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 beats. The bottom number (diastolic) measures the pressure when your heart rests between beats. Both are recorded in millimeters of mercury, abbreviated as mmHg.
What Each Number Tells You
The top number, systolic pressure, captures the peak force against your artery walls at the moment your heart contracts and pushes blood out. It’s the higher of the two because your arteries are under the most stress during that squeeze. A normal systolic reading is below 120 mmHg.
The bottom number, diastolic pressure, reflects the lowest pressure your arteries experience while your heart relaxes and refills with blood. A normal diastolic reading is below 80 mmHg. Together, the two numbers paint a picture of how hard your cardiovascular system is working both during effort and at rest.
There’s also a third number you might hear about: pulse pressure. That’s simply the top number minus the bottom number. A typical pulse pressure is around 40 mmHg. If the gap between your two numbers is noticeably wider than that, it can signal that your large arteries have stiffened. This happens naturally with age as artery walls lose elasticity and accumulate calcium, a process sometimes called “hardening of the arteries.” A widening pulse pressure is worth mentioning to your doctor even if both individual numbers look acceptable.
The Blood Pressure Categories
The American Heart Association breaks blood pressure into four main categories based on where your numbers land. Your reading falls into whichever category is higher. So if your systolic is normal but your diastolic is elevated, the elevated category applies.
- 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
Notice the word “or” in the hypertension stages. You only need one of the two numbers to be high to qualify. A reading of 145/78, for instance, is Stage 2 hypertension even though the bottom number looks fine.
When Numbers Are Too Low
Blood pressure below 90/60 mmHg is generally classified as low (hypotension). But low numbers aren’t automatically a problem. Many people walk around with readings in that range and feel perfectly fine. The numbers only become concerning when pressure drops low enough that your organs aren’t getting adequate blood flow. Symptoms like dizziness, fainting, blurred vision, or unusual fatigue are what signal trouble, not the number alone. If your blood pressure tends to run low and you have no symptoms, it typically doesn’t need treatment.
When Numbers Are Dangerously High
A reading above 180 systolic or above 120 diastolic crosses into what’s called a hypertensive crisis. This is split into two situations. In a hypertensive urgency, the numbers are severely high but your organs are still functioning normally. In a hypertensive emergency, those extreme numbers are actively damaging organs, potentially causing stroke, heart attack, kidney failure, or fluid in the lungs. If you see numbers above 180/120 and you’re experiencing chest pain, shortness of breath, severe headache, vision changes, or confusion, that’s a 911 call.
Why Your Reading Might Be Wrong
A single blood pressure reading is a snapshot, and it’s surprisingly easy to get an inaccurate one. One of the biggest sources of error is cuff size. In a randomized trial, people who needed an extra-large cuff but were measured with a standard cuff got readings that were nearly 20 mmHg too high on the systolic side. That’s enough to make a perfectly normal reading look like Stage 2 hypertension. Even for people who needed a large cuff, a regular cuff inflated systolic readings by about 5 mmHg. On the flip side, using a cuff that’s too big on a smaller arm can underestimate pressure by 3 to 4 mmHg.
Your mental state matters too. Roughly one in five people with borderline or mildly high readings in a doctor’s office actually have normal pressure at home. This is called white coat hypertension, and research shows that clinic readings run about 7 to 8 mmHg higher for systolic and about 5 mmHg higher for diastolic compared to home monitoring, on average. The reverse also happens: about one in five people with prehypertensive readings at the clinic have higher numbers at home that go undetected, a pattern called masked hypertension.
For the most reliable picture, take your blood pressure at home at the same time each day, sitting quietly for five minutes first, with your arm supported at heart level. Two or three readings a minute apart, averaged together, give you a much more trustworthy number than any single measurement.
Blood Pressure Targets for Older Adults
Current guidelines recommend a systolic target below 130 mmHg for adults 65 and older who are living independently and in reasonably good health. That’s the same threshold used for younger adults with Stage 1 hypertension. But for older adults managing multiple serious health conditions or with limited life expectancy, the target becomes more flexible. In those cases, doctors weigh the risks of aggressive blood pressure lowering (falls, dizziness, kidney strain) against the benefits, and the “right” number becomes a conversation rather than a fixed rule.