Systolic blood pressure (the top number) measures the force in your arteries when your heart pumps blood out to your body. Diastolic blood pressure (the bottom number) measures the pressure in your arteries when your heart is resting between beats. A reading of 120/80 mmHg means 120 mmHg of pressure during the pump and 80 mmHg of pressure during the rest phase.
Both numbers matter, but they tell you different things about your cardiovascular health, they change differently as you age, and they carry distinct risks when they’re out of range.
What Each Number Measures
Your heart works in a two-phase cycle, over and over, roughly 100,000 times a day. During the contraction phase, the heart’s main pumping chamber squeezes and pushes blood into your arteries. The peak pressure created by that squeeze is your systolic number. It reflects how forcefully your heart is working and how stiff or flexible your large arteries are.
During the relaxation phase, your heart refills with blood before the next beat. Pressure in the arteries drops but doesn’t fall to zero because the elastic walls of your blood vessels maintain some baseline tension. That lower, resting pressure is your diastolic number. It tells you about the resistance in your smaller blood vessels and how well your arteries relax between beats.
Current Blood Pressure Categories
Updated 2025 guidelines classify blood pressure into four categories:
- Normal: below 120 systolic and below 80 diastolic
- 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
Notice that “elevated” blood pressure is defined entirely by the systolic number. You can have a perfectly normal diastolic reading and still be in an elevated category if your top number creeps above 120. A reading above 180/120 is considered a hypertensive crisis and requires emergency medical attention.
Why Systolic Pressure Rises With Age
In younger adults, diastolic pressure tends to be the first number that climbs when blood pressure starts to rise. But after about age 50, the pattern flips. Systolic pressure becomes the bigger concern because your large arteries gradually lose their elasticity. Stiffer arteries can’t absorb the shock of each heartbeat as well, so peak pressure climbs even while diastolic pressure may stay the same or actually drop.
This pattern, called isolated systolic hypertension, is extremely common in older adults. It’s more than a cosmetic number shift. The combination of stiffening arteries and a harder-working heart leads to thickening of the heart muscle, accelerated plaque buildup, and greater strain on the cardiovascular system as a whole. For people over 50, systolic pressure is generally the more important predictor of heart attacks, strokes, and heart failure.
Why Diastolic Pressure Still Matters
Diastolic pressure plays a unique role that systolic pressure doesn’t: your heart muscle itself gets most of its blood supply during the relaxation phase, when diastolic pressure is driving blood through the coronary arteries. If diastolic pressure drops too low, the heart can actually starve for oxygen.
This is especially relevant for people with coronary artery disease. Research published by the American Heart Association found a J-curve relationship, meaning that cardiovascular risk goes up not only when diastolic pressure is too high but also when it falls below about 70 mmHg in people with existing artery blockages. Patients with diastolic pressure below 50 mmHg had dramatically worse outcomes: 30-day mortality of 2.5% compared to 0.7% in those with higher diastolic readings, and long-term mortality roughly tripled over a median follow-up of 3.6 years.
This matters practically because aggressive blood pressure treatment focused on bringing the systolic number down can sometimes push diastolic pressure too low as a side effect, creating a different kind of risk.
Pulse Pressure: The Gap Between the Numbers
The difference between your systolic and diastolic readings is called pulse pressure. If your blood pressure is 130/80, your pulse pressure is 50 mmHg. This gap carries its own health information.
A normal pulse pressure is roughly 40 mmHg, though it varies. A wide pulse pressure of 60 mmHg or more can signal stiff arteries, a leaky heart valve, or other cardiovascular changes. A very narrow pulse pressure, where the gap is less than one-quarter of your systolic number, can indicate poor heart function or significant blood loss. If you track your blood pressure at home and notice the gap between numbers widening or narrowing significantly over time, that trend is worth bringing up with your doctor.
How Age Affects Normal Ranges
Blood pressure norms aren’t one-size-fits-all. Children have much lower baseline readings than adults. A typical one-year-old boy has a systolic pressure around 80 and diastolic around 34 mmHg. By age 17, those numbers rise to roughly 114/65. Girls follow a similar but slightly lower trajectory, averaging about 108/64 by age 17.
In adults, the standard categories listed above apply regardless of age. But clinicians often weigh the two numbers differently depending on the patient. For a 35-year-old with early hypertension, a rising diastolic number may be the first warning sign. For a 65-year-old, systolic pressure climbing past 140 while diastolic stays at 75 is the more typical, and more dangerous, pattern.
Getting an Accurate Reading
Both numbers are sensitive to how you take the measurement. Small errors in positioning or preparation can shift your reading by 10 mmHg or more, enough to move you from one category to another. The American Heart Association recommends specific steps to get reliable numbers.
Sit quietly for 3 to 5 minutes before taking a reading. Don’t talk or move around during this rest period. Your arm should be supported on a flat surface like a desk, not hanging at your side or held up by your own effort. Holding your arm unsupported counts as isometric exercise and will raise the reading. The cuff should sit at heart level, roughly at the midpoint of your breastbone. If your arm is below heart level, readings will be artificially high.
Cuff size also makes a real difference. A cuff that’s too small for your arm will give you a falsely high reading, and one that’s too large will read falsely low. The bladder inside the cuff should wrap around 75% to 100% of your arm’s circumference. Most home monitors come with a standard cuff that fits arms roughly 9 to 13 inches around. If your arm is larger, you need a wide-range or large cuff to get accurate results.
Taking two or three readings a minute apart and averaging them gives a more reliable picture than any single measurement. Blood pressure fluctuates naturally throughout the day, so consistent technique matters more than any one number.