Blood pressure is measured by a device called a sphygmomanometer, which is the technical name for the inflatable cuff system you’ve seen at every doctor’s office. The most common versions are manual (used with a stethoscope) and digital automatic monitors. Both work by inflating a cuff around your upper arm, temporarily restricting blood flow, then detecting the pressure at which blood starts and stops making audible or detectable pulses in your artery. The two numbers you get, systolic over diastolic, represent the pressure on your artery walls when your heart squeezes and when it relaxes between beats.
Manual Blood Pressure Monitors
A manual sphygmomanometer has three main parts: an inflatable cuff, a rubber squeeze bulb, and a pressure gauge (either a dial or a mercury column). To use one, a clinician wraps the cuff around your bare upper arm, places the flat disk of a stethoscope just under the cuff on the inner side of your arm, and inflates the cuff by squeezing the bulb. This temporarily stops blood flow through the artery.
The clinician then slowly releases air from the cuff, about 2 millimeters per second on the gauge dial, while listening through the stethoscope. The first thumping sound they hear is the point where blood begins pushing past the cuff. The gauge reading at that moment is your systolic pressure, the higher number. They keep listening as the cuff deflates further until the thumping disappears entirely. That reading is your diastolic pressure, the lower number.
Manual monitors are still considered the gold standard for accuracy, which is why they remain common in clinical settings. The tradeoff is that they require training and a quiet environment to use correctly.
Digital (Automatic) Monitors
Digital monitors work on the same cuff-based principle but replace the stethoscope and human ear with an electronic sensor. You press a button, the cuff inflates automatically, and the device detects oscillations in pressure caused by your pulse. A microprocessor converts those oscillations into systolic and diastolic readings displayed on a screen.
These are the devices most people use at home. Upper-arm models are more reliable than wrist models because the arm artery is closer to heart level and easier to measure consistently. Many pharmacies also have sit-down kiosks with large digital cuffs that work the same way.
Why Cuff Size Matters
Using the wrong cuff size is one of the most common sources of inaccurate readings. A cuff that’s too small will overestimate your blood pressure, and one that’s too large will underestimate it. Recommended sizes are based on your mid-arm circumference: small adult cuffs fit arms 26 cm or less, standard adult cuffs fit 27 to 34 cm, large adult cuffs fit 35 to 44 cm, and extra-large cuffs are for arms above 44 cm. If you’re buying a home monitor, measure around the midpoint of your upper arm with a tape measure and match it to the cuff range on the packaging.
24-Hour Ambulatory Monitors
When a doctor needs a fuller picture of your blood pressure throughout the day, they may send you home with an ambulatory blood pressure monitor. This is a small device worn on a belt or strap, connected to a cuff on your arm, that inflates automatically at set intervals: typically every 15 to 30 minutes during the day and every 60 minutes at night. It records readings for a full 24 hours while you go about your normal routine, sleep included.
This type of monitoring is especially useful for detecting patterns that a single office reading can miss, like blood pressure that spikes only at night or stays elevated throughout the day despite appearing normal during appointments.
Arterial Lines in Critical Care
In intensive care units and high-risk surgeries, doctors sometimes need continuous, beat-by-beat blood pressure data. For this, they insert a thin catheter directly into an artery, usually at the wrist. This arterial line connects to an electronic sensor that displays a live pressure waveform on a bedside monitor. It’s the most precise method of measuring blood pressure, but it’s invasive and only used when moment-to-moment accuracy is medically necessary.
Cuffless Sensors in Wearables
Newer smartwatches and wearable devices are beginning to estimate blood pressure without a cuff. Most use light-based sensors on your wrist that shine LED light into the skin and measure how the reflected light changes with each heartbeat. As your arteries expand and contract, the amount of light absorbed by blood in the tissue shifts slightly. Software algorithms analyze these tiny changes, sometimes combined with the time it takes a pulse wave to travel between two points on the body, to estimate blood pressure.
These devices are improving but are not yet as reliable as cuff-based monitors. Most still require periodic calibration against a traditional cuff reading, and accuracy can vary with skin tone, motion, and how tightly the device sits on your wrist.
How to Get an Accurate Reading
Regardless of which device you use, preparation matters more than most people realize. The CDC recommends sitting in a comfortable chair with your back supported for at least 5 minutes before taking a reading. Both feet should be flat on the ground with your legs uncrossed. Rest the arm wearing the cuff on a table at chest height. Don’t talk during the measurement. Even casual conversation can raise your reading by several points.
Caffeine, exercise, and a full bladder can all temporarily elevate blood pressure. For the most reliable results, avoid caffeine for 30 minutes beforehand and empty your bladder first. If you’re tracking blood pressure at home, take readings at the same time each day and record two or three measurements a minute apart, then average them.
What the Numbers Mean
Blood pressure is recorded in millimeters of mercury (mmHg) as two numbers: systolic over diastolic. The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories for adults based on office readings:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 and diastolic below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher, or diastolic 90 or higher
If your systolic and diastolic numbers fall into two different categories, the higher category applies. A reading of 135/75, for example, counts as stage 1 hypertension because the systolic number is in that range, even though the diastolic number is normal.