Your monitor beeps, the cuff lets go, and two numbers appear on the screen. It is natural to want to know straight away whether they are good or bad. This page explains what the numbers are and how the American Heart Association (AHA) in the US and the NHS in the UK group them, in their own words.
One thing first, because both organizations say it too: a single reading is not a diagnosis. Only your doctor or nurse can tell you what your numbers mean for you. We are not doctors, and nothing here replaces that conversation.
The two numbers
A reading is written as two numbers, such as 130/80. The NHS explains them like this:
- The first, higher number is systolic pressure. It is the highest level your blood pressure reaches when your heart pumps blood around the body.
- The second, lower number is diastolic pressure. It is the lowest level your blood pressure reaches when your heart relaxes between beats.
Both are measured in millimeters of mercury, written mm Hg or mmHg. Many monitors also show your pulse, which is a separate number and not part of the blood pressure reading.
The AHA categories (US)
The AHA publishes five categories. Your reading falls into the highest category that either number reaches. Here they are as the AHA lists them:
| Category | Systolic (top number) | Diastolic (bottom number) | |
|---|---|---|---|
| Normal | Less than 120 | and | Less than 80 |
| Elevated | 120 to 129 | and | Less than 80 |
| Stage 1 hypertension | 130 to 139 | or | 80 to 89 |
| Stage 2 hypertension | 140 or higher | or | 90 or higher |
| Severe hypertension | Higher than 180 | and/or | Higher than 120 |
The AHA adds a note under its chart: only a doctor or other medical professional can confirm a diagnosis of high blood pressure, and they can also evaluate low blood pressure numbers.
The NHS numbers (UK)
The NHS does not use the same bands. It gives a single line for high blood pressure, and it draws that line in a different place depending on where the reading was taken. Your blood pressure is usually considered high if it is:
- 140/90 or higher when checked by a healthcare professional, or
- 135/85 or higher when checked at home.
At the other end, the NHS describes low blood pressure as a reading of less than 90/60. It notes that low blood pressure does not always cause symptoms, and suggests seeing a GP if you keep getting symptoms such as dizziness or fainting.
You may notice that the AHA and the NHS do not agree on where “high” begins. They are different bodies writing for different health systems, and your own doctor will tell you which targets apply to you. That is one more reason not to diagnose yourself from a chart.
Why the home number is lower
The 135/85 home line comes from NICE, which writes the guidance NHS doctors follow. NICE tells clinicians that home readings, and readings from a 24-hour ambulatory monitor, run 5 mmHg lower than the equivalent clinic readings. So a home average of 135/85 is treated as matching a clinic reading of 140/90.
Several things can make the clinic number higher. Some people’s blood pressure rises when it is measured by a doctor or nurse, which NICE calls the white-coat effect. For others it is the other way round, and readings are normal in the clinic but higher at home. We explain both, and what NICE and the AHA say to do about them, in our guide to white coat hypertension and home monitoring.
There is a second difference that matters more than the threshold: doctors look at averages, not single readings. When NICE uses home monitoring to confirm a diagnosis, it asks for two readings, at least a minute apart, morning and evening, for at least four days and ideally seven. The first day is thrown away and the rest are averaged. One reading on one evening is not what the 135/85 line was written for.
When a reading needs urgent attention
This is the part to read carefully. The AHA and the NHS put it differently, so here is each in turn.
The AHA: higher than 180/120
The AHA says that if your blood pressure is higher than 180/120:
- Wait at least 1 minute.
- Take your blood pressure again.
If the second reading is just as high, check for these symptoms: chest pain, shortness of breath, back pain, numbness, weakness, change in vision, or difficulty speaking.
- If you have none of these symptoms, or any other new and concerning symptoms, the AHA says you likely have severe hypertension. It says this usually does not need hospitalization, but you should contact your health care professional as soon as possible. On its home monitoring page it puts it more strongly: contact your healthcare professional immediately.
- If you have any of these symptoms, the AHA calls it a hypertensive emergency. Its words are: “This is a medical emergency. Do not wait to see whether your blood pressure comes down on its own. Call 911.”
The NHS: it goes by symptoms
The NHS pages on blood pressure do not give a reading at which to call for help. They point to symptoms instead, and note that high blood pressure rarely causes any.
- Get help from NHS 111 if you often have headaches or blurred vision, you have chest pain that comes and goes, or you have other symptoms you are worried could be caused by high blood pressure. You can call 111 or use 111 online.
- Call 999 if you get sudden pain or discomfort in your chest that does not go away, pain that spreads to your arm, neck, jaw, stomach or back, or chest pain with sweating, feeling sick, light-headed or short of breath. The NHS says you could be having a heart attack, and that you should not drive yourself to A&E.
NICE’s guidance for clinicians treats a clinic reading of 180/120 or higher as severe hypertension. If you are in the UK and worried about symptoms, the NHS says to get help from NHS 111, online or by calling 111.
A single high reading
The AHA is clear that one high reading is not an immediate cause for alarm. If a reading is higher than normal, take it a second time, write down both results, and check with your healthcare professional whether there is a health concern or a problem with your monitor.
That last point is worth remembering. A cuff that is the wrong size, a reading taken straight after coffee, or a monitor that has never been validated can all give a misleading number. Our guide to the best home blood pressure monitors covers the validated ones and the AHA’s routine for an accurate reading.
What to do with your numbers
The AHA recommends home monitoring for everyone with high blood pressure, so their healthcare professional can see whether treatment is working, and says it can also help confirm a diagnosis. It also says something worth repeating: home monitoring does not replace regular doctor visits, and you should not stop taking blood pressure medication without checking with your healthcare professional, whatever your home readings say.
So the most useful thing you can do with your readings is to write them all down and take them to your doctor. If you are not sure the monitor you have is easy enough to use every day, our guide to the easiest blood pressure monitors to use may help.
Two numbers, two sets of categories, and one rule that both the AHA and the NHS agree on: your doctor decides what your readings mean. Your job is to take them carefully and bring the record along.