A blood pressure monitor displays two numbers stacked on top of each other or separated by a slash. The top (larger) number is your systolic pressure, and the bottom (smaller) number is your diastolic pressure, both measured in millimeters of mercury (mm Hg). A reading of 120/80, for example, means 120 systolic over 80 diastolic. Understanding what these numbers mean, what ranges are healthy, and how to get an accurate reading at home takes just a few minutes to learn.
What the Two Numbers Mean
The top number, systolic pressure, measures the force in your arteries when your heart beats and pushes blood out. The bottom number, diastolic pressure, measures the force in your arteries between beats, when your heart is resting. Both numbers matter, but systolic pressure tends to get more attention because it rises steadily with age and is a strong predictor of cardiovascular risk in adults over 50.
Most home monitors also display a third number: your pulse rate, usually shown below or beside the blood pressure reading with a small heart icon. A normal resting pulse for adults falls between 60 and 100 beats per minute.
Blood Pressure Categories
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories:
- Normal: below 120/80 mm Hg
- Elevated: 120 to 129 systolic with diastolic still below 80
- 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 two numbers fall into different categories, the higher category applies. So a reading of 135/75 counts as Stage 1 hypertension because the systolic number is in that range, even though the diastolic number is normal.
Symbols on the Display
Beyond the numbers, your monitor may flash icons that are easy to overlook or misunderstand.
A heart symbol typically flashes while the device is actively measuring. On many monitors, if that heart icon stays on the screen after the reading is complete, or a separate irregular heartbeat indicator (sometimes labeled “IHB”) appears, it means the device detected an uneven rhythm during the measurement. Specifically, it flags a rhythm that varied by 25% or more from the average beat-to-beat interval. This can be triggered by something as simple as moving your arm, so it doesn’t necessarily signal a heart problem. If the symbol shows up repeatedly across multiple readings when you’re sitting still, bring it up with your doctor.
An “Err” or “E” code means the monitor couldn’t complete the reading. The most common cause is movement during the measurement. If you’re getting errors more than half the time, try loosening the cuff slightly, as overtightening is a frequent culprit.
How to Get an Accurate Reading
The numbers on your screen are only useful if the measurement itself is reliable. Small mistakes in preparation or positioning can skew a reading by 20 or 30 points, enough to make a normal blood pressure look dangerously high.
A full bladder alone can inflate your systolic number by up to 33 mm Hg. Having your arm hanging at your side or resting in your lap instead of at chest height can add 4 to 23 mm Hg. Even the stress of being in a medical setting (the “white coat effect”) can push systolic readings up by as much as 26 mm Hg. These aren’t small margins. They can shift a reading from one category to another entirely.
Before You Measure
Sit in a chair with your back supported for at least five minutes before starting. Put both feet flat on the floor and don’t cross your legs. Avoid caffeine, exercise, and smoking for at least 30 minutes beforehand. Empty your bladder. Don’t talk during the measurement.
Cuff Placement
Place the cuff on your bare upper arm, about one inch above the bend of your elbow. The tubing should run down the center of your inner arm. Tighten it so you can just slip two fingertips under the top edge of the cuff. Rest your arm on a table so the cuff sits at the same height as your heart. If your arm hangs lower, the reading will run artificially high.
Cuff size matters too. If the cuff is too small for your arm, it will overestimate your blood pressure. Most monitors come with a standard cuff that fits arm circumferences up to about 17 inches. If your upper arm is larger, you’ll need a wide-range or large cuff.
How to Track Your Numbers Over Time
A single reading is a snapshot, not a diagnosis. Blood pressure fluctuates throughout the day based on stress, food, hydration, and activity. What matters is the pattern over time.
The American Heart Association recommends taking readings twice a day: once in the morning before eating or taking medication, and once in the evening before bed. At each session, take two readings about one minute apart and record both. In the days leading up to a doctor’s appointment, do this for at least three days, preferably a full week. This gives your doctor a much clearer picture than any single office measurement.
Many digital monitors store 60 to 200 readings in memory or sync to a phone app. If yours doesn’t, a simple notebook works. Write down the date, time, systolic number, diastolic number, and pulse for each reading. Note anything unusual, like a stressful day or a skipped meal, that might explain an outlier.
When a Reading Is Dangerously High
A reading of 180/120 mm Hg or higher is considered a hypertensive crisis. If you see numbers in that range and have symptoms like chest pain, shortness of breath, severe headache, blurred vision, or seizures, call 911 immediately. These symptoms can signal life-threatening organ damage.
If your reading hits 180/120 but you feel fine, sit quietly for a few minutes and measure again. A single spike can result from stress, a full bladder, or poor cuff positioning. If the second reading is still that high, seek medical care even without symptoms.
Keeping Your Monitor Accurate
Home monitors can drift out of calibration over time. The American Heart Association recommends bringing your device to a medical appointment once a year so a healthcare professional can compare its readings against the equipment in the office. This also gives them a chance to check that you’re using the correct technique. Some monitor manufacturers recommend annual calibration as well, so check your device’s manual for specific guidance.