Diastolic pressure is the lower number in a blood pressure reading, and it measures the force your blood exerts against artery walls between heartbeats, when your heart is resting and refilling with blood. A normal diastolic reading is below 80 mmHg. While it gets less attention than the upper number (systolic pressure), diastolic pressure plays an important role in heart health, particularly in how blood flows to the heart muscle itself.
What Happens in Your Body During Diastole
Your heart works in a two-phase cycle: squeezing (systole) and relaxing (diastole). During the squeeze phase, blood is pumped out into your arteries, stretching them like elastic tubes. Then the heart relaxes. The valves leading out of the heart snap shut, and the heart chambers begin refilling with blood from the veins.
Here’s the key part: even though the heart isn’t pumping during this phase, blood is still moving through your arteries. The energy stored in the stretched artery walls pushes blood forward, much like a rubber band snapping back into shape. The pressure created by that elastic recoil is your diastolic pressure. It represents the lowest point of pressure in your arteries before the next heartbeat.
What the Numbers Mean
Blood pressure is written as two numbers, such as 120/80. The top number is systolic (pressure during a heartbeat), and the bottom number is diastolic (pressure between beats). According to the American Heart Association, diastolic pressure falls into these categories:
- Normal: less than 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Hypertensive crisis: higher than 120 mmHg
These categories always consider the systolic number too. If your systolic and diastolic readings fall into different categories, the higher category applies. So a reading of 118/85 would be classified as Stage 1 hypertension because of the diastolic number, even though the systolic number is normal.
What Controls Your Diastolic Pressure
Three main factors determine diastolic pressure: how much blood your heart pumps per minute, how elastic your arteries are, and how much resistance the smaller blood vessels create. Of these, the resistance in small blood vessels is the biggest driver of diastolic readings, especially in younger and middle-aged adults.
This is why diastolic pressure follows a distinct pattern over a lifetime. It tends to rise steadily until about age 50, driven mainly by increasing resistance in small blood vessels. After 60, diastolic pressure typically begins to decline. The reason is that large arteries stiffen with age and lose their ability to stretch and recoil. Stiffer arteries push systolic pressure up but actually pull diastolic pressure down, because they store less energy between beats. This is why older adults often have a wide gap between their systolic and diastolic numbers.
Diastolic vs. Systolic: Which Matters More
For decades, doctors focused heavily on diastolic pressure. That changed as large studies consistently showed that systolic pressure is a better predictor of heart attacks, strokes, and other cardiovascular events. Isolated systolic hypertension (high systolic with normal diastolic) carries clear risk, and treating it reduces cardiovascular events. There are no comparable treatment studies for isolated diastolic hypertension.
That said, diastolic pressure isn’t irrelevant. When diastolic pressure exceeds 100 mmHg, it has a clear linear relationship with cardiovascular disease risk. And considering both numbers together improves risk prediction compared to looking at either one alone. The practical takeaway: systolic pressure is the more important number for most people, but an abnormal diastolic reading still warrants attention.
When Diastolic Pressure Is Too High
Isolated diastolic hypertension, where the bottom number is elevated but the top number stays normal, is most common in younger adults. Its cardiovascular significance has been debated. At the traditional threshold of 90 mmHg or higher, recent large studies have more consistently shown an association with cardiovascular events. At the newer, lower threshold of 80 mmHg, the evidence is weaker, and most studies have not found a clinically meaningful increase in risk.
One thing is clear: isolated diastolic hypertension at any threshold is a strong predictor of developing full-blown hypertension (both numbers elevated) in the future. So even if the immediate risk is modest, it signals that blood pressure is trending in the wrong direction and lifestyle changes like reducing sodium, increasing physical activity, and maintaining a healthy weight can help prevent progression.
When Diastolic Pressure Is Too Low
Low diastolic pressure, generally below 60 mmHg, carries its own risks. Your coronary arteries, the vessels feeding the heart muscle itself, receive most of their blood supply during diastole. When diastolic pressure drops too low, blood flow to the heart muscle can become inadequate.
A study of U.S. adults found that diastolic pressure below 60 mmHg was associated with a 30% higher risk of death from any cause and a 34% higher risk of cardiovascular death, compared to people with diastolic readings between 70 and 80 mmHg. This creates what researchers call a J-shaped curve: risk increases at both the high and low ends of the diastolic range. Low diastolic pressure is more common in older adults and in people with heart failure, prior heart attacks, or diabetes.
How Diastolic Pressure Is Measured
When blood pressure is taken with a manual cuff and stethoscope, the clinician inflates the cuff to cut off blood flow, then slowly releases it while listening for sounds in the artery. These are called Korotkoff sounds. The diastolic reading is recorded at the moment those sounds disappear completely. In rare cases where the sounds never fully disappear (sometimes seen in pregnancy or in children), the point where the sounds become muffled is used instead.
Automatic blood pressure monitors use sensors rather than sound to detect the same information, but the principle is identical. For the most accurate readings at home, sit quietly for five minutes before measuring, keep your arm supported at heart level, and avoid caffeine or exercise for 30 minutes beforehand. Taking two or three readings a minute apart and averaging them gives a more reliable number than any single measurement.