A blood pressure cuff displays two numbers: the top number (systolic) is the pressure in your arteries when your heart squeezes, and the bottom number (diastolic) is the pressure when your heart relaxes between beats. A normal reading falls below 120/80 mmHg. Whether you’re using a manual gauge with a dial or a digital monitor with a screen, the process of reading those numbers is straightforward once you know what to look for.
What the Two Numbers Mean
The top number, systolic pressure, captures the peak force of blood pushing against your artery walls each time your heart contracts. The bottom number, diastolic pressure, reflects the lowest pressure in your arteries just before the next heartbeat. Together they’re written as a fraction: systolic over diastolic, measured in millimeters of mercury (mmHg).
The gap between the two numbers also matters. Subtracting the bottom from the top gives you your pulse pressure. For a reading of 120/80, the pulse pressure is 40, which is considered healthy. A pulse pressure consistently above 60 is a risk factor for heart disease, particularly in older adults, because it suggests the arteries have stiffened and lost some of their natural elasticity.
Reading a Manual (Dial) Cuff
A manual blood pressure cuff has a round gauge with a needle, an inflatable cuff, a rubber bulb for pumping, and a stethoscope. The dial is marked in increments of 2 mmHg, with small lines representing each step. Larger numbered markings typically appear every 10 or 20 mmHg, depending on the gauge.
To take a reading, you inflate the cuff until the needle rises above where you expect the systolic number to be (usually around 180 mmHg for a first measurement). Then you release the air slowly, letting the needle fall at a rate of about 2 lines per second, while listening through the stethoscope placed over the artery on the inside of your elbow.
The first clear tapping sound you hear marks the systolic pressure. Note where the needle is pointing at that moment. As the cuff continues to deflate, the tapping sounds change: they soften, then return sharply, then muffle into a soft blowing quality. When the sounds disappear completely, note the needle position again. That’s your diastolic pressure. If the needle is between two lines, round up to the nearest even number.
What the Sounds Tell You
Those tapping and swishing sounds go through five distinct phases. Phase I, the first clear taps heard for at least two consecutive beats, gives you the systolic reading. Phases II through IV involve changes in the quality of the sound as blood flow gradually resumes through the compressed artery. Phase V, when all sound vanishes, gives you the diastolic reading. You only need to identify the first appearance and final disappearance of sound to get your two numbers.
Reading a Digital Cuff
Digital monitors do the listening for you. After the cuff inflates and deflates automatically, the screen displays your systolic pressure on top, your diastolic pressure below it, and your heart rate (pulse) as a separate number. Most devices show a small flashing heart symbol while the cuff is deflating, which pulses in time with your heartbeat to confirm it’s detecting a signal.
Some digital monitors include additional indicators: an irregular heartbeat symbol (often a heart with an uneven line through it), a color-coded bar along the side of the screen that corresponds to blood pressure categories, or a memory icon showing previous readings. If the cuff can’t get a reliable measurement, it will typically display an error code. This usually means the cuff was too loose, you moved during the reading, or the batteries are low. Reposition the cuff and try again.
What Your Numbers Mean
Blood pressure categories, based on guidelines from the American College of Cardiology and American Heart Association:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 with diastolic still below 80
- High blood pressure (Stage 1): systolic 130 to 139, or diastolic 80 to 89
- High blood pressure (Stage 2): systolic 140 or higher, or diastolic 90 or higher
Only one of the two numbers needs to be in a higher category for the overall reading to count as that category. So a reading of 135/75 is Stage 1 high blood pressure, even though the diastolic number looks fine. A single high reading doesn’t mean you have hypertension. The diagnosis is based on consistently elevated numbers across multiple measurements taken on different days.
Getting an Accurate Reading
The numbers on your cuff are only useful if the measurement is taken correctly. Small errors in setup can swing a reading by 10 to 20 mmHg, enough to push a normal result into the high range or hide a genuinely elevated one.
Cuff Size and Placement
Using the wrong cuff size is one of the most common sources of error. The American Heart Association recommends selecting cuff size based on your arm circumference:
- Small adult cuff: arm circumference 22 to 26 cm
- Standard adult cuff: 27 to 34 cm
- Large adult cuff: 35 to 44 cm
A cuff that’s too small will give a falsely high reading. A cuff that’s too large will read falsely low. You can measure your arm circumference with a flexible tape at the midpoint of your upper arm, between the shoulder and elbow. Place the bottom edge of the cuff about 2 to 3 cm (roughly three-quarters of an inch) above the bend of your elbow. Most cuffs have an arrow or marker that should line up with the artery on the inner side of your arm.
Body Position and Timing
Sit in a chair with your back supported, feet flat on the floor, and your arm resting on a table so the cuff is level with your heart. Crossing your legs can raise the reading. Rest quietly for 3 to 5 minutes before taking the measurement. If you’ve had caffeine, exercised, or smoked recently, wait at least 30 minutes. An international consensus panel recommends repeating the measurement later in the visit if you weren’t able to rest adequately beforehand.
Take two or three readings, each separated by about a minute, and average them. It’s common for the first reading to run a bit higher than the ones that follow. If you’re tracking blood pressure at home over time, measuring at the same time each day gives you the most consistent picture of your actual numbers.
Left Arm vs. Right Arm
Your blood pressure can differ slightly between arms. When you first start monitoring, take a reading on both sides. Use whichever arm gives the higher number for future measurements, since that’s the more conservative estimate. A difference of more than 10 mmHg between arms consistently is worth mentioning to your healthcare provider, as it can signal differences in the blood vessels supplying each arm.