What Do Blood Pressure Numbers Mean? Top vs Bottom Number

A blood pressure reading is two numbers written with a slash, such as 120/80 mm Hg. The first number is the systolic pressure, the force of the blood flow when blood is pumped out of the heart. The second is the diastolic pressure, measured between heartbeats while the heart fills with blood. A healthy systolic pressure is less than 120 mm Hg and a healthy diastolic pressure is less than 80 mm Hg, and blood pressure is high when readings are consistently 130 or higher systolic, or 80 or higher diastolic, according to NHLBI: What Is High Blood Pressure?.

The unit, mm Hg, stands for millimeters of mercury. The same page says a reading of 120/80 mm Hg can be spoken as "120 over 80." It also says blood pressure changes throughout the day based on your activities.

What do the systolic and diastolic numbers measure?

MedlinePlus describes systolic pressure as the first and higher number, the pressure inside your arteries when your heart beats. Diastolic pressure is the second and lower number, the pressure when your heart rests between beats.

As the pressure falls, the reading when the sound of blood pulsing is first heard is recorded as the systolic pressure. The point at which the sound stops is recorded as the diastolic pressure, as the MedlinePlus encyclopedia entry on blood pressure measurement explains.

One or both numbers can be too high, the same entry notes. For older adults, the first number is often 130 or higher while the second number is less than 80. This is called isolated systolic hypertension, it is due to age-related stiffening of the major arteries, and it is the most common form of high blood pressure in older adults, according to the National Institute on Aging: High Blood Pressure and Older Adults.

The 10-year heart attack risk tool on this site has an input labeled "Systolic Blood Pressure." Its title is "Risk Assessment Tool for Estimating Your 10-year Risk of Having a Heart Attack."

Which category does a blood pressure reading fall into?

Two of the pages cited here print a five-category chart. The table puts them side by side: the first four categories read the same on both, and the fifth is joined by a different word on each.

The joining word decides the category. Normal and elevated need both numbers in range, because their rows say "and." Stage 1 and stage 2 need only one number in range, because their rows say "or." Take a reading of 126/84 and check it against the table:

1. The systolic number, 126, falls in the 120 to 129 band, which is the elevated row.
2. The diastolic number, 84, falls in the 80 to 89 band, which is the stage 1 row.
3. The elevated row requires 120 to 129 and a diastolic number less than 80. Because 84 is not less than 80, the reading fails that row.
4. The stage 1 row requires 130 to 139 or 80 to 89. The diastolic number meets the second half, so 126/84 is stage 1.

The two charts word the crisis category differently. The institute's chart joins "Higher than 180" and "Higher than 120" with OR. The MedlinePlus chart joins them with "and." This guide reports both.

The MedlinePlus encyclopedia entry writes the stages as pairs: 130/80 or higher but lower than 140/90 mm Hg is stage 1, and 140/90 mm Hg or higher is stage 2. The chart on the MedlinePlus test page, shown in the table above, lists stage 1 as 130 - 139 or 80 - 89 and stage 2 as 140 or higher or 90 or higher.

The JNC 7 Express slide text kept on this site says the JNC 7 Express was published in JAMA on May 21, 2003. Its slide 10, "Blood Pressure Classification," prints these rows: normal <120 and <80, prehypertension 120–139 or 80–89, stage 1 hypertension 140–159 or 90–99, and stage 2 hypertension 160 or 100. A reading of 135/85 sits in the prehypertension row of that slide (135 is in 120–139) and in the stage 1 row of the table above (135 is in 130 to 139).

Two more pages on this site give figures. One, headed High Blood Pressure in the United States, says a blood pressure reading of 140/90 mmHg or higher is considered high and that normal blood pressure is less than 120/80 mmHg. The other, headed "Join the Mission and Know Your Numbers for National High Blood Pressure Education Month," says of your blood pressure numbers: "Hopefully, they are less than 120/80 mmHg, which is normal."

Where does low blood pressure start?

The aging institute's page defines low blood pressure, or hypotension, as systolic pressure lower than 90 or diastolic pressure lower than 60. The MedlinePlus test page says that if you were tested for low blood pressure, a reading of 90/60 mmHg or lower is considered abnormal.

