What is blood pressure?
Blood pressure is created by the heart pumping blood through the arteries. It's measured in millimetres of mercury (mmHg), and given as two numbers – 120/80 mmHg, for example, or '120 over 80'.
The first number is what's called systolic pressure, caused by your contracting (beating) heart. The second number is what's called diastolic pressure, and is the pressure between beats when your heart relaxes.
Your blood pressure can vary by as much as 30 to 50 mmHg over the course of a day and it's influenced by a range of factors, such as what you've been eating or drinking, your emotional state, your level of activity, or even whether you're standing, sitting or lying down.
It's higher if you're stressed, have had a caffeinated drink and after exercise. It's usually higher in the afternoon than in the morning. And it gets higher as you get older.
So, to get comparable, consistent measurements, you need to measure your blood pressure at the same time of day and under the same conditions every time.
Eight simple steps to accurate measurement
- Don't exercise, have a bath, smoke, or drink coffee, tea, cola or other stimulating drinks for a couple of hours before you plan to take a measurement.
- Sit comfortably at a table.
- Remove watches, jewellery or anything that may interfere with the measurement, and have a bare arm or wrist (depending on the type of monitor you're using).
- Make sure you're comfortable; relax for a few minutes before you start.
- Ensure you take measurements using the same arm each time, as your blood pressure is different in each arm.
- If you're using an arm monitor, put on the cuff so that its centre is at heart height and your arm is resting on the table. If you're using a wrist monitor, put on the cuff and raise the wrist to heart height.
- Following the monitor's instructions, and take two measurements within a couple of minutes of each other. If they differ by more than 10 mmHg (e.g. one reading of 120/80 and another of 135/80, a difference of 15mmHg), take a third measurement.
- Keep a record of all results, and make a note of events that might explain an unusual result (for example, if you had an argument earlier that day, or if the grandkids were running around you while you took the measurement).
Using your blood pressure monitor
- To ensure accuracy, manufacturers strongly recommend visiting your GP for your first measurement, where you should measure your blood pressure using both the GP's monitor and your own (we recommend testing at least three times and working out an average). According to Dr Brian Morton from the Australian Medical Association, a fluctuation of about +/- 10mm/Hg is acceptable between different monitors, and we haven't found an average fluctuation beyond this for any monitors we've reviewed.
- You should go back and check your monitor's readings against your GP's machine every six months or so.
- Some monitor instructions also recommend sending the monitor back to the manufacturer every year for calibration.
- Your GP can advise you on the best time to measure your blood pressure and how often you should measure it, and they can check that you're using your own blood pressure monitor correctly.
What cuff size do you need?
Most blood pressure monitors come with a medium sized cuff already fitted. While this will be suitable for the majority of users, there are different sized cuffs available for several of the monitors on our test if your arm is exceptionally large or small.
To get the right cuff size for an arm model, measure your arm's circumference halfway between your shoulder and elbow, while standing with your arm hanging at your side. A circumference of 18–22cm requires a small cuff, 22–32cm requires a medium cuff, and above 32cm you'll need a large cuff – but check the measurements against the manufacturer's instructions on the product you intend to buy.
- For other tips on what to look for in a blood pressure monitor, have a look at our buying guide.
Why measure your blood pressure
High blood pressure, or hypertension, can increase your risk of heart attack, stroke, heart failure and kidney disease. When weighing up your risk, your GP will consider factors such as your age, sex, family history and weight, and whether or not you smoke.
High blood pressure is usually asymptomatic, so you may not even be aware you have it. That's why the disease is often called "the silent killer". A home blood pressure monitor is a good way to keep track of this stealthy problem.
What's a normal blood pressure, and what's high?
Here's how The Heart Foundation defines normal and high blood pressure:
- Less than 120/80: normal
- Between 120/80 and 140/90: normal to high
- Equal or greater than 140/90: high
- Equal or greater than 180/110: very high
Managing high blood pressure
Your lifestyle can have an impact on your blood pressure. To prevent high blood pressure, or to manage it once you've been diagnosed, follow these tips:
- Visit your GP regularly for a check-up, especially when you know you have high blood pressure.
- Quit smoking.
- Limit the amount of alcohol you drink.
- Exercise regularly (but note that there are certain sports to avoid when you have high blood pressure, such as lifting heavy weights) and consider stress-reducing techniques like yoga and meditation.
- Eat less red meat, and avoid salty and fatty foods.
- Eat more cereals, fish, fruit and vegetables.
- Maintain a healthy body weight.
Medication for high blood pressure
If lifestyle changes don't lower your blood pressure enough, your doctor may prescribe medication. Make sure you take it, and don't adjust the dosage yourself based on your own blood pressure measurements. This is known as "self diagnosis", and fiddling with your medication yourself can have serious consequences.
Low blood pressure
If you have low blood pressure, you probably won't have to do anything about it. It may cause some people to faint or feel tired, but most don't have any associated health problems.