Normal Blood Pressure: What Systolic and Diastolic Actually Mean

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Blood pressure is the force of blood pushing against the walls of the arteries, written as two numbers, systolic over diastolic, in millimetres of mercury (mmHg), and the US National Heart, Lung, and Blood Institute publishes normal adult blood pressure as less than 120 systolic and less than 80 diastolic. That sentence hides a complication worth knowing about. Expert bodies do not all agree on where the lines fall. A reading that one classification calls stage 1 high blood pressure sits below the threshold another uses to define hypertension at all. This page sets out what the numbers mean and what the main published categories actually say.

What the two numbers in a blood pressure reading mean

A blood pressure reading is written as two numbers, one over the other, such as 118/76 mmHg. The unit mmHg stands for millimetres of mercury.

The US National Heart, Lung, and Blood Institute describes the two phases this way. Systolic is the phase of circulation when the heart pumps and the pressure of blood against the vessels is highest. Diastolic is the phase when the heart fills and the pressure of blood against the vessels is lowest.

• Systolic is the first, upper number.
• Diastolic is the second, lower number.

Both numbers count. A reading can fall outside the normal band because of the upper number, the lower number, or both. People sometimes watch only the top figure, which means half the information is being ignored.

The adult blood pressure categories published in the United States

The National Heart, Lung, and Blood Institute, part of the US National Institutes of Health, publishes these adult categories:

• Normal — less than 120 systolic and less than 80 diastolic
• Elevated — 120 to 129 systolic and less than 80 diastolic
• High blood pressure stage 1 — 130 to 139 systolic or 80 to 89 diastolic
• High blood pressure stage 2 — 140 or higher systolic or 90 or higher diastolic
• Hypertensive crisis — higher than 180 systolic or higher than 120 diastolic

The small joining words matter. The first two categories use "and", so both numbers have to be low for a reading to qualify. The remaining categories use "or", so one high number is enough to place a reading in that band.

Why the UK and WHO draw the line at different numbers

There is no single worldwide table. Different expert bodies classify the same reading differently, and this is a genuine difference in published guidance rather than a mistake by one of them.

The World Health Organization states that hypertension is when the pressure in your blood vessels is too high, at 140/90 mmHg or higher. WHO adds that hypertension is diagnosed if, when blood pressure is measured on two different days, the systolic readings on both days are 140 mmHg or above and/or the diastolic readings on both days are 90 mmHg or above.

The UK National Institute for Health and Care Excellence (NICE) defines its stages using clinic readings paired with readings taken away from the clinic:

• Stage 1 hypertension — clinic blood pressure from 140/90 mmHg to 159/99 mmHg, with a daytime ambulatory average or home average from 135/85 mmHg to 149/94 mmHg
• Stage 2 hypertension — clinic blood pressure of 160/100 mmHg or higher but less than 180/120 mmHg, with a daytime ambulatory average or home average of 150/95 mmHg or higher
• Stage 3 or severe hypertension — clinic systolic of 180 mmHg or higher, or clinic diastolic of 120 mmHg or higher

So a clinic reading of 135/85 mmHg falls into stage 1 on the US categories, yet sits below the threshold WHO and NICE use to define hypertension. Knowing which classification a chart is built on is as important as the number printed on it.

Clinic readings and home readings are not judged by the same numbers

The NHS states that high blood pressure is 140/90 or higher when checked by a healthcare professional, and 135/85 or higher when checked at home. NICE makes the same point in general terms, noting that the corresponding ambulatory and home measurements are 5 mmHg lower than clinic measurements.

Two well described patterns explain why the setting changes the number:

• White-coat effect — NICE defines this as a difference of more than 20/10 mmHg between the clinic reading and the average daytime ambulatory or home reading.
• Masked hypertension — clinic readings are normal, less than 140/90 mmHg, but readings taken outside the clinic are higher.

If you measure at home, technique decides whether the number means anything at all. Our guide to checking blood pressure at home walks through the steps that authoritative bodies actually specify.

One reading is never the whole picture

Blood pressure moves constantly. It responds to rest, activity, pain, a full bladder, caffeine, cold, worry and the time of day. Published guidance therefore builds an assessment from several readings rather than one.

NICE advises that when a clinic reading is 140/90 mmHg or higher, a second measurement is taken during the same consultation, and if the second differs substantially from the first, a third is taken. The lower of the last two is recorded as the clinic blood pressure.

For home monitoring used to confirm a diagnosis, NICE describes two consecutive measurements at least one minute apart with the person seated, recorded twice daily, ideally morning and evening, continuing for at least four days and ideally seven. The first day is discarded and the remaining measurements are averaged.

WHO similarly describes diagnosis using readings taken on two different days. A single number, on its own, is not a diagnosis of anything.

Ranges vary, so read the range printed on your own report

Reference ranges and thresholds vary by the body that publishes them, by the measurement method used, whether clinic, ambulatory or home, and by age, sex, pregnancy and clinical context. Laboratory tests ordered alongside a blood pressure check vary by laboratory and method as well.

Read the range printed on your own report, and take that report to the doctor who ordered it. Your treating doctor interprets your readings, your symptoms and your history together, and that combination is what gives a number meaning. A reference page cannot do that, and this page is not attempting to.

Whether the thresholds themselves shift as people get older is a separate and frequently misunderstood question, covered in our guide to blood pressure by age.

When a blood pressure reading needs urgent attention

NICE advises same-day specialist assessment for a clinic blood pressure of 180/120 mmHg and higher accompanied by signs such as retinal haemorrhage or papilloedema, or by life-threatening symptoms such as new onset confusion, chest pain, signs of heart failure or acute kidney injury. The US categories describe a reading higher than 180 systolic or higher than 120 diastolic as a hypertensive crisis.

A high number with no symptoms is a different situation. It still needs medical review, and promptly, but contact your doctor rather than emergency services and follow the advice you are given. This guide supports a conversation with your doctor. It does not replace an emergency assessment.

Support with blood pressure at home

Some families prefer readings to be taken by a trained person, recorded the same way every time and shared with the treating doctor. Varolyn offers vitals monitoring at home and hypertension care at home in Bangalore.

Clinical decisions stay with your doctor. A home care team supports the plan your doctor has set, and keeps the record clear.

Frequently asked questions

Sources

National Heart, Lung, and Blood Institute (NIH) - High Blood Pressure · World Health Organization - Hypertension fact sheet · NICE guideline NG136 - Hypertension in adults: diagnosis and management · NHS - Blood pressure test

Last checked 2 September 2026. How we write and check these pages: editorial policy.