A blood pressure reading has two numbers separated by a slash. The top number (systolic) measures the pressure in your arteries when your heart contracts and pushes blood out. The bottom number (diastolic) measures the pressure when your heart relaxes between beats. A normal reading is below 120/80 mmHg.
What the Top Number Means
The top number, called systolic pressure, captures the peak force of blood against your artery walls at the moment your heart squeezes. It’s the higher of the two numbers because the heart is actively pumping. In a reading of 120/80, the systolic pressure is 120.
This number tends to rise with age. As you get older, your large arteries, especially the aorta, gradually stiffen. In younger people, the aorta expands like a balloon during each heartbeat to absorb some of the force. When it loses that flexibility, more of that force transfers directly to your smaller blood vessels, pushing the top number higher while the bottom number stays the same or even drops. This is why many older adults have a pattern called isolated systolic hypertension, where the top number is 130 or above but the bottom stays below 80.
For people over 50, the top number is the stronger predictor of heart disease, stroke, and other cardiovascular problems. That doesn’t make the bottom number irrelevant, but if your doctor is focused on one number in particular, it’s likely this one.
What the Bottom Number Means
The bottom number, called diastolic pressure, reflects the lowest pressure in your arteries, recorded in the brief pause just before the next heartbeat. It tells you how much residual pressure remains in your blood vessels even when the heart is at rest. In a reading of 120/80, the diastolic pressure is 80.
For adults under 50, the bottom number actually matters more than the top. Research published in the Journal of General Internal Medicine found that diastolic pressure was the stronger predictor of mortality in younger adults, while providing relatively little independent risk information in people over 50. Readings above 100 on the bottom number were associated with significant increases in death risk regardless of age. So if you’re younger and your doctor flags your diastolic number, take it seriously.
Blood Pressure Categories
The American Heart Association’s 2025 guidelines break blood pressure into four 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
If your two numbers fall into different categories, the higher category applies. For example, a reading of 135/75 counts as stage 1 hypertension because the top number is in that range, even though the bottom number is normal.
On the low end, blood pressure below 90/60 is considered hypotension. Some people naturally run low without any symptoms, but if low readings come with dizziness, fainting, or fatigue, that’s worth investigating.
A reading of 180/120 or higher is a hypertensive crisis. If you see those numbers along with chest pain, shortness of breath, or stroke symptoms like sudden weakness or slurred speech, that requires emergency care.
When Only One Number Is High
It’s common for just the top number to creep up while the bottom stays normal. This pattern, isolated systolic hypertension, is especially frequent in older adults due to arterial stiffening. It can also be linked to an overactive thyroid, diabetes, heart valve problems, or obesity. It still counts as hypertension and still raises cardiovascular risk.
Less commonly, some people have an elevated bottom number with a normal top number. This pattern, called isolated diastolic hypertension, is more typical in younger adults and often reflects increased resistance in smaller blood vessels. Either pattern warrants attention, not just readings where both numbers are high.
Getting an Accurate Reading
The position of your arm during measurement can meaningfully shift both numbers. A 2024 study from Johns Hopkins Medicine found that resting your arm on your lap instead of a desk overestimated the top number by about 4 points and the bottom by 4 points. Letting your arm hang unsupported at your side was even worse, inflating the top number by 6.5 points and the bottom by 4.4 points. That’s enough to push a borderline reading into hypertension territory.
For the most accurate result, sit with your back supported, feet flat on the floor, legs uncrossed, and the middle of the cuff positioned at heart level on an arm resting on a desk or table. Use a cuff that fits your arm. A cuff that’s too small will read artificially high. Taking two or three readings a minute apart and averaging them gives a more reliable picture than a single measurement.