A blood pressure reading is two numbers written as a fraction, like 120/80 mmHg. The top number (systolic) measures the force of blood against your artery walls when your heart beats. The bottom number (diastolic) measures that same force when your heart rests between beats. Both numbers matter, and understanding what they mean puts you in a much better position to track your heart health over time.
What the Two Numbers Tell You
Think of your cardiovascular system as a pressurized loop. Every time your heart contracts, it sends a wave of blood through your arteries, and the walls of those arteries stretch to absorb the force. That peak moment of pressure is your systolic number. In the brief pause before the next beat, pressure drops but never hits zero because the elastic recoil of your arteries keeps blood moving forward. That resting pressure is your diastolic number.
The unit “mmHg” stands for millimeters of mercury, a holdover from the original mercury-column gauges used to measure pressure. You don’t need to worry about the unit itself. What matters is where your numbers fall in the categories below.
Blood Pressure Categories for Adults
The American Heart Association and American College of Cardiology updated their guidelines in 2025. Here are the current categories:
- 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
One important rule: if your systolic and diastolic numbers fall into two different categories, the higher category applies. So a reading of 135/72 counts as stage 1 hypertension even though the bottom number looks normal, because the top number lands in the 130 to 139 range.
A reading of 180/120 or higher is a hypertensive crisis. If you see numbers that high and you’re experiencing symptoms like chest pain, blurred vision, confusion, or severe anxiety, that’s a medical emergency. If you have no symptoms, wait five minutes and measure again. Persistently high readings at that level still need same-day medical attention.
Pulse Pressure: A Hidden Third Number
Subtracting your diastolic number from your systolic number gives you a value called pulse pressure. For a reading of 120/80, pulse pressure is 40 mmHg. This number reflects how much force each heartbeat generates and how stiff or flexible your arteries are.
A normal pulse pressure sits around 40 mmHg. Once it climbs to 50 or above, your risk of heart disease, stroke, and irregular heart rhythms starts to increase. Every 10 mmHg increase in pulse pressure raises the risk of coronary artery disease by about 23%. A pulse pressure above 100 is considered truly widened and can signal problems like a leaky heart valve or significant arterial stiffness.
On the other end, a very narrow pulse pressure (one-quarter or less of your systolic number, so 30 or below for someone with a systolic of 120) can indicate the heart isn’t pumping enough blood. This is less common in everyday home monitoring but worth knowing if your numbers ever look unusually close together.
Why a Single Reading Can Be Misleading
Blood pressure fluctuates constantly. Stress, a full bladder, a recent cup of coffee, or even sitting in a doctor’s office can temporarily push your numbers higher. This is why a diagnosis of high blood pressure is never based on one reading.
White coat hypertension, where your numbers run high in a clinical setting but are normal at home, affects 15% to 30% of people with elevated office readings. The reverse also happens: some people have normal readings at the doctor’s office but consistently high readings at home, a pattern called masked hypertension. Both situations make home monitoring essential for getting an accurate picture.
How to Get an Accurate Reading
The CDC recommends a specific routine to minimize variables that can skew your numbers:
- Avoid food, drinks, caffeine, alcohol, and smoking for 30 minutes before measuring.
- Sit quietly for at least 5 minutes with your back supported in a comfortable chair.
- Keep both feet flat on the floor with your legs uncrossed.
- Rest your arm on a table at chest height with the cuff positioned on bare skin.
Cuff size makes a real difference. A cuff that’s too small will give falsely high readings, and one that’s too large will read low. Measure around the midpoint of your upper arm and match it to the right size: 22 to 26 cm needs a small adult cuff, 27 to 34 cm fits a standard adult cuff, 35 to 44 cm requires a large adult cuff, and 45 to 52 cm calls for an extra-large.
How Often to Monitor at Home
If you’re tracking your blood pressure for a new concern or after a medication change, the recommended protocol is to take two readings at least one minute apart, both in the morning and evening. That’s four readings per day. Do this for seven days (28 total readings), with a minimum of three days if seven isn’t practical. Many guidelines suggest throwing out the first day’s readings because they tend to run higher due to unfamiliarity with the process, so planning for eight days total is ideal.
Once your blood pressure is stable and well controlled for several months, measuring one to three days per week is generally enough to catch any trends. Write down every reading or use a monitor that stores results automatically. The pattern over days and weeks matters far more than any individual number.
Blood Pressure in Children
The adult categories above don’t apply to children. For anyone under 18, blood pressure is evaluated using percentile tables that account for age, sex, and height. A child’s reading is compared against what’s typical for other children of the same size and age, rather than against a fixed cutoff like 120/80. Your pediatrician uses these tables at every well-child visit, so there’s no need to interpret a child’s numbers using adult standards.