How to Read Your Blood Pressure Numbers (Systolic vs Diastolic)

To read blood pressure, you look at two numbers written as one over the other, such as 120/80 mmHg. The top number is your systolic pressure and the bottom number is your diastolic pressure. Compare both against the standard categories, and remember that a reading is judged by whichever number falls in the higher band.

If those two numbers have ever left you guessing, you are not alone. Once you know what each one measures and where it sits on the official scale, a blood pressure reading becomes easy to understand at a glance. This guide walks through how to read your numbers, which one matters most, and what to do with the result.

The two numbers: systolic and diastolic

Blood pressure is measured in millimetres of mercury, shown as mmHg, and it always comes as a pair.

The top number is your systolic pressure. According to the American Heart Association (AHA), this is the force in your arteries each time your heart beats and pushes blood out. It is the higher of the two figures.

The bottom number is your diastolic pressure. This is the force in your arteries while your heart rests and refills between beats, so it is the lower figure. A reading of “120 over 80” simply means a systolic of 120 and a diastolic of 80, written 120/80 mmHg.

Both numbers are measured every time because they describe two different moments in each heartbeat, and each one carries information about how hard your heart and arteries are working.

How to read the blood pressure categories

The AHA sorts readings into five categories. To place your reading, find the band that either number falls into, and use the higher of the two.

• Normal: below 120/80 mmHg, meaning a systolic under 120 and a diastolic under 80.
• Elevated: 120 to 129 systolic and below 80 diastolic.
• Stage 1 high blood pressure: 130 to 139 systolic, or 80 to 89 diastolic.
• Stage 2 high blood pressure: 140 or higher systolic, or 90 or higher diastolic.
• Hypertensive crisis: higher than 180 and, or, 120, which needs urgent care.

The “higher band wins” rule is the part people miss. If your systolic is a normal 118 but your diastolic is 84, the reading counts as stage 1, because the diastolic sits in that band. For a fuller walk through each category with examples, see our guide to understanding your blood pressure readings.

Which number matters more?

Both numbers matter, and doctors consider them together. That said, the AHA notes that systolic pressure, the top number, tends to get more attention as a risk indicator, particularly in adults over 50, because it often rises with age as arteries stiffen. A high systolic is a well-recognised warning sign even when the diastolic is normal, a pattern called isolated systolic hypertension.

This does not mean you can ignore the diastolic. A raised bottom number, especially in younger adults, still signals that your arteries are under strain. The safest habit is to read both numbers each time and look at how they behave over days and weeks rather than fixating on one figure from one reading.

The gap between the two numbers is called your pulse pressure. It is not something to diagnose yourself with, but if you are curious, it is simply the systolic minus the diastolic, and your doctor may mention it in the wider context of your health.

How to read your monitor screen

Most home monitors show three numbers. The largest two are your systolic and diastolic, usually stacked with systolic on top, and the third is your pulse, or heart rate, often marked with a small heart symbol. Do not mistake the pulse for part of your blood pressure. A screen showing 122, 78 and 68 means a blood pressure of 122/78 mmHg with a pulse of 68 beats a minute.

For the reading to mean anything, technique matters. Sit quietly for five minutes first, keep your back supported and feet flat on the floor, rest your arm on a table at heart height, and stay still and silent while the cuff inflates. Taking two readings a minute apart and noting both gives a more reliable result than a single measurement. Our complete guide to measuring blood pressure at home and our tips on the best time to take your blood pressure cover the routine in full.

Reading one number in context

A single reading is only a snapshot. Blood pressure naturally rises and falls through the day with activity, stress, caffeine, posture and temperature, so one high or low result is rarely the full story. The AHA and the UK’s National Health Service (NHS) both stress that diagnosis is based on a pattern of readings over time, not one measurement.

That is why keeping a log is so useful. When you read your numbers at consistent times and record them, you can see your real trend, spot patterns such as higher mornings, and bring solid data to your next appointment instead of a single figure. It also helps you tell the difference between a one-off spike and a genuine shift worth acting on.

When to see a doctor

Reading your own numbers is a habit worth building, and it helps you know when to seek help.

• Call emergency services if a reading is higher than 180/120 mmHg together with chest pain, shortness of breath, weakness or numbness, vision changes, trouble speaking or a severe headache, as these can signal a medical emergency.
• Contact your doctor soon if your readings are consistently 130/80 mmHg or above, or if they are rising over time.
• Check in if you feel unwell or if your readings swing widely and you are unsure what is normal for you.

This guide is for general information and is not a substitute for personalised medical advice. Your doctor can interpret your numbers alongside your full health history.

Frequently asked questions

Which number is the systolic and which is the diastolic? The top number is systolic, the pressure when your heart beats, and the bottom number is diastolic, the pressure when your heart rests. So in 120/80 mmHg, 120 is systolic and 80 is diastolic.

Which blood pressure number is more important? Both matter and are read together, but the AHA notes the systolic (top) number gets particular attention as a risk indicator in adults over 50, because it tends to rise with age. A raised diastolic still signals strain and should not be ignored.

What is a normal blood pressure reading? The AHA defines normal as below 120/80 mmHg, meaning both a systolic under 120 and a diastolic under 80. A reading is placed in the higher of the two numbers’ categories.

Log this reading in CardioVibe

Reading your numbers is easier when you can see them build into a trend. Log this reading in CardioVibe, record your systolic and diastolic in seconds, and watch your pattern take shape over time.