood against the walls of your arteries, measured in millimetres of mercury and written mmHg. Each heartbeat sends a wave of pressure through the arteries, 60 to 100 times a minute at rest, and the reading captures the two ends of that wave. The first number, systolic pressure, is the peak: the pressure while the heart contracts and pushes blood out. The second number, diastolic pressure, is the trough: the pressure that remains while the heart relaxes and refills between beats. A reading of 120/80 therefore means a peak of 120 mmHg and a trough of 80 mmHg.

The gap between them, 40 mmHg in this example, is the pulse pressure. It widens with age as the large arteries stiffen, which is why older people often have a high first number and a normal second one. For predicting risk, however, the gap is a weak guide. The largest pooled analysis of blood pressure and mortality, which followed 1 million adults in 61 studies, found that the average of the systolic and diastolic values was slightly more informative than either alone, and that pulse pressure was much less informative. So read both numbers, and treat either one crossing a line as meaningful.

Both numbers move all day, lowest in sleep and higher with exertion, pain, caffeine or a full bladder, which is why every guideline defines high blood pressure by repeated readings under standard conditions, never by one value.

Where normal ends and high begins

Two sets of categories are in wide use, and they disagree on one line. The US National Institutes of Health, following the 2017 American guideline, call anything below 120/80 normal, a systolic of 120 to 129 with a diastolic below 80 "elevated", 130 to 139 or 80 to 89 stage 1 hypertension, and 140/90 or higher stage 2. The World Health Organization, the NHS in England and the NICE guideline for England and Wales draw the line at 140/90 mmHg in a clinic, or 135/85 as an average of home readings, which run lower; WHO wants the readings over the line on 2 separate days. Under the NICE definition, stage 1 hypertension runs from 140/90 to 159/99 in the clinic (135/85 to 149/94 at home), stage 2 is 160/100 or higher (150/95 at home), and 180/120 or higher counts as severe.

So a reading of 132/84 is stage 1 hypertension in the United States and below the line for WHO, the NHS and NICE. Neither system treats it as harmless: both expect lifestyle changes, and both use it to spot people whose other risk factors tip the balance towards medication. At the other end, the NHS uses 90/60 as the threshold for low blood pressure, which only needs attention if it comes with dizziness, fainting or blurred vision.

Hypertension is common and mostly silent. WHO estimated that 1.4 billion adults aged 30 to 79, 33 percent of that age group, had hypertension in 2024. About 600 million of them (44 percent) did not know, 630 million (44 percent) were diagnosed and treated, and only 320 million (23 percent) had it under control. Most people feel nothing until the pressure is very high, usually 180/120 or more, when severe headaches, chest pain, blurred vision or nosebleeds can appear.

What 20 mmHg is worth in risk

The best evidence on what the numbers mean comes from the Prospective Studies Collaboration, published in The Lancet in 2002. It pooled individual data on 1 million adults with no previous vascular disease from 61 prospective studies, followed for 12.7 million person-years, during which there were about 56,000 vascular deaths (12,000 from stroke, 34,000 from ischaemic heart disease) and 66,000 other deaths at ages 40 to 89.

The result is a straight line on a logarithmic scale. At ages 40 to 69, each difference of 20 mmHg in usual systolic pressure, or roughly 10 mmHg in usual diastolic pressure, was associated with more than a twofold difference in the death rate from stroke and a twofold difference in deaths from ischaemic heart disease. The relationship held down to at least 115/75 mmHg, with no sign of a threshold, and was similar in men and women. At ages 80 to 89 the proportional differences were about half as extreme as at 40 to 49, but because deaths are far more frequent in old age, the absolute difference in risk was larger, not smaller.

Two limits. These are observational cohorts: they show how closely pressure and mortality travel together, not what happens when a given person lowers it, which is a question for randomised trials. And the analysis used "usual" blood pressure, corrected for the noise in a single measurement, one more reason to judge yourself on repeated readings.

How to take a reading that counts

Technique changes the number more than most people expect, and the US National Library of Medicine and NICE agree on the essentials. Do not measure within 30 minutes of caffeine, tobacco, a meal or exercise, and empty your bladder first. Sit in a chair with your back supported, legs uncrossed, feet flat on the floor, and rest for at least 5 minutes; 10 is better. Rest your arm on a table so that the upper arm is at heart level, with the cuff on bare skin and its lower edge about 2.5 cm above the bend of the elbow. Use a cuff that fits your arm, and do not talk.

Then take 2 or 3 readings 1 minute apart and record all of them. NICE's home protocol for confirming a diagnosis is 2 consecutive readings, at least 1 minute apart, in the morning and again in the evening, for at least 4 days and ideally 7, discarding the first day and averaging the rest. MedlinePlus suggests before breakfast and before dinner for 5 days, at least 10 readings to average. A single reading, high or low, tells you very little.

