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 is hypertension for WHO, the NHS and NICE (the NHS and NICE use 135/85 for home readings), and the risk does not start at a threshold: in a pooled analysis of 1 million adults, every 20 mmHg of systolic pressure went with more than a doubling of the death rate from stroke between the ages of 40 and 69.

What the two numbers measure

Blood pressure is the force of blood against your artery walls, measured in millimetres of mercury, mmHg. Each heartbeat sends a pressure wave through the arteries, 60 to 100 times a minute at rest, and the reading captures both 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.

The gap between them, 40 mmHg in this example, is the pulse pressure. It widens with age as the large arteries stiffen, but it predicts vascular death much less well than the 2 numbers themselves. Read both: either one over the line counts.

Both numbers move all day, lowest in sleep and higher with exertion, caffeine or a full bladder, so every guideline defines high blood pressure by repeated readings, 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 and the NICE guideline for England draw the line at 140/90 mmHg in a clinic; the NHS and NICE add 135/85 for home readings, which run lower, and WHO wants 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 runs from 160/100 to 179/119 (150/95 or higher at home), and severe hypertension, stage 3, starts at a clinic systolic of 180 or a diastolic of 120, either number on its own.

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. At the other end, the NHS calls a reading below 90/60 low blood pressure; it often causes nothing at all, but see a doctor if it comes with dizziness, fainting, blurred vision, weakness or confusion. All of these bands are for adults: under 18, blood pressure is judged against percentiles for age, sex and height, so a child's reading is for a paediatrician to interpret.

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, and only 320 million (23 percent) had it under control. Most people feel nothing at all, and even at 180/120 or more there may be no symptoms at all.

What 20 mmHg is worth in risk

The clearest picture of 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, with about 56,000 vascular deaths (12,000 stroke, 34,000 ischaemic heart disease) and 66,000 other deaths at ages 40 to 89.

At ages 40 to 69, each difference of 20 mmHg in usual systolic pressure, or roughly 10 mmHg in usual diastolic pressure, went with more than a twofold difference in the stroke death rate 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 more frequent in old age, the absolute difference in risk was larger.

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.

How to take a reading that counts

Technique changes the number more than most people expect. The US National Library of Medicine sets out the preparation: 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 with the upper arm at heart level, the cuff on bare skin and its lower edge about 2.5 cm above the bend of the elbow. NICE adds the setting: a quiet, temperate room, a cuff sized for your arm, and a validated device.

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, morning and evening, for at least 4 days and ideally 7, discarding the first day and averaging the rest. 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 more than 20/10 mmHg above your home or daytime ambulatory average, NICE calls that the white-coat effect: it confirms a clinic reading between 140/90 and 180/120 with ambulatory monitoring or the home protocol above.

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 symptoms need emergency care whatever the cuff says. Chest pain or pressure, breathlessness, new confusion, a drooping face, sudden weakness, numbness or slurred speech: call your emergency number (112, 911 or 999) at once, without stopping to take a reading first. A reading of 180/120 or higher alongside any of those, or with a severe headache or vision changes, is a hypertensive emergency and the same call applies. The same reading with no symptoms is not an emergency, but it is not something to sit on either: rest 5 minutes, measure again, and if it holds, contact your doctor that day. NICE asks for tests for organ damage as soon as possible and a clinical review within 7 days.

A fall of 20 mmHg or more systolic, or 10 mmHg or more diastolic, after standing for at least 1 minute, is what NICE counts as postural hypotension; with dizziness or falls it is worth reporting. If you are under 40 with diagnosed hypertension, NICE recommends a specialist look for a secondary cause. Pregnancy follows separate rules and has its own NICE guideline: the bands and targets above do not apply, and ACE inhibitors and angiotensin receptor blockers are not used in pregnancy, so tell your doctor at once if you take one and are pregnant or planning to be. A reading of 140/90 or higher, a severe headache, vision changes, pain under the ribs or sudden swelling of the face, hands or feet all need reporting to your midwife or maternity unit the same day, as possible signs of pre-eclampsia.

Otherwise the NHS suggests a check at least every 5 years from the age of 40. What lowers the numbers: 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 just above the normal band. In the US categories normal means below 120 and below 80, so a diastolic of exactly 80 already falls in the 80 to 89 range the 2017 American guideline calls stage 1. For WHO, the NHS and NICE, who diagnose hypertension from 140/90 in a clinic, the same reading is comfortably below the line, and 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: NICE counts a gap of more than 20/10 mmHg between clinic and home or daytime ambulatory readings as the white-coat effect. That is why the NHS and NICE set the home threshold 5 mmHg lower (135/85 instead of 140/90), and why NICE confirms a diagnosis with home or ambulatory 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 NICE 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 (33 percent of the age group); 600 million (44 percent) unaware, 630 million (44 percent) diagnosed and treated, 320 million (23 percent) controlled; 140/90 on 2 separate days, targets, 150 or 75 minutes of activity, strength work on 2 days. The fact sheet gives the limit as under 2 grams a day, which is the WHO sodium limit, about 5 grams of salt
• 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; the page tells readers to call the emergency number for sudden chest pain rather than wait; page last reviewed 19 July 2024. The 90/60 figure and its symptom list (dizziness, fainting, blurred vision, weakness, confusion) are from the NHS page on low blood pressure (hypotension)
• NICE guideline NG136: Hypertension in adults, diagnosis and management (2019, last updated 26 February 2026) · Measurement technique, both arms and 15 mmHg, stages 1 to 3, home and ambulatory protocol, postural drop, targets by age and kidney disease, same-day referral at 180/120 with symptoms and review within 7 days without, specialist evaluation under 40, separate NICE guideline for pregnancy
• 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 and PubMed (PMID 12493255)

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