A blood pressure reading consists of two numbers written as a fraction, like 120/80 mmHg. The top number (systolic) measures the pressure in your arteries when your heart beats, and the bottom number (diastolic) measures the pressure between beats, when your heart is resting. Together, they tell you how hard your blood is pushing against your artery walls.
What the Two Numbers Mean
Systolic pressure is always the higher number because it captures the peak force of each heartbeat. Diastolic pressure is always lower because it reflects the baseline pressure your arteries maintain while the heart refills with blood. Both numbers matter, and they’re measured in millimeters of mercury (mmHg), a unit that dates back to the earliest pressure gauges.
The 2025 guidelines from the American Heart Association and American College of Cardiology 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, the higher category applies. So a reading of 138/78 would be classified as Stage 1 hypertension because of the systolic number, even though the diastolic is normal.
How a Manual Reading Works
When a nurse or doctor takes your blood pressure with a stethoscope and an inflatable cuff, they’re using a technique called auscultation. The cuff wraps around your upper arm and is inflated until it’s tight enough to completely stop blood flow through the brachial artery, the main artery in your inner arm. Standard protocol calls for inflating the cuff 30 mmHg above the point where your pulse disappears at the wrist.
Then the cuff slowly deflates. The person listening places the stethoscope directly over the brachial artery, just below the cuff’s edge. As pressure drops, blood starts forcing its way through the partially compressed artery, creating a thumping sound with each heartbeat. The pressure reading at the moment that first thump appears is your systolic number. As the cuff continues to loosen, the sounds eventually fade and disappear. The reading at that moment is your diastolic number.
How Digital Monitors Work
Automated monitors skip the stethoscope entirely. Instead, they detect tiny vibrations in the cuff itself. As the cuff deflates at a rate of about 2 to 4 mmHg per second, blood pulsing through the compressed artery causes small volume changes that ripple through the cuff’s air bladder. The device’s sensor picks up these oscillations and uses an algorithm to calculate your systolic and diastolic values from the pattern of increasing and then decreasing vibration strength.
This oscillometric method is what powers the digital monitors you find in pharmacies and for home use. It’s generally reliable, but the algorithms vary between manufacturers, which is why readings can differ slightly between devices.
Why Cuff Size Matters
The most common source of inaccurate readings is a poorly fitting cuff. A cuff that’s too small for your arm can overestimate systolic pressure by 5 to 20 mmHg, according to the American College of Cardiology. That’s enough to make a perfectly normal reading look like Stage 1 or even Stage 2 hypertension. A cuff that’s too large tends to read slightly low, typically by 1 to 6 mmHg.
Most cuffs are labeled with an arm circumference range. Measure around the midpoint of your upper arm to find your size. If you’re between sizes, go with the larger cuff. The bladder inside the cuff should wrap around at least 80% of your arm.
Getting an Accurate Reading
Your body position and recent activity can shift a reading by enough to change which category you fall into. The CDC recommends sitting in a chair with your back supported for at least five minutes before measuring. Both feet should be flat on the floor with your legs uncrossed. Rest the arm wearing the cuff on a table at chest height. If your arm hangs at your side or sits too low, gravity adds extra pressure and inflates the reading.
Wait at least 30 minutes after smoking, drinking caffeine or alcohol, or exercising. All of these temporarily raise blood pressure. An empty bladder also helps, since a full one can add several points to your reading. Don’t talk during the measurement.
Upper Arm vs. Wrist Monitors
The American Heart Association recommends upper arm cuffs as the most reliable option for home monitoring. Wrist monitors can be accurate when used exactly as directed, but they’re more sensitive to arm position. Your wrist needs to be held precisely at heart level, and even small deviations can throw off the reading.
Wrist monitors make sense in specific situations. If you have a very large arm and can’t find a well-fitting upper arm cuff, a wrist monitor is a reasonable alternative. They’re also useful for people who’ve had lymph nodes removed from the armpit area, where an upper arm cuff could cause discomfort or complications.
When Office and Home Readings Disagree
It’s common for blood pressure to read differently at a doctor’s office than at home, and the direction of that difference matters. White coat hypertension describes a situation where your readings run high in the clinic (130/80 or above) but normal at home (below 130/80). The stress of a medical visit is usually the cause. This pattern affects a meaningful portion of people who are told their blood pressure is elevated.
The opposite pattern, called masked hypertension, is more concerning. Your readings look normal in the office but run high the rest of the time. Because the elevated pressure goes undetected during routine visits, it can silently damage your heart, kidneys, and blood vessels. Home monitoring is the main way to catch it. If your doctor suspects either pattern, they may recommend ambulatory monitoring, where you wear a cuff that takes readings automatically throughout a full 24-hour day.
What a Single Reading Can and Can’t Tell You
Blood pressure fluctuates constantly. It rises when you’re stressed, active, or dehydrated, and drops when you’re relaxed or asleep. A single high reading doesn’t mean you have hypertension. Diagnosis typically requires elevated readings on at least two separate occasions, or a pattern of high readings from home monitoring over days or weeks.
If you’re tracking at home, take two or three readings one minute apart at the same time each day, ideally in the morning before medications and again in the evening. Record all the numbers. The average over several days gives a much more accurate picture of your cardiovascular health than any single measurement.