A blood pressure reading has two numbers written as one over the other, like 120/80. The top number (systolic) is the pressure inside your arteries when your heart beats and pushes blood out. The bottom number (diastolic) is the pressure when your heart relaxes between beats. Both numbers are measured in millimeters of mercury, abbreviated as mmHg.
What the Two Numbers Mean
Think of your arteries as flexible tubes. Every time your heart contracts, it sends a wave of blood through those tubes, stretching them outward. The peak pressure during that squeeze is your systolic number. Between beats, when your heart refills, the pressure drops to its lowest point. That’s your diastolic number.
Both numbers matter, but they can tell you different things. A high systolic number with a normal diastolic number is common as arteries stiffen with age. A high diastolic number can signal that your blood vessels are under constant tension even when the heart is resting. Your overall cardiovascular risk depends on whichever number falls into the higher category.
Blood Pressure Categories
The American Heart Association and American College of Cardiology define four main ranges for adults:
- Normal: below 120 systolic and below 80 diastolic
- 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 is the one that applies. So a reading of 135/75 counts as Stage 1 hypertension because of the systolic number, even though the diastolic is normal.
A single high reading doesn’t mean you have hypertension. Guidelines recommend averaging at least two readings taken on two or more separate occasions to get a reliable picture. Blood pressure fluctuates throughout the day, so one snapshot can be misleading.
Common Mistakes That Skew Your Reading
Small errors in how you take a reading can shift the numbers enough to push you into a different category entirely. Some of the most common culprits, with their approximate effects:
- Placing the cuff over clothing: can add up to 50 mmHg depending on sleeve thickness
- Using a cuff that’s too small: can add 2 to 10 mmHg
- Talking or actively listening: can add 10 mmHg
- Crossing your legs: can raise systolic pressure by 2 to 8 mmHg
- Sitting without back support (like on an exam table instead of a chair): can increase diastolic pressure by about 6 mmHg
- Letting your arm hang at your side or rest below heart level: can inflate the reading by 10 mmHg or more, roughly 2 mmHg for every inch below heart level
Caffeine, recent exercise, and a full bladder can also push readings higher, though the exact amount varies from person to person. The safest bet is to sit quietly for five minutes before measuring, with both feet flat on the floor, your back against a chair, and your arm resting on a flat surface at chest height.
Choosing and Fitting a Monitor
For home use, an automated (digital) upper-arm monitor is the standard recommendation. Manual monitors with a squeeze bulb and stethoscope require trained technique and tend to produce systolic readings 13 to 15 mmHg higher than automated monitors in people with hypertension. Current blood pressure guidelines are actually based on automated readings, so using a digital monitor at home gives you numbers that line up more closely with the thresholds your doctor uses.
Cuff size is one of the most overlooked factors. Four adult cuff sizes are available in the United States, based on mid-arm circumference: small (20 to 25 cm), regular (25.1 to 32 cm), large (32.1 to 40 cm), and extra-large (40.1 to 55 cm). To find your size, wrap a flexible tape measure around the midpoint of your upper arm, halfway between your shoulder and elbow. A cuff that’s too tight will artificially raise your reading.
How to Track Your Numbers Over Time
A single pair of numbers is less useful than a pattern. The CDC recommends taking your blood pressure at the same time each day, with at least two readings spaced one to two minutes apart. Record both readings and note the date, time, and which arm you used. Many digital monitors store this data automatically or sync with a phone app.
Morning readings, taken before coffee or exercise, tend to give the most consistent baseline. If your doctor has asked you to monitor at home, they may also want an evening reading. Keeping a log over one to two weeks gives a much clearer picture than any single visit to a clinic, where anxiety alone can temporarily spike your numbers (a phenomenon known as white coat hypertension).
When a Reading Is an Emergency
A reading of 180/120 mmHg or higher is considered a hypertensive crisis. If you see numbers in that range and also have symptoms like chest pain, severe headache, vision changes, confusion, difficulty speaking, sudden weakness on one side of your body, or heart palpitations, call 911. These signs suggest the extreme pressure is actively damaging organs like the brain, heart, or kidneys.
If your reading is that high but you feel fine, wait five minutes and measure again. A single very high reading without symptoms still warrants a call to your doctor the same day, but it’s not the same level of emergency as one paired with chest pain or neurological symptoms.