Heart & Metabolic Health - Know your numbers
How to understand blood pressure
Last updated 30 September 2026. How we write this .
Blood pressure is two numbers: the peak as your heart beats and the low point between beats. In the UK it counts as high at 140/90 or more in a clinic, or 135/85 or more at home.
Start here Blood pressure is two numbers: the peak as your heart beats and the low point between beats. In the UK it counts as high at 140/90 or more in a clinic, or 135/85 or more at home.
By the end, you will understand What do the two numbers mean? Why won't my GP diagnose me from one reading? Why do a few points matter so much? Key words used in this lesson Meta-analysis A statistical method for combining the results of multiple studies into a single estimate. More reliable than any one study, but bounded by the quality of what went into it. Full plain-English glossary
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Blood pressure is the force your blood puts on the walls of your arteries, written as two numbers such as 128/82. In the UK, a reading counts as high if it is 140/90 or more when a nurse or doctor takes it, or 135/85 or more when you take it at home. Those cut-offs are the starting point. What a reading means depends on how it was taken, how many readings sit behind it, and how far above the line it is.
High blood pressure is common and usually silent. A Public Health England briefing estimated that more than 1 in 4 adults in England had it in 2015, around 12.5 million people. Most feel nothing at all. As the NHS puts it, the only way to find out is to get it checked.
What do the two numbers mean?
The first number is systolic pressure, the peak reached each time your heart squeezes. The second is diastolic pressure, the lowest point as the heart relaxes and refills between beats. Both are measured in millimetres of mercury (mmHg), a unit left over from the old mercury gauges.
Doctors look at both. A clinic reading of 150/80 still sits in the high range because of the top number, even though the bottom one looks fine. Your pulse is a separate measurement. It counts heartbeats per minute and tells you little about pressure.
Why won't my GP diagnose me from one reading?
Because blood pressure moves all day with activity, stress and the time of day. Some people also run higher in the surgery than anywhere else, which doctors call the white-coat effect. NICE defines it as a gap of more than 20/10 mmHg between clinic readings and readings taken away from the clinic.
So NICE guideline NG136 sets out a process. If a clinic reading falls between 140/90 and 180/120, the next step is ambulatory monitoring: a cuff you wear through a normal day that inflates at least twice an hour while you are awake, giving an average of at least 14 readings. If that is not possible, you measure at home instead: two readings at least a minute apart, sitting down, morning and evening, for at least four days and ideally seven. The diagnosis is confirmed when the clinic reading is 140/90 or more and the daytime average is 135/85 or more.
The home and ambulatory thresholds are set 5 mmHg lower than the clinic ones. NICE then grades the result. Stage 1 is a home or ambulatory average of 135/85 to 149/94. Stage 2 is 150/95 or more. A clinic reading of 180/120 or more counts as severe.
If you are 40 or over and have not had a check in more than five years, the NHS suggests getting one. Many pharmacies offer free checks for people in that age group.
Why do a few points matter so much?
Because risk climbs steadily rather than jumping at a cut-off. The Prospective Studies Collaboration pooled data on one million adults from 61 studies and published the results in The Lancet in 2002. In people aged 40 to 69, every 20 mmHg higher usual systolic pressure went with more than double the death rate from stroke, and double the death rate from heart disease. That pattern held all the way down to about 115/75, below which there was little data.
Bringing pressure down reverses much of that risk. A 2016 Lancet meta-analysis led by Dena Ettehad combined 123 trials with 613,815 participants. Every 10 mmHg fall in systolic pressure cut major cardiovascular events by 20%, stroke by 27%, heart failure by 28% and deaths from any cause by 13%. The same Public Health England briefing noted that only 35% of adults with high blood pressure in England were both diagnosed and treated to target, against 65% in Canada.
How low should blood pressure go?
This is where the honest answer is less tidy. The SPRINT trial, published in the New England Journal of Medicine in 2015, took 9,361 adults at raised heart risk but without diabetes. One group aimed for a systolic pressure under 120, the other under 140. After a year, the groups averaged 121.4 and 136.2. The trial stopped early, after a median of 3.26 years, because the lower-target group had fewer heart attacks, strokes, cases of heart failure and cardiovascular deaths: 1.65% a year against 2.19%. Deaths from any cause were 27% lower.
The lower target came at a price. Episodes of low blood pressure, fainting, abnormal blood salt levels and kidney injury were all more common in that group. SPRINT also studied one particular kind of patient, so its result does not carry over neatly to everyone. NICE has not adopted 120 as a general target. For most people with hypertension under 80, it aims for a clinic reading below 140/90, and below 150/90 for people aged 80 and over, with clinical judgement for anyone who is frail or living with several conditions.
What lowers blood pressure without medicine?
Food makes a measurable difference. In the DASH-Sodium trial, 412 people ate set diets for 30 days at a time. Cutting salt lowered systolic pressure at each step, in people with and without hypertension. A diet rich in fruit, vegetables and low-fat dairy, known as the DASH diet, combined with low salt brought systolic pressure down by 7.1 mmHg in people without hypertension and by 11.5 mmHg in people with it, compared with a typical American diet high in salt.
NHS advice follows the same lines: eat a balanced diet, keep salt to no more than 6g a day for adults, aim for at least 150 minutes of exercise a week, lose weight if you are overweight, stop smoking and keep alcohol under 14 units a week. Around three quarters of the salt people eat is already in bought foods such as bread, breakfast cereals and ready meals, so labels count for more than the salt shaker. Many people still need tablets as well, and that is a decision for a doctor based on your readings and your overall risk.
When is a high reading an emergency?
One high number on a home monitor is rarely an emergency by itself. Take a second reading. When a first clinic reading is high, NICE asks clinicians to take another and to record the lower of the last two.
A clinic reading of 180/120 or more is different. NICE says it needs same-day specialist assessment if it comes with new confusion, chest pain, signs of heart failure or bleeding at the back of the eye. Without those signs, it still calls for tests for organ damage as soon as possible and a review within 7 days.
The NHS advises contacting 111 if you often get headaches or blurred vision, or chest pain that comes and goes. Call 999 for chest pain that does not go away, pain spreading to your arm, neck, jaw, stomach or back, or chest pain with sweating, sickness, light-headedness or breathlessness.
This lesson is general education about blood pressure, not medical advice. For questions about your own readings or treatment, speak to your GP, practice nurse or pharmacist.
Related in this classroom Related in other classrooms The studies behind this page Hypertension in adults: diagnosis and management. Recommendations - NICE guideline NG136, updated 2023High blood pressure - NHSHealth matters: combating high blood pressure - Public Health England, 2017Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies - The Lancet, 2002Blood pressure lowering for prevention of cardiovascular disease and death: a systematic review and meta-analysis - The Lancet, 2016A randomized trial of intensive versus standard blood-pressure control (SPRINT) - New England Journal of Medicine, 2015Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet - New England Journal of Medicine, 2001Salt in your diet - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.