A blood pressure meter displays two numbers stacked on top of each other, and understanding what each one means takes about 30 seconds. The larger number on top is your systolic pressure, the smaller number on the bottom is your diastolic pressure, and most digital monitors also show a third number for your pulse rate. Here’s how to make sense of all three, plus the symbols and error codes you might see.
What the Two Main Numbers Mean
The top number (systolic) measures the pressure inside your arteries when your heart squeezes and pushes blood out. The bottom number (diastolic) measures the pressure between beats, when your heart is relaxed. Both are displayed in millimeters of mercury, abbreviated as mm Hg, though your monitor may not show the unit on screen.
You’ll see these written together with a slash, like 120/80. Most digital monitors display the systolic number in larger font near the top of the screen and the diastolic number below it in slightly smaller font. Some models label them “SYS” and “DIA.” The third number, usually at the bottom or off to one side and marked with a heart icon, is your pulse rate in beats per minute.
What Your Numbers Actually Tell You
The American College of Cardiology and the American Heart Association use these ranges:
- 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
Only one number needs to be in a higher range to place you in that category. So a reading of 138/75 counts as stage 1 hypertension even though the bottom number is normal. A single high reading doesn’t mean you have high blood pressure. What matters is the pattern across multiple readings taken over days or weeks.
Symbols and Icons on the Screen
Many monitors display a small flashing heart icon that blinks in time with your pulse during the reading. This is normal. A different symbol to watch for is an irregular heartbeat indicator, which looks like a heart with a jagged line through it or a small “IHB” label. The monitor triggers this when it detects pulse intervals that deviate more than 25% from your average during the reading. If you see it once, rest for two to three minutes and measure again. If it appears on two out of three consecutive readings, it’s worth mentioning to your doctor.
Some monitors also display a movement error icon (often a crossed-out body or arm) if you shifted during the measurement. Any movement can throw off the reading, so treat it as a prompt to sit still and try again.
Common Error Codes
If your monitor shows “Err” or a numbered error code instead of a reading, the problem is almost always cuff-related or movement-related. Here are the most common causes:
- Unstable pressure at start (code 000 on some models): you moved as the cuff began inflating. Stay still and retry.
- No systolic or diastolic detected (codes 001, 002): the monitor couldn’t pick up a clear signal. Usually caused by arm movement or a loose cuff.
- Cuff too tight or blocked (code 003): reposition the cuff so it sits snugly but not painfully, about one finger-width above your elbow crease.
- Air leak or cuff too loose (code 004): tighten the cuff and make sure the tubing isn’t kinked.
If you keep getting errors, check that you’re using the right cuff size. CDC data shows four standard adult sizes based on mid-arm circumference: small (26 cm or less), standard adult (27 to 34 cm), large adult (35 to 44 cm), and extra-large (over 44 cm). A cuff that’s too small will give artificially high readings, while one that’s too large may give low readings or fail to register at all. Measure around the middle of your upper arm with a flexible tape to find your size.
How to Get an Accurate Reading
The numbers on your screen are only useful if the measurement itself was taken correctly. Position matters more than most people realize. Sit in a chair with your back supported for at least five minutes before pressing start. Put both feet flat on the floor with your legs uncrossed. Rest the arm wearing the cuff on a table so the cuff sits at chest height. If your arm dangles at your side or rests in your lap, the reading can be off by 5 to 10 points.
Don’t talk during the measurement. Don’t check your phone. The cuff will inflate, pause, then slowly deflate. Your numbers appear once deflation is complete, usually within 30 to 60 seconds.
How Often to Take Readings
When you first start tracking, take your blood pressure at least twice a day: once in the morning before eating or taking any medication, and once in the evening. Each time, take two or three readings spaced one to three minutes apart and note all of them. The first reading is often a bit higher, so the average of the second and third tends to be more representative.
Once your blood pressure is stable and well-controlled, you can usually reduce to once a day or even a few times a week. Keep a simple log with the date, time, and both numbers from each reading. Many digital monitors store your last 60 to 100 readings automatically, and some sync to a phone app, but a written log is just as effective for spotting trends.
Reading a Manual Monitor
If you’re using a manual (aneroid) monitor with a stethoscope, the process is different. You inflate the cuff by hand, then slowly release the valve while listening through the stethoscope placed over the artery inside your elbow. The first clear tapping sound you hear marks the systolic pressure. Read the number on the dial at that exact moment. As you continue deflating, the tapping sounds change character and eventually disappear completely. The point where all sound vanishes is your diastolic pressure. Manual monitors require practice and good hearing, which is why most home users prefer digital models.
Keeping Your Monitor Accurate
Digital monitors can drift out of calibration over time. The American Heart Association recommends bringing your monitor to a healthcare appointment once a year so the readings can be compared against a clinical-grade device. If your home readings consistently run 5 or more points higher or lower than your doctor’s office reading (taken on the same arm, same day), your monitor may need recalibration or replacement. Most manufacturers offer calibration checks, and some pharmacies do as well.