What is blood pressure and how do I measure it?

Blood pressure is the pressure that blood exerts on the walls of blood vessels. 

You can think of the circulatory system as a series of pipes and the heart as a tap. When the heart beats faster, more blood flows through the pipes, and the pressure increases. In the same way, if the pipes are narrowed or stiff, this can also cause pressure to rise, which is what happens naturally as we age. 

As the heart is constantly pumping, blood pressure goes up and down all the time. The pressure needs to be high enough that oxygen-rich blood can flow to the vital organs where it’s needed, but not too high that it puts stress on the blood vessels and increases the risk of heart disease and stroke.

What do the two numbers mean?

When you measure your blood pressure, you’ll be given two numbers — a top number and a bottom number. The top number is always higher and there’s a reason for this. 

Systolic blood pressure (top number) — This is the highest level your blood pressure reaches. It’s when your heart is contracting and forcing blood through your blood vessels. 

Diastolic blood pressure (bottom number) — This is the lowest level your blood pressure goes as your heart relaxes in between beats. 

Blood pressure is measured in millimetres of mercury (mmHg). It’s an old-fashioned unit that’s been carried forward from the 17th century when mercury pressure gauges were used.

An example would be 132/82 mmHg (“132 over 82”), where 132 is the systolic blood pressure, and 82 is the diastolic blood pressure. 

Why measure two numbers?

You might be wondering why we bother recording two numbers. Don’t the two go hand in hand? 

The answer is usually, but that’s not always the case. Both systolic and diastolic blood pressure values are important when it comes to your health — just because one figure is normal, does not mean we should ignore the other.

Using both values give us an idea of what’s happening when your heart is contracting as well as relaxing. And sometimes, the gap between the two numbers is minimal or quite pronounced. This can point to certain conditions, like valve disease, an overactive thyroid, or severe iron deficiency.

What should your blood pressure be?

A textbook blood pressure is often quoted as 120/80 mmHg, but as we’re all different, normal blood pressure varies quite a bit — what’s normal for you may be abnormal for someone else.

Also, as you get older, your blood vessels begin to stiffen and blood pressure gradually increases. Because of this, target blood pressures are higher for people aged 80 and over. 

As a general guide:

• Ideal blood pressure is between 90/60 mmHg and 120/80 mmHg
• High blood pressure is 140/90 mmHg or higher
• Low blood pressure is 90/60 mmHg or lower [1]

However, your target may be different depending on your age, how your blood pressure was taken, and whether you have an underlying medical condition, such as diabetes. We have included a more detailed table below, but you should always follow your doctor’s advice who will consider your medical history. 

Recommended blood pressure targets [2]:

How to take your blood pressure

When taking your blood pressure, it’s important to follow the instructions specific to your device. These steps are given as a guide. 

Follow these steps to take your blood pressure at home:

• Check your kit. Make sure you have an upper arm monitor that’s been listed as clinically validated in the UK — this means it will have been tested to prove it’s accurate. Here is a list of blood pressure monitors validated for home use.
• Check the cuff size. If the cuff is too small, your reading will be falsely elevated, and vice versa. 
• Apply the cuff two to three centimetres above the elbow joint. The centre of the bladder should be positioned over the line of your artery — most cuffs have this marked on them. 
• Rest your arm at heart level. You may need a cushion for support. Having your arm too high or too low can lead to a false reading. 
• Be still and silent whilst the reading is taken. Talking and moving can affect the accuracy of the reading. 
• Take at least two readings. The first reading is often higher. Record your numbers clearly. 
• If your reading is high, repeat after five minutes. If it remains high, organise a follow-up with your doctor or nurse.
• If your reading is very high (160/100 mmHg or above) and you’re showing symptoms (such as headaches or blurred vision), seek urgent medical attention.

You can check your blood pressure reading using the NHS blood pressure checker, which gives advice on what to do next. 

Factors that may influence your blood pressure reading

When taking your blood pressure, it’s useful to be aware of some of the factors that might impact your reading. 

Manual vs automatic

You can either measure your blood pressure automatically using a machine that calculates the numbers or a healthcare professional may measure your blood pressure manually using a cuff and stethoscope.

Manual readings give very accurate results when measured correctly by a trained person, however, some individuals may have a blood pressure reading higher than usual due to the stress of being in a clinic. 

Automatic blood pressure machines are now much more widely available and affordable than before, and they don’t require training to use. Provided it is calibrated properly, an automatic machine is a reliable way to measure your blood pressure at home. 

Clinic vs home

Travelling to the GP and worrying about your blood pressure being high is, ironically, sometimes enough to raise your blood pressure. This is known as white coat syndrome. 

If this is the case, your GP or nurse may recommend measuring your blood pressure at home, where you’re likely to feel more relaxed. Generally, readings are about 5 mmHg lower at home.

One-off vs repeated measurements

Because blood pressure is influenced by so many factors, a one-off reading is not always reliable. It’s best to take three measurements, a few minutes apart, around the same time each day. 

You may be referred for ambulatory monitoring by your doctor. This is where you have a cuff attached to your arm, usually for 24 hours, that measures your blood pressure at set intervals. It can be used to calculate your average blood pressure and to look at your highest and lowest readings. 

Time of day

In the morning, looking at your to-do list or the morning commute might cause your blood pressure to spike. Your blood pressure is likely to be lowest when you’re winding down in the evenings and when you’re asleep. 

Stress

If you’re going through a particularly stressful period, such as moving house, your blood pressure is likely to be a lot higher than before — see our ten most common causes of stress throughout life. But unless your readings are way above normal, this is not generally harmful — your blood pressure will likely improve as your life situation settles. If it doesn’t, check in with your GP. 

Caffeine, nicotine, and big meals

You should avoid taking your blood pressure immediately after your morning coffee or a large meal as these can temporarily raise blood pressure. The same is true for smoking, which is also one of the biggest long-term contributors to longstanding raised blood pressure. NHS stop smoking services can help you quit.

Know your numbers

Measuring your blood pressure has never been easier. You can either buy a machine to use at home, use the automatic machine at your local practice, or book an appointment with your GP or practice nurse. A blood pressure test takes minutes and might just save your life.

References

1. NHS, 2021. What is blood pressure?. [online] Available at: <https://www.nhs.uk/common-health-questions/lifestyle/what-is-blood-pressure/> [Accessed 4 January 2022].
2. CKS, NICE. 2021. Hypertension | Health topics A to Z. [online] Available at: <https://cks.nice.org.uk/topics/hypertension/> [Accessed 4 January 2022].
3. Blood Pressure UK. 2020. Guidelines for Home Blood Pressure Testing. [online] Available at: <https://www.bloodpressureuk.org/media/bpuk/docs/TESTING-GUIDELINES_2020_web.pdf> [Accessed 4 January 2022].