Blood pressure is measured by detecting the force of blood pushing against artery walls, expressed as two numbers in millimeters of mercury (mmHg). The top number (systolic) captures pressure when the heart beats, and the bottom number (diastolic) captures pressure when the heart rests between beats. A normal reading falls below 120/80 mmHg. The measurement itself can happen several ways, from an inflatable cuff at your doctor’s office to a sensor inside an artery during surgery.
What the Two Numbers Mean
The 2025 guidelines from the American College of Cardiology and American Heart Association break blood pressure into four categories:
- Normal: below 120/80 mmHg
- Elevated: 120 to 129 systolic and below 80 diastolic
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic or 90 or higher diastolic
If your systolic and diastolic numbers fall into different categories, you’re classified in the higher one. So a reading of 135/75 counts as Stage 1 hypertension even though the bottom number looks fine.
Why We Still Use “Millimeters of Mercury”
The unit mmHg sounds oddly specific for a modern measurement. It dates back to a time before electronic sensors, when pressure was measured by how high it could push a column of liquid mercury in a glass tube. Mercury is 13.6 times denser than water, which kept the tubes a practical size. A blood pressure of 120 mmHg literally meant the pressure could support a mercury column 120 millimeters tall. Electronic devices have replaced the mercury, but the unit stuck because it works well for the range of pressures found in the human body.
Manual Measurement With a Stethoscope
The traditional method uses a cuff, a pressure gauge, and a stethoscope. A clinician wraps the cuff around your upper arm, inflates it until it temporarily stops blood flow through the artery, then slowly releases the pressure while listening through the stethoscope placed over the artery at the inside of your elbow.
As the cuff deflates, blood begins forcing its way through the compressed artery, producing distinct sounds called Korotkoff sounds. These move through five phases. Phase I is a clear, rhythmic tapping, and the pressure reading at that moment is your systolic number. As deflation continues, the tapping softens, gains a swishing quality, then returns sharper and louder. In Phase IV, the sounds become muffled and blowing. Phase V is silence: blood is now flowing freely with no turbulence. The pressure at that point of silence is your diastolic number.
How Automatic Monitors Work
Most home monitors and many clinic devices are oscillometric, meaning they detect vibrations rather than sounds. When the cuff inflates above your systolic pressure, no blood flows through the compressed artery. As the cuff slowly deflates, blood begins pushing the arterial wall open with each heartbeat, creating tiny vibrations. These vibrations travel through the air inside the cuff into an electronic sensor that converts them into electrical signals.
The vibrations grow stronger as cuff pressure drops into the range between systolic and diastolic, then fade once cuff pressure falls below diastolic and blood flows smoothly again. The device’s software analyzes the pattern of these vibrations to calculate both numbers. Newer models use adaptive algorithms to decide how high to inflate the cuff for each person, typically aiming for about 20 mmHg above the estimated systolic pressure. This keeps inflation time short and the measurement more comfortable.
Upper-Arm Cuffs vs. Wrist Monitors
Upper-arm cuffs are the standard for home monitoring because the brachial artery in the upper arm is large and close to heart level when your arm rests on a table. Wrist monitors measure the radial artery, which is smaller and farther from the heart. That distance matters: even small changes in wrist position relative to your heart can shift the reading significantly, and bending the wrist introduces additional error. Wrist monitors often produce falsely high readings because of poor positioning.
Wrist devices do serve a purpose, though. People with very large upper arms who can’t find a well-fitting arm cuff, or those who’ve had lymph nodes removed from the armpit, may need a wrist monitor. If you use one, keep your wrist at heart level, place the sensor directly over your pulse, and avoid bending the wrist during the reading.
Choosing the Right Cuff Size
Cuff size is one of the most overlooked sources of error. A cuff that’s too small for your arm will give artificially high readings, potentially by 10 mmHg systolic or more. Four adult sizes are available: small (for arm circumferences of 20 to 25 cm), regular (25 to 32 cm), large (32 to 40 cm), and extra-large (40 to 55 cm). You can measure your arm circumference at the midpoint between your shoulder and elbow with a flexible tape measure. Most home monitors come with a regular or large cuff, but if your arm doesn’t fit within the stated range, you’ll need a different size for accurate readings.
How to Get an Accurate Reading
The numbers on the screen are only as good as the conditions under which you measure. Research on common errors shows just how much everyday factors can throw off a reading:
- Talking or listening to someone talk: adds about 10 mmHg to both numbers
- Full bladder: raises systolic by roughly 15 mmHg and diastolic by 10
- Cuff placed over clothing: can add anywhere from 5 to 50 mmHg systolic
- Crossing your legs: raises systolic by 5 to 8 mmHg and diastolic by 3 to 5
- Back unsupported (sitting on an exam table): adds 6 to 10 mmHg systolic
- Arm hanging at your side instead of resting at heart level: inflates readings by up to 7 systolic and 11 diastolic
- Smoking within 30 minutes: raises systolic by 6 to 20 mmHg
To minimize these effects, the American Heart Association recommends sitting quietly in a chair with your back supported and feet flat on the floor for three to five minutes before measuring. Place the cuff on bare skin at the midpoint of your upper arm, and rest your arm on a surface so the cuff sits at heart level. Don’t talk during the measurement.
Invasive Monitoring in Critical Care
In intensive care units and major surgeries, a cuff isn’t precise or fast enough. Instead, a thin catheter is placed directly inside an artery, most commonly the radial artery at the wrist. This provides a continuous, beat-by-beat pressure reading on a monitor and doubles as a port for drawing blood samples. It’s reserved for patients on medications that rapidly change blood pressure, those undergoing high-risk surgery, or anyone whose condition demands constant tracking. You wouldn’t encounter this type of monitoring outside a hospital setting.