Systolic blood pressure is the peak pressure in your arteries when your heart contracts and pushes blood out. Diastolic blood pressure is the lowest pressure in your arteries between beats, when your heart relaxes and refills. On a blood pressure reading of 120/80, the top number (120) is systolic and the bottom number (80) is diastolic. Both numbers matter, but they tell you different things about your heart and blood vessels.
What Each Number Measures
Every heartbeat has two phases. During the first phase, your left ventricle squeezes and ejects blood into the aorta, your body’s largest artery. That surge of blood raises pressure to its highest point, and that peak is your systolic pressure. During the second phase, the ventricle relaxes and refills with blood from the lungs. Pressure in the aorta gradually drops during this pause, and the lowest point it reaches, just before the next beat, is your diastolic pressure.
Systolic pressure reflects how forcefully your heart pumps and how much resistance your arteries put up against that force. Diastolic pressure reflects the baseline tension in your artery walls when the heart isn’t actively pushing. Together, they give a snapshot of both your heart’s workload and the condition of your blood vessels.
Normal Ranges and What’s Too High
The American Heart Association defines five categories based on both numbers. Only one number needs to be elevated for you to land in the higher category:
- 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 the “elevated” category only involves the top number. There’s no equivalent category for a mildly elevated bottom number alone. This reflects the fact that systolic pressure tends to climb first in many people, especially as arteries stiffen with age.
Why Diastolic Pressure Matters for Your Heart
Your heart muscle gets its own blood supply through the coronary arteries, and most of that blood flows during the relaxation phase between beats. The driving force behind coronary blood flow is your diastolic pressure. If the bottom number drops too low, your heart muscle may not receive enough oxygen, which is why very low diastolic readings (below 60) can be a concern, particularly in people who already have coronary artery disease.
This is one reason doctors pay attention to both numbers rather than focusing only on systolic. A reading of 150/60, for example, signals two problems at once: excessive force during beats and potentially inadequate blood flow to the heart between them.
How Both Numbers Change With Age
Systolic and diastolic pressure don’t age the same way, which is part of why the two numbers carry different weight at different life stages. Among Western populations over 40, systolic pressure rises by roughly 7 mmHg per decade, eventually averaging around 140 by the late 70s. Diastolic pressure also rises with age but more slowly, at about 1 mmHg per decade in women. In men, diastolic pressure actually declines continuously with age after early adulthood.
This divergence explains why isolated systolic hypertension, where only the top number is high, is common after age 50. The arteries gradually stiffen from plaque buildup and loss of elasticity. Stiff arteries can’t expand to absorb the force of each heartbeat, so the systolic peak climbs while the diastolic trough stays the same or falls. Women show a sharper rise in systolic pressure between ages 40 and 55, around menopause, after which their systolic readings often surpass men’s.
Isolated systolic hypertension is also increasing among younger adults, particularly men with a higher body mass index who smoke and don’t exercise regularly.
Pulse Pressure: The Gap Between the Two Numbers
Subtracting the bottom number from the top gives you your pulse pressure. For a reading of 120/80, that’s 40 mmHg, which is considered healthy. A pulse pressure above 40 generally signals stiffer arteries, and a pulse pressure above 60 is an independent risk factor for heart attacks and strokes, especially in older adults.
Pulse pressure widens when systolic rises but diastolic doesn’t follow, which is exactly what happens as arteries lose their flexibility. A reading of 160/70 gives a pulse pressure of 90, suggesting significant arterial stiffness even if the diastolic number looks fine on its own.
Which Number Predicts Heart Disease Better
For decades, doctors debated whether the top or bottom number mattered more. The answer depends partly on age. In older adults, systolic pressure is the stronger predictor of heart attacks and strokes because arterial stiffness is the dominant problem.
In younger adults, the picture is more balanced. A large study published in Circulation found that among young adults, having only the top number elevated and having only the bottom number elevated carried similar cardiovascular risk, each raising the likelihood of a cardiovascular event by roughly 30 to 36 percent compared to normal blood pressure. But having both numbers elevated at once was worse, increasing risk by about 67 percent. The takeaway: neither number is safe to ignore at any age, and the combination of both being high is more dangerous than either alone.
Getting an Accurate Reading
Blood pressure is surprisingly easy to measure wrong, and many common mistakes inflate the systolic reading far more than people realize. Research from the American College of Cardiology quantifies some of the biggest errors:
- No rest period before measurement: can add 10 to 20 mmHg to the top number
- Full bladder: can add up to 15 mmHg
- Talking or texting during the reading: can add 10 to 15 mmHg
- Sitting without back support (like on an exam table): can add 5 to 15 mmHg
- Wrong cuff size: a too-small cuff can add 5 to 20 mmHg, and measuring over clothing can swing the reading by up to 50 mmHg in either direction
- Caffeine within 30 minutes: roughly 200 to 300 mg (about two cups of coffee) can add around 8 mmHg
- Crossing your legs or letting feet dangle: can add 5 to 8 mmHg
These errors stack. If you skip the rest period, have a full bladder, and sit on an exam table with legs dangling, your systolic reading could be 30 or more points higher than your true resting pressure. That’s enough to turn a normal reading into what looks like stage 2 hypertension. For the most reliable numbers, sit quietly for five minutes with your back supported, feet flat on the floor, arm resting at heart level on a table, and use a properly sized cuff on bare skin.
Common pain relievers can also nudge blood pressure upward. Ibuprofen has been shown to raise systolic pressure by about 4 mmHg with regular use, and even acetaminophen at high daily doses raises it by a similar amount. These effects are small for a single reading but meaningful over months or years of daily use.