What do the diastolic and systolic numbers mean

Only one of your numbers needs to be higher than it should be to be diagnosed with high blood pressure, and only one needs to be lower than it should be to be diagnosed with low blood pressure.

So if your top number is over 140 or the bottom number is over 90, you may be diagnosed with high blood pressure, regardless of the other number. If your top number is under 90 or your bottom number is under 60, you may be diagnosed with low blood pressure. Use the chart to see where your numbers sit.

If your top number (systolic pressure) is consistently higher than 140mmHg, but the bottom number is healthy - this is known as Isolated Systolic Hypertension. If the bottom number (diastolic pressure) is consistently higher than 90mmHg but the top number is healthy - this is known as Isolated Diastolic Hypertension.

Making sure your readings aren’t a one-off

A single high reading doesn’t necessarily mean you have high blood pressure, as many things can affect your blood pressure throughout the day, such as the temperature, when you last ate, and if you’re feeling stressed.

Your doctor or nurse will probably want to measure your blood pressure a number of times over a few weeks to make sure the reading wasn’t just a one off and that your blood pressure stays high over time. 

Read about how high blood pressure is diagnosed, getting a blood pressure check, the further tests you might have if you have a high blood pressure reading, and what it means if you’re diagnosed with high blood pressure.

When reading your blood pressure numbers, the top number is the systolic pressure and the bottom number is the diastolic pressure. The systolic pressure is the maximum pressure your heart exerts while beating. The diastolic pressure is the amount of pressure in your arteries between beats. Think of this number as the one you get when your heart relaxes - it should be lower. The numeric difference between the two numbers is called your pulse pressure.

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Systolic and diastolic blood pressure make up the two numbers in a blood pressure reading (e.g., 120/80).

Systolic blood pressure, or the top number, is the amount of pressure experienced by the arteries while the heart is beating. Diastolic blood pressure (the bottom number) is the amount of pressure in the arteries while the heart is resting in between heart beats.

This article delves into the differences between systolic and diastolic blood pressure, why both numbers are important, and what your blood pressure readings mean.

Is My Blood Pressure Normal?

Why Your Blood Pressure Matters

When the heart beats, blood pulses through the arteries to travel throughout the body. However, it is not a steady stream like you might see from a garden hose.

The pulse of the blood flow and the pressure it exerts change from moment to moment. It's highest during the heartbeat (systolic pressure) and lowest between heartbeats (diastolic pressure).

Providers measure blood pressure using these numbers because it is a standard way of describing the force of the pulsing blood.

Your systolic and diastolic blood pressures are both important. If the readings are too high, you could have high blood pressure. If the readings are too low, there may not be enough blood flowing to your brain and other organs.

Furthermore, if there are changes in the difference between the two numbers, it's a clue that there could be a heart condition or other health problem.

How Can You Tell If You Have High Blood Pressure?

Systolic Blood Pressure

During a heartbeat, the heart pushes blood out into the arteries. Systolic pressure is the measure of this pressure within the arteries while the heart beats. This phase, known as systole, is the point at which blood pressure is the highest.

Systolic blood pressure is considered normal when the reading is below 120 mmHg (millimeters of mercury) while a person is sitting quietly at rest.

Systolic pressure below 90 mmHg is considered low and may require intervention and management from your healthcare provider. If you get multiple systolic pressure readings above 180 mmHg, it is considered dangerously high and should be addressed by your healthcare provider.

High Systolic Blood Pressure

The heart muscle pushes out blood with higher pressure when a person is exercising, under stress, or at any time when the heart rate is increased. The systolic pressure goes up with it.

In these cases, the increased pressure is normal. However, when the pressure is high while a person is resting, that is not normal and is considered high blood pressure.

Since your blood pressure can go up when you're active, it's important to take your blood pressure during periods of quiet rest to accurately diagnose high blood pressure (hypertension).

High systolic blood pressure is usually caused by the stiffening of the arteries, which makes the heart have to work harder to push blood through them.

Even within the high systolic pressure range, there are different stages:

• Systolic BP of 130-139 is Stage 1 hypertension, which may be reversed with temporary meds and lifestyle changes.
• Systolic BP of 140 or higher is Stage 2 hypertension, which can drastically increase the risk of stroke or heart attack, may require a prolonged regimen of medication.
• Systolic BP of 180 or higher means that you're in hypertensive crisis and should call your healthcare provider right away.

Your healthcare provider will help you determine the best course of action for treating your high systolic BP.

What Does It Mean If Only Your Systolic Blood Pressure Is High?

Low Systolic Blood Pressure

There is such a thing as too-low systolic pressure, however. When the reading is significantly below 90 mmHg, it's called hypotension. This can cause lightheadedness, dizziness, or fainting. If low blood pressure is not treated, it may cause organs like the kidneys to start shutting down.

Systolic hypotension can happen if the amount of blood in your body becomes too low. For example, low blood pressure can happen when you are severely dehydrated or when you have major bleeding. In these cases, there just isn't enough blood to push through the body.

Low blood pressure can also happen if the heart muscle is too weak to push blood normally—for example if the heart muscle is damaged (cardiomyopathy) or if the arteries suddenly widen too much (as in vasovagal syncope, a reflex that causes fainting).

Sometimes, low blood pressure happens when you change positions suddenly. You may feel dizzy when you stand up because gravity is pulling blood down toward your feet. This is a common condition called orthostatic hypotension.

