Systolic and diastolic are the two numbers in a blood pressure reading, and they measure two different moments in your heart’s pumping cycle. The top number (systolic) captures the peak pressure in your arteries when your heart contracts, while the bottom number (diastolic) captures the lowest pressure when your heart relaxes between beats. A normal reading is less than 120/80 mmHg.
What Each Number Measures
Your heart doesn’t push blood in a steady stream. It pumps in rhythmic bursts, squeezing and then relaxing roughly 100,000 times a day. Each squeeze-and-relax cycle creates two distinct pressure levels in your arteries, and that’s exactly what a blood pressure reading captures.
Systolic pressure (the top number) is the maximum force your blood exerts against artery walls when the left ventricle contracts and pushes a burst of blood into your circulatory system. Think of it as the pressure at the moment of the pump. In a healthy person, this peaks around 120 mmHg in the aorta, the body’s largest artery.
Diastolic pressure (the bottom number) is the pressure that remains in your arteries during the pause between heartbeats, when the heart muscle is relaxed and refilling with blood. It reflects the baseline tension in your blood vessels. In a healthy person, this sits around 80 mmHg.
The unit “mmHg” stands for millimeters of mercury, a holdover from early blood pressure devices that used a column of mercury to measure force. Modern digital cuffs no longer contain mercury, but the unit stuck.
Blood Pressure Categories
The American Heart Association breaks blood pressure into four main categories based on where your systolic and diastolic numbers fall. Only one number needs to be elevated for the higher category to apply.
- Normal: systolic below 120 and diastolic below 80
- Elevated: systolic 120 to 129 and diastolic below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher or diastolic 90 or higher
Notice that elevated blood pressure is defined only by a rising systolic number while diastolic stays normal. This is often the earliest warning sign that blood vessels are beginning to stiffen, particularly in people over 50.
Why Both Numbers Matter
For years, doctors focused almost exclusively on the systolic number, treating it as the better predictor of heart attacks and strokes. Systolic pressure does tend to climb with age as arteries lose elasticity, and it remains the stronger risk marker for cardiovascular events in most adults. But diastolic pressure carries its own risks, especially in younger people.
When only the top number is high (130 or above) while the bottom stays below 80, that’s called isolated systolic hypertension. It’s the most common form of high blood pressure in older adults, typically driven by stiffening arteries. Over time, it raises the risk of stroke, heart disease, dementia, and chronic kidney disease. Other contributing factors include diabetes, obesity, overactive thyroid, and heart valve problems.
Isolated diastolic hypertension, where the bottom number is elevated but the top is normal, is less common and less well understood. The biggest risk factors appear to be excess weight, sleep apnea, and smoking. While it may not cause immediate symptoms, it raises the lifetime risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are greatest for women and people under 60.
Pulse Pressure: The Gap Between the Two
Subtracting your diastolic number from your systolic gives you a third metric called pulse pressure. In a reading of 120/80, the pulse pressure is 40. That’s considered the upper edge of a healthy gap. A pulse pressure above 60 is a risk factor for heart disease, particularly in older adults, because it signals that the large arteries have become stiff and less able to absorb the force of each heartbeat.
A very narrow pulse pressure (below 25 or so) can point in the other direction, suggesting the heart isn’t pumping with enough force. Both extremes give your doctor useful information that neither number alone provides.
Why Your Numbers Change Throughout the Day
Blood pressure is not a fixed value. It follows a daily rhythm, starting to rise a few hours before you wake up, climbing through the morning, peaking around midday, and then gradually dropping in the late afternoon and evening. It reaches its lowest point during sleep.
Physical activity, stress, caffeine, a full bladder, even crossing your legs during a reading can temporarily push both numbers up. This is why a single high reading doesn’t automatically mean you have hypertension. Doctors typically want to see a pattern across multiple readings taken on different days and at different times. Home monitoring, where you check your pressure at roughly the same time each morning and evening, gives a much clearer picture than a one-off reading in a clinic.
One pattern worth knowing about: most people see at least a 10% drop in blood pressure overnight. When that nighttime dip doesn’t happen, a pattern called “nondipping,” it’s associated with higher cardiovascular risk. This is something a 24-hour ambulatory monitor can detect if your doctor suspects it.
How to Read Your Own Numbers
When you see a reading like 118/76, the first number is always systolic (the heart contracting) and the second is always diastolic (the heart resting). You’ll sometimes hear them called the “top” and “bottom” numbers because of how they appear on a monitor or in written records.
If your systolic and diastolic fall into different categories, the higher category is the one that applies. A reading of 135/75, for example, counts as Stage 1 hypertension because the systolic is in that range, even though the diastolic looks perfectly normal. Similarly, a reading of 118/92 would also be Stage 2 hypertension based on the diastolic alone.
Tracking both numbers over time, rather than fixating on a single reading, gives you the most useful information about your cardiovascular health. Small fluctuations day to day are completely normal. What matters is the trend.