Systolic vs. Diastolic: Explaining the Top and Bottom Numbers in a Blood Pressure Reading
Key takeaways:
Your blood pressure includes two numbers. The top number is your systolic pressure. The bottom number is your diastolic pressure.
Systolic blood pressure is measured when your heart is actively squeezing. Diastolic blood pressure is measured when your heart is relaxed and refilling with blood.
If either number in your blood pressure is high — especially over repeated measurements — you may have high blood pressure (hypertension).
Table of contents
Over 40% of the U.S. population has high blood pressure (hypertension). But many people don’t even realize they have it. So, experts recommend learning how to measure blood pressure at home. This way, you don’t have to wait until your next healthcare visit to check on your heart.
To know if your blood pressure is high, it helps to understand the differences between systolic and diastolic blood pressure. We’ll explain the difference between these two numbers and why it matters.
What are systole and diastole?
The top and bottom numbers in your blood pressure reading are based on the two phases of a heartbeat:
Systole is the phase when your heart contracts. During this phase, it squeezes blood out to the rest of your body.
Diastole is the phase when your heart relaxes. During this phase, it fills with blood to prepare for the next beat.
Systolic blood pressure
Systolic blood pressure is the top number of your blood pressure reading, measured in millimeters of mercury (mmHg). So, if your blood pressure is 120/80, your systolic pressure is 120 mmHg.
The systolic blood pressure measures the pressure in your blood vessel walls during the systole phase. Healthy blood vessels stretch easily to make room for blood flow. But pressure is higher in stiff vessels — which can happen with age or cardiovascular disease. This is one of the reasons systolic blood pressure rises steadily with age.
Systolic blood pressure is always higher than diastolic blood pressure. A normal range for systolic blood pressure for most people is 90 to 120.
Diastolic blood pressure
Diastolic pressure is the bottom number of your blood pressure reading, also measured in mmHg. So, if your blood pressure is 120/80, your diastolic pressure is 80 mmHg.
Diastolic blood pressure measures the pressure on blood vessel walls when your heart is relaxed between contractions. This number is lower because the heart isn’t actively pumping blood through your blood vessels during this phase. Like systolic pressure, high diastolic pressure can happen when blood vessel walls become more rigid and stiff.
The normal range for diastolic blood pressure is 60 to 80.
What is a ‘normal’ blood pressure?
It’s important to emphasize that a “normal” blood pressure reading depends on your individual health. For most people, a normal blood pressure is less than 120/80. A diagnosis of hypertension requires repeated measurements of blood pressures above 130/80. Your primary care provider can tell you what’s healthy and safe in your personal health circumstances.
This table shows guidelines from the American Heart Association. Keep in mind that your blood pressure can be temporarily high or low at times (like with exercise, stress, or caffeine). This chart applies to your typical blood pressure, which is why it’s helpful to take an average of several readings.
Blood pressure category | Systolic blood pressure | Diastolic blood pressure | |
Normal | Below 120 | and | Below 80 |
Elevated | 120-129 | and | Below 80 |
High blood pressure (stage 1) | 130-139 | or | 80-89 |
High blood pressure (stage 2) | Above 140 | or | Above 90 |
Hypertensive crisis (call your doctor’s office immediately) | Above 180 | and/or | Above 120 |
Which is more important, diastolic or systolic blood pressure?
Both measurements are important, although high systolic and diastolic blood pressures can have different treatments. There are actually a few different types of hypertension:
Isolated systolic hypertension: This is a systolic blood pressure of 130 or higher with a diastolic pressure under 80.
Isolated diastolic hypertension: This is a diastolic blood pressure of 80 or higher with a systolic pressure under 130.
Mixed hypertension: This means that both systolic blood pressure is 130 or higher and the diastolic blood pressure is 80 or higher.
Systolic blood pressure (the top number) often gets more attention. This is because a high systolic blood pressure is a greater risk factor for cardiovascular disease in people over 50. A higher top number means a higher risk of having a stroke or developing heart disease. But the bottom number is still important.
Should you worry if you have high diastolic blood pressure?
High diastolic blood pressure can also increase your risk for heart disease and stroke. So, it should also be managed. According to the American Heart Association, for every 10-point increase in diastolic blood pressure, the risk of death from heart attacks and strokes doubles in those 40 to 89 years old. (The same applies to every 20-point increase in systolic blood.)
Isolated diastolic hypertension often gets less attention because it’s less common. And it’s easier to manage with medications. But treating any kind of hypertension is important because it can lower the risk of developing serious disease and even death.
How to take your blood pressure at home
When taking your blood pressure at home, make sure you’re resting in a calm environment. Avoid anything that could falsely raise your blood pressure in the 30 minutes before taking it (like caffeine, exercise, or smoking).
Here’s the best way to measure your blood pressure at home:
Sit in a chair with your back straight, feet flat on the ground, and legs and ankles uncrossed.
Place the blood pressure cuff around your upper arm, directly against your skin — not over clothing.
Relax your arm at your side to release any tension in your muscles.
Let the cuff inflate. It may squeeze your arm pretty tightly. Breathe in and out normally and avoid talking during the measurement.
Once the cuff deflates, take note of the blood pressure reading.
It’s best to take your blood pressure at the same time each day and keep a record of those measurements. This will help you and your primary care provider decide if and how your blood pressure should be treated.
Healthcare professionals recommend taking 2 measurements — at least 1 minute apart — at these times:
In the morning before taking medications
In the evening before eating dinner
This provides a log of four measurements each day.
The bottom line
Hypertension is defined as blood pressure above 130/80. But there are differences between systolic and diastolic hypertension — and both measurements matter. If you’re wondering whether you have high blood pressure, it’s helpful to track your blood pressure at home. This allows you and your primary care provider to figure out if your blood pressure is in a healthy range on a daily basis. And this can make a big difference for your long-term health.
Why trust our experts?
References
American Heart Association. (n.d.). High blood pressure.
American Heart Association. (2020). Blood pressure categories.
American Heart Association. (2024). Understanding blood pressure readings.
Strandberg, T. E., et al. (2003). What is the most important component of blood pressure: Systolic, diastolic or pulse pressure? Current Opinion in Nephrology and Hypertension.