Why Does My Blood Pressure Go Down When I Stand Up?

Diastolic Blood Pressure

The heart rests between beats so it can refill with blood. The pause between beats is called diastole. Your diastolic blood pressure is the measurement during this pause before the next heartbeat.

Normal diastolic blood pressure during quiet rest is below 80 mmHg. If you have high blood pressure, the diastolic number is often higher even during quiet rest. Diastolic blood pressure is considered dangerously low when it is 60 mmHg or lower and dangerously high when it is 110 mmHg or over. If you receive multiple readings with these numbers, it's a good idea to call your healthcare provider.

Low diastolic pressure may be seen with dehydration or with severe bleeding. It also may happen if the arteries relax and widen.

There are also multiple stages of high diastolic blood pressure:

• Diastolic BP of 80-89 is Stage 1 hypertension, which may be reversed with temporary meds and/or lifestyle changes.
• Diastolic BP of 90 or higher is Stage 2 hypertension, which can drastically increase the risk of stroke or heart attack may require a prolonged regimen of medication.
• Diastolic BP of 120 or higher means that you're in hypertensive crisis and should call your healthcare provider right away.

Your healthcare provider will help you determine the best course of action for treating your high systolic BP.

Improving Blood Pressure Reading Accuracy

Your systolic and diastolic pressures change depending on your activity level, stress, fluid intake, and other factors.

You need to do your best to limit how these other factors might change your pressure when you're taking a blood pressure reading.

For the most accurate reading, check your blood pressure when you are in a calm, warm space after you have been able to rest quietly for at least five minutes.

You should be relaxed, with your arms at your sides, and the cuff should be placed on your arm at about the level of your heart. Your legs should be uncrossed, and your bladder should be empty, as both of these factors can affect your reading.

Measuring blood pressure this way is a challenge in a busy provider's office. Your provider might suggest you take your blood pressure at home.

What Time of Day Should I Check My Blood Pressure?

Many experts suggest recording blood pressures that are taken over an extended time (which can include repeating the measurements at home) before diagnosing high blood pressure.

You should also know that your blood pressure will be different throughout the day. It tends to be highest in the morning and lower at night.

Your provider may want you to take your use a blood pressure monitor multiple days a day—usually once in the morning and once in the afternoon or evening. Try to avoid taking it early in the morning right when you wake up or right after dinner.

Instead, try to take your morning reading before your breakfast (especially if you drink coffee or take medication) and your evening reading when you're getting ready for bed (and again, before you take any medications).

Once you decide what time you'll check your blood pressure, it's important that you're consistent. You'll get the most accurate results and comparisons if you take your blood pressure at about the same time every day.

What Should My Blood Pressure Be?

Summary

Your blood pressure is a measurement of the pressures in your arteries while your heart is beating (systolic) and between beats (diastolic). Both of these values are important for diagnosing and managing high blood pressure.

Systolic and diastolic blood pressure are also important to know for treating a range of conditions, including heart disease or severe blood loss. It's important to get an accurate blood pressure reading under calm, quiet conditions.

Why Your Blood Pressure Changes Throughout the Day

Frequently Asked Questions

• Which is more important: Systolic blood pressure or diastolic blood pressure?

  Both systolic and diastolic blood pressure readings give important information about your health. However, systolic pressure is the number your doctor pays the greatest attention to if it is high.

• What is normal blood pressure by age?

  A blood pressure below 120/80 mmHg is ideal for most healthy adults. Children and teens may have slightly lower normal blood pressure. For example, a blood pressure of 100/65 could be considered normal for a toddler and a healthy, active teen could have a blood pressure of around 115/70.

  Older adults may also have higher or lower blood pressures that are considered normal. For example, a person who is in their 60s may have a blood pressure of around 130/60.

  Learn More: Chart: Average Blood Pressure by Age

• What is considered dangerously high blood pressure?

  Dangerously high blood pressure, also known as a hypertensive crisis, is when systolic blood pressure (the top number) is 180 or higher or diastolic blood pressure (the bottom number) is 120 or more.
  

• What is the best time to take your blood pressure?

  The specific time you take your blood pressure is not as important as being consistent about it, day after day. That said, it is best to avoid taking a reading first thing in the morning, after you have a meal, or right after you take medication.

  Which is more important your systolic or diastolic blood pressure?

  Over the years, research has found that both numbers are equally important in monitoring heart health. However, most studies show a greater risk of stroke and heart disease related to higher systolic pressures compared with elevated diastolic pressures.

  How many numbers should be between systolic and diastolic?

  The top number (systolic) minus the bottom number (diastolic) is the pulse pressure. For example, if the resting blood pressure is 120/80 millimeters of mercury (mm Hg), the pulse pressure is 40 — which is considered a healthy pulse pressure.

  What should be the lower systolic or diastolic?

  The first, or upper, number measures the pressure in the arteries when the heart beats (systolic pressure). The second, or lower, number measures the pressure in the arteries between beats (diastolic pressure). Normal blood pressure is below 120/80 mm Hg.

  What do the systolic and diastolic values represent?

  Systolic blood pressure, the top number, measures the force the heart exerts on the walls of the arteries each time it beats. Diastolic blood pressure, the bottom number, measures the force the heart exerts on the walls of the arteries in between beats.