The diastolic is the bottom number in a blood pressure reading. It measures the pressure inside your arteries between heartbeats, during the brief moment when your heart relaxes and refills with blood. In a reading like 120/80, the 80 is your diastolic pressure, measured in millimeters of mercury (mm Hg). A normal diastolic reading is below 80 mm Hg.
What Happens in Your Heart During Diastole
Your heart works in a two-phase cycle: squeezing and relaxing. The squeezing phase (systole) pumps blood out into your body. The relaxing phase (diastole) is when the lower chambers of your heart, called ventricles, loosen up and fill with blood from the upper chambers. This refilling step is critical because it loads the heart for its next pump.
Even though your heart is resting during diastole, your arteries still have pressure in them. That residual pressure exists because your blood vessels are elastic and maintain tension between beats, similar to how a garden hose still has some water pressure even when the faucet pulses on and off. The diastolic number captures that baseline arterial pressure.
This phase also matters because it’s when your heart muscle receives most of its own blood supply. Your coronary arteries, the vessels that feed oxygen to the heart itself, fill most effectively during diastole. Your heart uses 70% to 80% of the oxygen in the blood passing through those arteries, so adequate diastolic pressure is essential for keeping the heart muscle nourished.
How Diastolic Pressure Is Measured
When a nurse or doctor takes your blood pressure manually with a cuff and stethoscope, they listen for specific sounds as the cuff slowly deflates. These are called Korotkoff sounds, and they’re created by blood pushing through a partially compressed artery. The systolic number is recorded when those sounds first appear. The diastolic number is recorded when the sounds suddenly muffle or disappear entirely, which signals that blood is now flowing freely through the artery without obstruction from the cuff. At that point, the pressure in the artery between heartbeats is greater than the cuff pressure, and the reading on the gauge corresponds to your diastolic pressure.
Automatic cuffs use sensors to detect the same changes, though the technology differs slightly. Either way, you get two numbers representing the peak and trough of pressure in your arteries.
Diastolic Blood Pressure Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology define these diastolic categories for adults:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
- Hypertensive crisis: 120 mm Hg or higher
The “elevated” blood pressure category (systolic 120 to 129) still requires a diastolic below 80, so there’s no separate elevated range for diastolic alone. A diastolic reading of 80 or above moves directly into Stage 1 hypertension territory.
A hypertensive crisis occurs when blood pressure reaches 180/120 or higher. If your diastolic hits 120 and you experience symptoms like chest pain, headache, or vision changes, that’s a medical emergency with potential organ damage to the heart, brain, kidneys, or eyes.
When Diastolic Is Too High
High diastolic pressure with a normal systolic number is called isolated diastolic hypertension. It tends to affect younger adults more than older populations, partly because arterial stiffness (which drives systolic numbers up) hasn’t set in yet. Instead, high diastolic pressure often reflects increased resistance in smaller blood vessels.
This pattern raises your lifetime risk of heart attack and makes death from cardiovascular disease more likely. It also increases the risk of congestive heart failure. These risks are greatest for women and people under 60. Because isolated diastolic hypertension rarely causes symptoms on its own, many people don’t know they have it until a routine check reveals the number.
When Diastolic Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mm Hg, so a diastolic consistently under 60 may qualify as hypotension. Many people with low diastolic pressure feel perfectly fine and never need treatment. But when symptoms do appear, they can include dizziness, lightheadedness, fainting, blurred vision, fatigue, nausea, and difficulty concentrating.
A very low diastolic number is particularly concerning for heart health because it can reduce the pressure that drives blood into the coronary arteries. If your heart muscle isn’t getting enough oxygen between beats, the risk of damage increases, especially in people who already have coronary artery disease.
Common causes of low diastolic pressure include dehydration, blood loss, certain medications, and conditions affecting the nervous system like Parkinson’s disease. Orthostatic hypotension, where your pressure drops when you stand up quickly, is one of the most common forms.
Diastolic vs. Systolic: Key Differences
One practical distinction worth knowing: diastolic pressure is remarkably stable compared to systolic. Your systolic number jumps around throughout the day in response to caffeine, anxiety, exercise, and even conversation. Diastolic pressure doesn’t fluctuate nearly as much under daily conditions. This stability is why a consistently elevated diastolic reading is taken seriously, because it’s less likely to be a temporary spike and more likely to reflect a real change in your vascular health.
For decades, doctors focused primarily on diastolic pressure as the more important number. That shifted in the early 2000s as research showed systolic pressure was a stronger predictor of heart disease and stroke, especially in older adults. Current guidelines treat both numbers as important. If either one falls into a higher category, your blood pressure classification follows the higher category. So a reading of 118/85 would be classified as Stage 1 hypertension based on the diastolic alone, even though the systolic is normal.