Blood pressure is measured in millimeters of mercury, written as mmHg. A reading is expressed as two numbers, such as 120/80 mmHg, where the top number represents pressure during a heartbeat and the bottom number represents pressure between beats.
Why Millimeters of Mercury
The unit traces back to the earliest blood pressure devices, which used a glass column filled with liquid mercury. When a cuff was inflated around a patient’s arm, the pressure pushed the mercury up the column. The height it reached, measured in millimeters, became the reading. Modern devices have largely replaced mercury columns with digital sensors or mechanical dials, but the unit stuck. When your reading says 120 mmHg, it means the pressure in your blood vessels is equivalent to the force needed to push a column of mercury up 120 millimeters.
What the Two Numbers Mean
The top number is your systolic pressure: the maximum force your blood exerts against artery walls when your heart contracts and pushes blood out. The bottom number is your diastolic pressure: the minimum force present just before the next contraction, when your heart is relaxed and refilling. Together, these two numbers describe the full cycle of pressure your cardiovascular system operates under.
Both numbers matter, but they can tell different stories. A high systolic number with a normal diastolic number is common in older adults as arteries stiffen with age. In younger people, an elevated diastolic reading can signal increased resistance in smaller blood vessels. Your doctor looks at both in context rather than focusing on one alone.
Normal Ranges and Categories
The American Heart Association defines normal blood pressure as a systolic reading below 120 mmHg and a diastolic reading below 80 mmHg. Here’s how the categories break down:
- Normal: below 120/80 mmHg
- Elevated: systolic between 120 and 129, with diastolic still below 80
- High blood pressure, stage 1: systolic between 130 and 139, or diastolic between 80 and 89
- High blood pressure, stage 2: systolic 140 or higher, or diastolic 90 or higher
- Hypertensive crisis: systolic above 180 and/or diastolic above 120, which can be life-threatening
Pregnancy uses slightly different thresholds. A reading of 140/90 mmHg or higher is classified as hypertension in pregnancy, and 160/110 mmHg or higher is considered severe.
How the Measurement Works
Whether you use a manual cuff in a clinic or an automatic monitor at home, the basic principle is the same. An inflatable cuff wraps around your upper arm and tightens until it temporarily stops blood flow through the artery beneath it. As the cuff slowly deflates, blood begins to push through again, and the device detects the pressure at two key moments.
With a manual device, a healthcare provider listens through a stethoscope placed over the artery just below the cuff. As the cuff loosens, they hear a series of sounds called Korotkoff sounds. The first clear tapping sound marks the systolic pressure, the point where blood first squeezes past the cuff. As deflation continues, those sounds shift through several phases, growing sharper, then suddenly muffling. The moment all sound disappears marks the diastolic pressure, the point where the artery is fully open and blood flows smoothly again.
Digital monitors use a sensor instead of a stethoscope, detecting tiny oscillations in cuff pressure caused by the pulse. They’re easier to use at home because they don’t require the trained listening skill that manual measurement demands. Both types report results in mmHg.
Why Cuff Size Affects Your Reading
One of the most common sources of error is using the wrong cuff size, and the impact is larger than most people realize. A randomized trial published in JAMA Internal Medicine found that using a cuff one size too small inflated the systolic reading by about 4.8 mmHg. Using a cuff two sizes too small pushed it up by nearly 20 mmHg, enough to make a person with normal blood pressure appear to have stage 1 hypertension. Going one size too large had a smaller but opposite effect, lowering the systolic reading by about 3.6 mmHg.
If you’re buying a home monitor, measure the circumference of your upper arm at its midpoint and check the sizing chart on the packaging. Most standard cuffs fit arms between about 22 and 36 centimeters. People with larger arms often need a wide-range or extra-large cuff to get accurate results.
Readings That Shift With the Setting
Your blood pressure isn’t a single fixed number. It fluctuates throughout the day based on activity, stress, caffeine, posture, and even the environment where it’s measured. Two well-recognized patterns can complicate diagnosis.
White-coat hypertension describes readings that come back high in a clinical setting but are normal when measured at home or over 24 hours with a portable monitor. The stress of being in a medical office is enough to temporarily raise blood pressure in some people. Masked hypertension is the opposite: readings look normal in the clinic but run high at home or during daily life. Both patterns are relatively common and can lead to missed diagnoses or unnecessary treatment if only clinic readings are considered.
This is one reason home monitoring is increasingly encouraged. Taking readings at the same time each day, sitting quietly for five minutes beforehand, with your arm supported at heart level, gives a much clearer picture of your true average blood pressure over time.