Systolic vs diastolic blood pressure: what the two numbers mean

Every blood pressure reading has two parts because your heart works in a cycle. When it contracts it pushes blood out into the arteries, and the pressure rises to a peak. Between beats the heart relaxes and refills, and the pressure falls to a low point. A blood pressure cuff measures both.

The first, higher number is systolic pressure. Blood Pressure UK explains it as the highest level your blood pressure reaches when your heart beats, forcing blood around your body. The British Heart Foundation describes it as the pressure against your arteries when your heart is pumping blood around the body. The second, lower number is diastolic pressure: the lowest level your pressure reaches as your heart relaxes between beats, or how much pressure is in the arteries when the heart rests.

Both numbers are written as millimetres of mercury, usually shortened to mmHg, with the systolic number first. The NHS notes that the first number is always the higher of the two, for example 130/80.

The two numbers describe different moments in the same heartbeat. The table summarises the differences, using the descriptions from Blood Pressure UK and the British Heart Foundation.

Blood Pressure UK gives general guide ranges for adults, noting that the cut-off for diagnosing high blood pressure does not change with age. A reading of 140/90mmHg or over suggests you may have high blood pressure. From 120/80 up to 140/90 it describes the range as pre-high or high-normal, which is not high blood pressure but is a little higher than it should be. From 90/60 up to 120/80 is described as ideal, and 90/60 or lower may be low blood pressure.

The British Heart Foundation publishes a slightly different banding. It says a normal, healthy reading is usually between 90/60 and 130/85, with 130/85 to 139/89 slightly raised, and it gives stage 1 as 140 to 159 over 90 to 99, stage 2 as 160 to 180 over 100 to 120, and above 180 over 120 as severe. It adds that these are guidance ranges and that you can agree a personal target with your healthcare team. The different charities draw the lower bands in slightly different places, but both agree on the 140/90 clinic threshold the NHS uses.

For readings taken at home, the NHS threshold is lower, 135/85 or higher. The BHF says that people over 80 should usually have a reading below 150/90 when checked by a healthcare professional or below 145/85 at home.

This is the most common source of confusion. Blood Pressure UK says that only one number needs to be higher than it should be to be diagnosed with high blood pressure. So a top number over 140 or a bottom number over 90 can lead to a diagnosis, regardless of the other number.

When the top number is consistently higher than 140mmHg but the bottom number is healthy, this is called isolated systolic hypertension. When the bottom number is consistently higher than 90mmHg but the top number is healthy, it is called isolated diastolic hypertension. Both count as high blood pressure.

The same applies in the other direction: a top number under 90 or a bottom number under 60 may be diagnosed as low blood pressure. Low blood pressure usually is not a problem, according to Blood Pressure UK, but it can sometimes make you feel faint or dizzy or be a sign of another health problem.

Both the BHF and Blood Pressure UK stress that a single high reading does not necessarily mean you have high blood pressure. Temperature, when you last ate and stress can all affect it, and readings in a clinic can be higher if you feel nervous. Your doctor or nurse will probably measure it a number of times over a few weeks.

The NHS says that if a test shows you might have high blood pressure you may be asked to use a monitor at home for a few days, to return to the surgery a few days or weeks later for another check, or to wear a portable monitor that takes regular readings over 24 hours. Averaging the readings is what confirms the picture.

High blood pressure usually has no symptoms, which is why measuring it is the only way to know. The NHS says it rarely causes headaches, blurred vision or chest pain, and that the only way to find out is to have it checked. The BHF estimates that as many as 5 million adults in the UK have undiagnosed high blood pressure.

The NHS says that if blood pressure is too high it puts extra strain on your blood vessels, heart and other organs such as your brain, kidneys and eyes, and that untreated it can increase the risk of heart disease, heart attacks, strokes, heart failure, kidney disease and vascular dementia. It also says lowering blood pressure even a small amount can help reduce these risks.

Lifestyle measures the NHS lists include a balanced diet, regular exercise (at least 150 minutes a week), losing weight if you are overweight, cutting salt, keeping alcohol to no more than 14 units a week, limiting caffeine and not smoking. A GP may prescribe medicine if your pressure is very high, if you are at higher risk of heart attack or stroke, or if it stays high after lifestyle changes.

The NHS says many pharmacies offer free blood pressure checks for people aged 40 or over, and that you should get checked if you are 40 or over and have not had it measured in more than 5 years. The BHF adds that healthy adults aged 40 to 74 in England are likely to be invited to a free NHS Health Check every 5 years, which includes a blood pressure check.

Most high readings are not emergencies, but the NHS lists symptoms that need a 999 call: sudden chest pain or discomfort that does not go away, pain spreading to the arm, neck, jaw, stomach or back, or chest pain with sweating, sickness, light-headedness or breathlessness. These could be a heart attack. The NHS says to get help from NHS 111 if you often have headaches or blurred vision, have chest pain that comes and goes, or are worried about symptoms you think are caused by high blood pressure. The BHF adds that if a reading is dangerously high you should seek urgent medical help.