Three refinements. Measure both arms the first time; if they differ by more than 15 mmHg on repeat, use the arm with the higher reading from then on. If your pulse is irregular, automated cuffs may misread, and NICE asks clinicians to measure manually. And if clinic readings run higher than home readings, that is the white-coat effect, which is why NICE confirms a clinic reading of 140/90 or above with 24-hour ambulatory monitoring (at least 2 readings an hour, 14 or more while awake) or the home protocol above. Use a validated upper-arm monitor; the resting heart rate it shows alongside is worth noting too.

Targets, and when to see a doctor

Targets depend on age and condition. NICE asks for a clinic pressure below 140/90 for adults under 80 (below 135/85 on home or ambulatory readings) and below 150/90 for adults aged 80 and over (145/85 at home), with clinical judgement for people who are frail. WHO's goal is below 140/90 for most people, and below 130/80 for anyone with cardiovascular disease, diabetes, chronic kidney disease or a high cardiovascular risk. If you take blood pressure medication, the target is set with your doctor: do not adjust doses on the strength of home readings.

Some readings need a doctor quickly. A pressure of 180/120 or higher with chest pain, breathlessness, new confusion, vision changes or a severe headache is an emergency. The same reading without symptoms still needs a clinical review within 7 days. A drop of 20 mmHg systolic or 10 mmHg diastolic within 1 minute of standing, with dizziness or falls, is postural hypotension and worth reporting. If you are under 40 with high readings, NICE recommends a specialist look for a secondary cause. In pregnancy separate rules apply; report a rise, a severe headache or sudden swelling to your midwife or doctor promptly.

Otherwise the NHS suggests a check at least every 5 years from the age of 40. What lowers the numbers is well established: WHO lists 150 minutes a week of moderate aerobic activity or 75 minutes of vigorous activity, strength work on 2 or more days, less than 2 grams of sodium a day (about 5 grams of salt), losing weight if you carry too much, and no tobacco. The Walking Calculator turns a day's steps into minutes against the 150. With diabetes, heart disease or kidney disease, agree the plan with your doctor first.

Before you press start on the cuff

Frequently asked

Is 120/80 a normal blood pressure?

It sits at the top of the normal band. The US categories used by the NIH call anything below 120/80 normal, and a systolic of 120 to 129 with a diastolic below 80 is labelled elevated. WHO, the NHS and NICE do not diagnose hypertension until 140/90 in a clinic, so 120/80 is unremarkable in every system, although the mortality data suggest risk keeps falling down to about 115/75.

Which number matters more, the top or the bottom?

Both. In the pooled analysis of 1 million adults, the average of the two was slightly more informative than either alone, and every category system counts you as over the line if either number crosses it. The gap between them, the pulse pressure, was much less informative.

Why is my reading lower at home than at the doctor's?

Nerves in a clinical setting push readings up, the white-coat effect. That is why the home and ambulatory thresholds are set 5 mmHg lower (135/85 instead of 140/90) and why NICE confirms a diagnosis with home or 24-hour readings rather than clinic values alone.

Is 90/60 too low?

The NHS treats a reading below 90/60 as low blood pressure. In a fit person without symptoms it is usually not a problem. Dizziness, fainting, blurred vision or a drop of 20 mmHg systolic on standing should be discussed with a doctor, especially if you take blood pressure or heart medicines.

How often should I check my blood pressure?

If your readings are normal and you are over 40, the NHS suggests a check at least every 5 years. If a reading has been high, follow the home protocol: morning and evening for 7 days, then discuss the average with your doctor. Do not check dozens of times a day; anxiety drives the number up.

Can I stop my tablets if my home readings are normal?

Not without your doctor. Normal readings on treatment usually mean the treatment is working. Targets and any dose change belong to a conversation with your prescriber, particularly if you also have diabetes, heart disease or kidney disease.

Sources

• World Health Organization: Hypertension fact sheet (25 September 2025) · 1.4 billion adults aged 30 to 79 with hypertension in 2024; 44 percent unaware, 23 percent controlled; 140/90 definition, goals and lifestyle advice
• NHS: High blood pressure (hypertension) · Clinic 140/90, home 135/85; a check if you are 40 or over and have not had one for 5 years
• NICE guideline NG136: Hypertension in adults, diagnosis and management (2019, last updated 2026) · Measurement technique, stages 1 to 3, home and ambulatory protocol, targets by age; page answers 200 to curl
• MedlinePlus Medical Encyclopedia: Blood pressure measurement · US National Library of Medicine; preparation, posture, 2 to 3 readings 1 minute apart, US categories
• Lewington et al. 2002, The Lancet: Age-specific relevance of usual blood pressure to vascular mortality, a meta-analysis of individual data for one million adults in 61 prospective studies · Prospective Studies Collaboration; 12.7 million person-years, about 56,000 vascular deaths; abstract read via Europe PMC

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This guide is for general information. It does not replace medical advice or an examination.