The MedlinePlus encyclopedia entry on vital signs lists the normal blood pressure range for the average healthy adult while resting as between 90/60 mmHg and 120/80 mmHg. That entry also says normal vital signs change with age, sex, weight, exercise capability, and overall health.

Do the same numbers apply to children?

For children younger than 13, readings are compared with readings that are common for children of the same age, sex and height, the institute's diagnosis page says. The MedlinePlus test page says the same: for a child younger than 13, the provider compares the readings with what is common for children the same age, sex, and height.

How is a blood pressure reading taken properly?

Several things can increase a blood pressure measurement, the MedlinePlus test page says. The checklist below combines steps from the institute's diagnosis page and the MedlinePlus measurement entry, both cited above, and names which page says what.

1. Clear 30 minutes. The institute's page says not to exercise, drink coffee or smoke cigarettes for 30 minutes before the test. The MedlinePlus entry lists stress, a meal, caffeine and tobacco in the past 30 minutes, and the MedlinePlus test page lists alcohol.
2. Empty your bladder. The institute's page says to go to the bathroom before the test, and the MedlinePlus entry says to empty your bladder beforehand.
3. Sit for at least 5 minutes. The institute's page says to sit in a chair and relax for at least 5 minutes. The MedlinePlus entry says to rest for at least 5 minutes and that 10 minutes is better.
4. Set your posture. The MedlinePlus entry says to sit with your back against the back of a chair, your feet flat on the floor and your legs uncrossed.
5. Stay quiet. The institute's page says not to talk while you are relaxing or during the test.
6. Bare and support the arm. The institute's page says to uncover your arm for the cuff and rest it on a table so it is supported and at the level of your heart. The MedlinePlus entry says the lower edge of the cuff should be 1 inch (2.5 cm) above the skin fold at the bend of the elbow.
7. Repeat. The MedlinePlus entry says to take 2 or 3 readings at a sitting, 1 minute apart, and to remain seated.

Readings can be different if the cuff is too small or too big, the institute's page says. For taking blood pressure at home, the MedlinePlus test page says a cuff that goes around the upper arm is more accurate than the ones that measure on the wrist or finger.

On the institute's page, "white coat hypertension" refers to readings that are higher in a provider's office than readings at home or in a pharmacy. "Masked high blood pressure" is its term for the case where people with elevated blood pressure levels show lower readings when measured at a provider's office, and it says a provider can recognize this by looking at readings taken at the office and at home.

For home checks, the MedlinePlus entry describes a typical schedule: before breakfast and before dinner for 5 days, which yields at least 10 readings. It also says blood pressure is usually lowest when you are sleeping and that it is normal for it to increase suddenly when you wake up.

A page on this site headed "General Guide for High Blood Pressure Screening, Referral, and Follow Up," the screening and referral guide, says that at screenings trained volunteers measure blood pressures and that those with elevated readings are referred to a doctor or clinic for further evaluation and diagnosis.

Which blood pressure reading is an emergency?

Call 911 or get immediate medical attention if your blood pressure is over 180/120 mmHg, or if you have chest pain, shortness of breath, back pain, numbness or weakness, trouble speaking, or vision changes. That instruction comes from the MedlinePlus test page. The institute's diagnosis page says readings above 180/120 mm Hg are dangerously high and require immediate medical attention.

The MedlinePlus test page also says to seek immediate medical attention or call 911 for symptoms of extremely low blood pressure, such as confusion, a weak and rapid pulse, or cold skin.

The institute's page says two or more readings at separate medical appointments are needed to diagnose high blood pressure. This guide is general education, and what a specific reading means for you is a question for your own health care provider.

How often should blood pressure be checked?

The institute's diagnosis page says everyone age 3 or older should have their blood pressure checked by a healthcare provider at least once a year. The MedlinePlus measurement entry lists once a year for adults aged 40 years and older and for people at increased risk for high blood pressure, and every 3 to 5 years for adults aged 18 to 39 years with blood pressure lower than 120/80 mm Hg who do not have other risk factors.

Half of all American adults have high blood pressure, and about 1 in 3 U.S. adults who have it are not aware of it, according to the institute's page cited in the first paragraph. The same page says symptoms from high blood pressure don't usually occur until it causes serious health problems. More pages are listed on this site's blood pressure topic hub.