Your Blood Pressure Numbers, Explained
What the two numbers mean, when they matter, and why the reading at the pharmacy kiosk keeps scaring people unnecessarily.
Blood pressure is the most measured number in medicine and one of the least explained. Here is what the two numbers are and when they actually matter.
What the numbers are
The reading is written as one number over another, for example 128 over 78.
The top number, systolic, is the pressure in your arteries when your heart contracts and pushes blood out.
The bottom number, diastolic, is the pressure that remains between beats, while the heart is refilling.
Both are measured in millimeters of mercury, which is why you sometimes see mm Hg after them.
The ranges
Current categories used by most clinicians in the United States:
- Normal: under 120 and under 80
- Elevated: 120 to 129, and under 80
- Stage 1: 130 to 139, or 80 to 89
- Stage 2: 140 or higher, or 90 or higher
- Hypertensive crisis: over 180, or over 120
Note the “or” in those. If your top number is 142 and your bottom is 76, that is stage 2, driven by the systolic alone.
Why one reading is not a diagnosis
Blood pressure moves constantly. It rises when you are talking, when you have just walked in from the parking lot, when you are anxious about being at a doctor’s office, after coffee, after a cigarette, and when your bladder is full. It falls when you sleep.
Diagnosing hypertension from one clinic reading would misclassify a large number of people in both directions. That is why a first high reading should be rechecked, and why a series of readings taken at home over a week or two is worth more than any single number taken here.
Measuring it properly at home
Most home readings are wrong in a predictable direction, because of how they are taken rather than the machine.
- Sit for five minutes first, feet flat on the floor, back supported
- Arm resting at heart level on a table, not held up
- Cuff on bare skin, not over a sleeve
- No talking during the measurement
- Nothing to eat, drink, smoke or exercise for 30 minutes beforehand
- Take two readings a minute apart and record both
Use an upper arm cuff. Wrist and finger devices are consistently less reliable. Bring the machine to your next visit and we can check it against ours.
When a number becomes a plan
Stage 1 in someone with otherwise low cardiovascular risk often starts with lifestyle changes and a recheck date rather than a prescription. Reducing sodium, moving regularly, limiting alcohol and losing modest weight all have measurable effects.
Stage 2, or stage 1 with diabetes, kidney disease or established cardiovascular disease, usually means starting medication alongside those changes.
The part that matters is that there is a recheck date. Blood pressure that gets measured once, called high, and never followed up is the most common failure in this entire area.
Why any of this matters
Untreated high blood pressure does its damage silently over years, to the heart, the kidneys, the brain and the eyes. There are no symptoms until there are, and by then the damage is often the symptom.
That is the entire argument for measuring it on a schedule even when you feel fine.
Our high blood pressure page covers how we manage it, and you can book a visit to get a real baseline.
Common questions
Which number matters more, the top or the bottom?
In adults over about 50, the top number generally carries more predictive weight, largely because arteries stiffen with age. Under 50 the bottom number matters relatively more. Both get treated when they are high.
My reading is always higher at the doctor's office. Is that real?
It is a well described effect and it is common. That is exactly why home readings over a couple of weeks are more useful than a single clinic number, and why we ask for them before starting a medication.
How high is high enough to worry about today?
A reading at or above 180 systolic or 120 diastolic, particularly with chest pain, shortness of breath, weakness, vision change or trouble speaking, is an emergency. Call 911. Without symptoms, recheck after resting and call the office promptly.
This article is general health information for adults age 16 and older, not medical advice for your situation, and reading it does not create a patient relationship. For advice about your own health, book a visit. If you think you are having an emergency, call 911.