Diastolic blood pressure measures the pressure inside your arteries during the brief pause between heartbeats, when your heart muscle is relaxed and refilling with blood. It’s the bottom number in a blood pressure reading, and it reflects how much resistance your blood vessels are creating even when the heart isn’t actively pumping. A normal diastolic reading is below 80 mmHg.
What Happens Between Heartbeats
Your heart works in two phases. During the contraction phase (systole), the heart squeezes blood out into your arteries, creating the higher, top number. During the relaxation phase (diastole), the heart stops squeezing and fills back up for the next beat. Diastolic pressure captures what’s happening in that relaxation window.
Even though the heart isn’t pumping during diastole, pressure doesn’t drop to zero. Your arteries are elastic tubes, and after each heartbeat they recoil inward, pushing blood forward. At the same time, smaller blood vessels throughout your body create resistance to blood flow. Diastolic pressure is essentially a snapshot of those two forces: how elastic your large arteries are and how much resistance your smaller vessels are putting up. Stiffer arteries and tighter small vessels both push the number higher.
How Diastolic Differs From Systolic
Systolic pressure (the top number) is heavily influenced by the stiffness of your large arteries, which is why it climbs steadily as people age and those arteries become more rigid. Diastolic pressure, by contrast, is driven more by peripheral vascular resistance, the tightness of the smaller arteries and arterioles spread throughout your body.
This distinction matters because the two numbers don’t always move in the same direction. National health data from the CDC show that diastolic pressure rises from early adulthood until roughly age 60, then starts to decline. Systolic pressure, on the other hand, keeps climbing with age. That’s why older adults often have a wide gap between their two numbers: a high systolic reading paired with a normal or even low diastolic reading. In younger adults, the pattern can reverse, with a normal systolic number but an elevated diastolic one.
What the Numbers Mean
Current guidelines break diastolic pressure into clear categories:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
These thresholds apply alongside your systolic number. If either number falls into a higher category, that’s the category that counts for your overall blood pressure classification.
When Only the Bottom Number Is High
Some people have a diastolic reading of 80 mmHg or above while their systolic number stays below 130. This is called isolated diastolic hypertension, and it’s more common in younger adults. The exact cause isn’t fully understood, but three of the biggest risk factors are carrying excess weight, sleep apnea, and smoking.
Isolated diastolic hypertension may not cause noticeable symptoms, but it isn’t harmless. It raises your lifetime risk of heart attack and makes death from cardiovascular disease more likely. It also increases the chance of developing heart failure. These risks are highest for women and for people under 60. Because it tends to fly under the radar (many people focus only on the top number), it’s worth paying attention to both numbers in your reading.
Why Low Diastolic Pressure Also Matters
Most of the conversation around blood pressure focuses on numbers that are too high, but a diastolic reading that drops too low carries its own risks, particularly for your heart muscle itself. The heart is unusual in that it receives most of its own blood supply during the diastolic phase, not while it’s contracting. When diastolic pressure falls too low, the pressure driving blood into the coronary arteries drops with it.
This becomes especially dangerous for people who already have narrowed coronary arteries. Research from the American Heart Association shows that aggressive lowering of diastolic blood pressure can impair coronary perfusion enough to promote ischemia, a condition where the heart muscle doesn’t get enough oxygen. For people with coronary artery disease, pushing diastolic pressure too low with medication can worsen symptoms rather than improve them. There’s no single cutoff that applies to everyone, but diastolic readings consistently below 60 mmHg generally warrant a conversation with your doctor, especially if you’re on blood pressure medication.
How Diastolic Pressure Is Measured
When a clinician uses a manual blood pressure cuff and stethoscope, they inflate the cuff to stop blood flow, then slowly release it while listening for sounds called Korotkoff sounds. The systolic number is recorded when tapping sounds first appear. The diastolic number is recorded when those sounds disappear completely, known as Korotkoff phase 5. Automated home monitors use sensors to detect the same transition points, which is why they sometimes give slightly different readings than a manual cuff.
For the most accurate home readings, sit quietly for five minutes before measuring, keep your arm supported at heart level, and don’t talk during the reading. Taking two or three readings a minute apart and averaging them gives a more reliable picture than any single measurement.