The top number in a blood pressure reading is called systolic blood pressure. It measures the maximum pressure inside your large arteries at the moment your heart contracts and pushes blood out to the rest of your body. A normal systolic reading is below 120 mm Hg, and this number is considered the single most important predictor of heart disease and stroke risk.
What the Top Number Tells You
Every time your heart beats, it squeezes blood into your arteries with considerable force. The top number captures that peak pressure. Between beats, when your heart relaxes and refills, the pressure drops to its lowest point, which is the bottom number (diastolic). Together, these two numbers give a snapshot of how hard your cardiovascular system is working.
Elevated systolic blood pressure is the leading modifiable risk factor for cardiovascular death worldwide. The risk isn’t a cliff edge at some arbitrary cutoff. It rises in a continuous, linear pattern starting at a systolic reading of just 120 mm Hg. For every 10 mm Hg increase above that point, the risk of a brain bleed roughly doubles, and the risk of stroke and major coronary events climbs significantly as well.
Blood Pressure Categories
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories based on your numbers:
- 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
If your systolic and diastolic numbers fall into different categories, the higher category applies. So a reading of 135/75 counts as stage 1 hypertension, even though the bottom number looks fine.
A systolic reading of 180 or higher is considered a hypertensive crisis. Symptoms can include severe headache, blurred vision, chest pain, shortness of breath, and seizures. That situation requires emergency medical attention, especially if those symptoms are present.
Why the Top Number Rises With Age
Your arteries are flexible when you’re young. They expand slightly with each heartbeat, absorbing the force of blood flow. Over decades, plaque buildup and calcium deposits gradually stiffen those artery walls, a process called atherosclerosis. Stiff arteries can’t stretch to absorb the pressure wave from each heartbeat, so the systolic number climbs.
Data from the Framingham Heart Study shows that systolic pressure rises continuously from age 30 through at least 84. Diastolic pressure, by contrast, tends to increase only until about age 50, then gradually drops. This widening gap between the two numbers is a hallmark of arterial aging. It’s also why isolated systolic hypertension, where only the top number is high, becomes increasingly common in older adults.
When Only the Top Number Is High
Isolated systolic hypertension means your systolic reading is 130 or above while your diastolic stays below 80. In older adults, arterial stiffness is almost always the driver. In younger people, it can sometimes point to hormonal imbalances or narrowing of the arteries that supply the kidneys.
Treatment typically starts with lifestyle changes: cutting sodium, eating more fruits and vegetables, maintaining a healthy weight, getting regular aerobic exercise, limiting alcohol, and managing stress. If those steps aren’t enough, medication may be added. One consideration specific to this condition is that lowering the top number too aggressively can sometimes pull the bottom number dangerously low, reducing blood flow to organs. Your provider will monitor both numbers and adjust accordingly.
When the Top Number Is Too Low
A systolic reading below 90 is generally considered low blood pressure. For some people, that’s normal and causes no symptoms. For others, it can mean dizziness, fainting, or fatigue, especially when standing up quickly. Dehydration is one of the most common causes, whether from illness, not drinking enough water, or overuse of water pills. Prolonged bed rest, pregnancy, and certain medications can also drop systolic pressure.
How Diet and Sodium Affect the Top Number
Dietary changes can produce surprisingly large drops in systolic blood pressure, particularly if your numbers are already elevated. In clinical trials, people who started with a systolic reading of 150 or higher and switched to a low-sodium version of the DASH diet (rich in fruits, vegetables, whole grains, and low-fat dairy) saw their systolic pressure drop by roughly 21 mm Hg on average. Even people starting below 130 saw reductions of about 5 mm Hg.
Sodium reduction alone made a meaningful difference. Cutting sodium intake from high to low levels lowered systolic pressure by about 7 to 9 mm Hg in people with readings between 130 and 150. The pattern is consistent: the higher your starting blood pressure, the more dramatic the effect of dietary changes. For someone with mildly elevated numbers, these changes can be enough to bring readings back into the normal range without medication.
Getting an Accurate Reading
A single high reading doesn’t necessarily mean you have high blood pressure. Technique matters more than most people realize. For an accurate measurement, you should sit quietly for at least five minutes beforehand with your back supported, feet flat on the floor, and legs uncrossed. Empty your bladder first. The cuff goes directly on bare skin on your upper arm, with the middle of the cuff positioned at heart level. Rolled-up sleeves bunched above the cuff can artificially raise the reading.
Talking, crossing your legs, or letting your arm dangle unsupported can each add several points to your systolic number. If you’re checking at home, take two or three readings a minute apart and average them for the most reliable result.