A blood pressure reading has two numbers, written as one over the other (like 120/80). The top number measures the force your blood pushes against artery walls when your heart beats. The bottom number measures that same force when your heart rests between beats. Together, they tell you how hard your cardiovascular system is working and whether that level of effort is putting your organs at risk.
The Top Number (Systolic)
The top number, called systolic pressure, captures the peak moment of each heartbeat. Every time your heart contracts, it sends a wave of blood into your arteries, and systolic pressure reflects the force of that wave. This is the higher of the two numbers because it represents the maximum pressure your arteries experience.
Systolic pressure tends to rise with age as arteries lose their elasticity. It’s the number doctors pay closest attention to in people over 50, since elevated systolic pressure is a stronger predictor of heart disease and stroke than the bottom number in that age group.
The Bottom Number (Diastolic)
The bottom number, called diastolic pressure, reflects what’s happening between heartbeats. Your heart relaxes briefly before each new contraction, and during that pause, your arteries still maintain some baseline pressure. That resting pressure is diastolic. A healthy diastolic number means your arteries aren’t under excessive strain even during the “quiet” phase of each heartbeat cycle.
What the Ranges Mean
Blood pressure is measured in millimeters of mercury, abbreviated mm Hg. The current treatment goal for all adults, set by the 2025 American Heart Association guidelines, is below 130/80 mm Hg. Here’s how the ranges break down:
- Normal: Below 120/80 mm Hg. Both numbers are in a healthy range, and no intervention is needed beyond maintaining good habits.
- Elevated: Systolic between 120 and 129, with diastolic still below 80. This is a warning zone. Without lifestyle changes, it tends to progress toward full hypertension.
- Stage 1 hypertension: Systolic 130 to 139, or diastolic 80 to 89. Lifestyle changes are the first step. Medication is typically added if you also have diabetes, chronic kidney disease, a history of stroke or heart disease, or a calculated 10-year cardiovascular risk of 7.5% or higher.
- Stage 2 hypertension: Systolic 140 or higher, or diastolic 90 or higher. Medication plus lifestyle changes are recommended for all adults at this level.
- Hypertensive crisis: 180/120 mm Hg or higher. This requires immediate emergency care, especially if accompanied by chest pain, shortness of breath, blurred vision, severe headache, or seizures.
Only one number needs to be elevated for the reading to count as high. If your systolic is 145 but your diastolic is 78, that’s still Stage 2 hypertension.
When Numbers Are Too Low
A reading below 90/60 mm Hg is generally considered low blood pressure. Not everyone with low numbers feels symptoms, but when they do, the experience can include dizziness, blurred vision, fainting, fatigue, and trouble concentrating. Even a relatively small drop matters: a systolic drop of just 20 mm Hg (say, from 110 to 90) can be enough to make you feel lightheaded or faint.
The Gap Between the Two Numbers
Subtracting the bottom number from the top gives you something called pulse pressure. If your reading is 130/70, your pulse pressure is 60. A pulse pressure above 40 mm Hg is generally considered unhealthy, and above 60 it becomes a risk factor for heart disease, particularly in older adults.
A wide gap usually signals that the body’s largest arteries have stiffened. Stiff arteries can’t expand and absorb the force of each heartbeat the way flexible ones do, so systolic pressure climbs while diastolic stays the same or drops. The wider the gap, the more damage and stiffening is likely present.
Why High Blood Pressure Damages Organs
Persistently high pressure acts like water blasting through a garden hose at too high a setting. Over time it damages artery walls, making them narrower and less flexible. The consequences show up in specific organs.
In the heart, narrowed arteries reduce blood flow to the heart muscle itself. This can cause chest pain, irregular heart rhythms, and heart attacks. The heart also has to pump harder against the increased resistance, which thickens and enlarges the left side of the heart. That thickening further raises the risk of heart failure.
In the brain, damaged blood vessels can narrow, leak, or rupture. High pressure also promotes blood clots in arteries that feed the brain. Both mechanisms raise the risk of stroke.
In the kidneys, damaged blood vessels prevent effective filtering of waste from the blood. High blood pressure is one of the most common causes of kidney failure, and having diabetes alongside hypertension accelerates the damage significantly.
Your Reading Can Vary More Than You Think
A single reading is a snapshot, not a diagnosis. Blood pressure fluctuates throughout the day based on stress, activity, caffeine, and even how full your bladder is. Two phenomena make this especially tricky.
White coat hypertension means your readings are high in a medical setting but normal at home. This affects roughly 20 to 25% of people diagnosed with hypertension in a clinic. The anxiety of being in a doctor’s office is enough to temporarily push numbers up. Masked hypertension is the opposite: your readings look normal at the doctor’s office but are actually elevated during everyday life. Studies estimate this affects 10 to 20% of people with normal office readings. Both patterns carry real health implications, which is why home monitoring or 24-hour ambulatory monitoring can give a much more accurate picture than office visits alone.
Getting an Accurate Reading at Home
Preparation matters more than most people realize. The CDC recommends a specific routine: avoid food, drinks, and caffeine for 30 minutes before measuring. Empty your bladder. Sit in a chair with back support for at least five minutes, both feet flat on the floor, legs uncrossed. Rest the arm with the cuff on a table so it sits at chest height. The cuff should go directly on bare skin, not over a sleeve, and fit snugly without being tight. Don’t talk during the measurement.
Skipping any of these steps can throw off your reading by 10 to 20 mm Hg or more, enough to push a normal result into the elevated category or mask a genuinely high one. Taking two or three readings a minute apart and averaging them gives a more reliable result than relying on a single measurement.
Targets Can Differ by Age and Health
The general goal of below 130/80 applies to most adults, but the specifics shift in certain situations. For people over 50 with type 2 diabetes, research shows that targeting a systolic pressure below 120 mm Hg leads to better cardiovascular outcomes than the standard target of below 140. For adults over 80, the 2025 guidelines recommend treatment starting at 130/80 or above, but with more flexibility. Frailty, side effects from medication, and limited life expectancy all factor into choosing how aggressively to lower the numbers. The target becomes a conversation rather than a fixed rule.