What Do the Two Blood Pressure Numbers Mean?
A blood pressure reading is written as two numbers separated by a slash — for example, 118/76. Both numbers describe pressure inside your arteries, measured in millimeters of mercury, abbreviated mm Hg. The unit is a leftover from the original instruments, which used a column of mercury: the higher the pressure, the higher the column rose.
The top number: systolic pressure
The first (top) number is systolic pressure — the pressure in your arteries at the moment the heart contracts and pushes blood out. It is the peak of each heartbeat's pressure wave, which is why it is always the larger of the two numbers.
The bottom number: diastolic pressure
The second (bottom) number is diastolic pressure — the pressure in your arteries between beats, while the heart is relaxing and refilling. Arteries never fully depressurize between beats, so this number is never zero; it is the floor the pressure falls to before the next contraction.
How readings are categorized
Clinicians do not judge a reading as simply "good" or "bad" — they place it in named categories. According to the National Heart, Lung, and Blood Institute, a reading below 120/80 mm Hg is considered normal for most adults, and high blood pressure (the medical term is hypertension) is generally diagnosed when readings are consistently 130/80 mm Hg or higher. Two things in that sentence carry real weight:
- Consistently. Blood pressure moves around all day — with activity, stress, caffeine, even talking during the measurement. A single elevated reading is a data point, not a diagnosis. Clinicians look at patterns across multiple readings, sometimes including measurements taken at home or worn monitors.
- Diagnosed. Categorizing a reading is a clinical judgment that accounts for your history, medications, and how the measurement was taken. That is why this page explains the vocabulary and stops there.
You may also hear elevated used as a category name for readings between normal and hypertension, and stage 1 and stage 2 used to describe degrees of high blood pressure. If a clinician uses those terms, they are referring to this same classification system, and it is completely reasonable to ask them where your readings fall and why.
Why one number can matter without the other
A reading can be high in only one of its two numbers. Clinicians have terminology for this too — for instance, isolated systolic hypertension describes a high top number with a normal bottom number. The existence of these terms is a useful reminder that both numbers are read together, in context, by someone trained to do it.
Terms you'll meet alongside the numbers
- Hypertension — the medical term for high blood pressure.
- Hypotension — abnormally low blood pressure.
- White coat effect — readings that run higher in a medical office than elsewhere, a recognized measurement phenomenon.
- Cuff — the inflatable band used to take the measurement; its size and placement affect accuracy, which we cover in our page on home blood pressure monitor specifications.
For the full clinical picture — causes, risk factors, and management — go to the primary sources: the NHLBI's high blood pressure overview and the American Heart Association's health topics library. And for what any particular reading means for you, ask the person taking it.