A blood pressure reading has two numbers, written as one over the other, like 120/80. The top number (systolic) measures the pressure in your arteries when your heart pumps blood out. The bottom number (diastolic) measures the pressure between beats, when your heart is relaxing and refilling with blood. Both numbers are recorded in millimeters of mercury (mmHg), and together they tell you how hard your blood is pushing against your artery walls.
What the Top Number Tells You
The top number, systolic pressure, captures the peak force your blood exerts each time your heart contracts. This is the moment of highest pressure in your cardiovascular system. It tends to climb with age because arteries gradually lose their flexibility, becoming stiffer and less able to absorb the surge of each heartbeat. People with diabetes or chronic kidney disease also tend to develop stiffer arteries earlier.
Systolic pressure is the number doctors pay the most attention to for cardiovascular risk, especially after age 50. A reading under 120 mmHg is considered normal. Once it consistently hits 130 or higher, it falls into the hypertension range.
What the Bottom Number Tells You
The bottom number, diastolic pressure, reflects the baseline pressure in your arteries during the brief pause between heartbeats. Think of it as the “resting” pressure your blood vessels experience. A normal diastolic reading is below 80 mmHg.
In younger adults, an elevated diastolic number can be an early sign of high blood pressure, often driven by factors like excess weight, high sodium intake, or stress. After about age 50, diastolic pressure tends to drift downward even as systolic pressure rises. This widening gap between the two numbers has its own health significance, which is covered below.
Blood Pressure Categories
The 2025 guidelines from the American Heart Association and the American College of Cardiology divide blood pressure into four categories:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 and diastolic below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher, or diastolic 90 or higher
If your two numbers fall into different categories, the higher category is the one that applies. For example, a reading of 138/76 would be classified as stage 1 hypertension because of the systolic number, even though the diastolic number is normal.
A reading of 180/120 or higher is a hypertensive crisis. If you see numbers that high along with symptoms like chest pain, blurred vision, confusion, or severe anxiety, that requires emergency medical attention.
The Gap Between the Two Numbers
The difference between your systolic and diastolic readings is called pulse pressure. You can calculate it by subtracting the bottom number from the top. For a reading of 120/80, the pulse pressure is 40 mmHg. A typical healthy pulse pressure falls in the range of 40 to 60.
A wide pulse pressure, where the gap between the two numbers is large, often shows up in older adults as arteries stiffen. It can also indicate problems with a heart valve or plaque buildup in the arteries. A very narrow pulse pressure, roughly one-quarter or less of your systolic number, can signal that the heart isn’t pumping enough blood. This sometimes occurs with heart failure or significant blood loss.
Why Your Numbers Can Vary
Blood pressure isn’t a fixed number. It fluctuates throughout the day based on activity, stress, caffeine, hydration, and even the temperature of the room. A single high reading doesn’t necessarily mean you have hypertension. Doctors typically base a diagnosis on the average of multiple readings taken on separate occasions.
Two common patterns can make your numbers misleading. White coat hypertension, which affects 15% to 30% of people with elevated office readings, happens when your blood pressure spikes in a medical setting but stays normal the rest of the time. The reverse pattern, called masked hypertension, is when your readings look fine in the clinic but run high during everyday life. Home monitoring or wearing a 24-hour ambulatory monitor can catch both of these.
Getting an Accurate Reading
The way you sit during a reading can shift your numbers by 10 mmHg or more, enough to change your category entirely. For the most accurate result, sit quietly for three to five minutes before measuring. Your back should be supported against a chair, feet flat on the floor, and your arm resting on a table at heart level. Place the cuff on your bare upper arm about one inch above the bend of your elbow, with your palm facing up.
Crossing your legs, talking, or letting your arm hang at your side during a reading all tend to inflate the numbers. If you’re tracking your blood pressure at home, take two or three readings a minute apart and use the average. Morning readings before coffee or exercise tend to give the most consistent baseline.
Blood Pressure Targets by Age
Current guidelines set a general target of below 130/80 mmHg for most adults with hypertension, regardless of age. In practice, though, this target is harder to reach for older adults, particularly those with isolated systolic hypertension, where the top number is high but the bottom number is already low. Pushing the systolic number down aggressively in that scenario can drop diastolic pressure to levels that leave organs without adequate blood flow.
For adults over 65, treatment decisions typically weigh the benefits of lower blood pressure against the risk of side effects like dizziness and falls. The ideal target for any individual depends on their overall health, other medications, and how well they tolerate treatment.