The diastolic number, the bottom number in a blood pressure reading, measures the pressure in your arteries between heartbeats, when your heart is relaxing and refilling with blood. A normal diastolic reading is below 80 mmHg. This number tells you something important about the condition of your blood vessels and how much resistance blood faces as it moves through your body.
What Happens During Diastole
Every heartbeat has two phases. During the first phase (systole), your heart contracts and pushes blood out into your arteries, producing the top number. During the second phase (diastole), the lower chambers of your heart relax and fill with blood from the upper chambers. The pressure that remains in your arteries during this resting phase is your diastolic number.
Think of it this way: the top number captures the peak force when your heart squeezes, while the bottom number captures the baseline pressure your arteries maintain even when the heart isn’t actively pumping. That baseline pressure is largely determined by how tight or relaxed your smaller blood vessels are. When those vessels are narrowed or stiff, blood meets more resistance, and your diastolic number climbs.
Diastolic Blood Pressure Ranges
The American Heart Association breaks blood pressure into these categories based on both numbers:
- Normal: diastolic below 80 mmHg
- Elevated blood pressure: diastolic still below 80, but systolic is between 120 and 129
- Stage 1 hypertension: diastolic 80 to 89 mmHg
- Stage 2 hypertension: diastolic 90 mmHg or higher
Your blood pressure falls into whichever category is higher. So if your reading is 118/86, the systolic is normal but the diastolic puts you in stage 1 hypertension. That single elevated number is enough to classify the whole reading.
Why a High Diastolic Number Matters
A high diastolic reading, especially in younger adults, reflects increased resistance in the small blood vessels throughout your body. This means your arteries are working harder than they should be, even during the moments your heart is at rest. Over time, that extra pressure takes a toll.
When only the diastolic number is elevated (80 or above) while the systolic stays below 130, the condition is called isolated diastolic hypertension. It may not cause immediate problems, but it raises your long-term risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are greatest for women and people under 60.
High diastolic pressure also affects the kidneys. A large study of young adults found that those with stage 1 isolated diastolic hypertension had a 19% higher risk of developing chronic kidney disease compared to people with normal blood pressure. For stage 2 (diastolic 90 or above), that risk nearly doubled. The kidneys are packed with tiny blood vessels, making them especially vulnerable to sustained high pressure.
When the Diastolic Number Is Too Low
A diastolic reading below 60 mmHg is generally considered low. Low blood pressure can cause dizziness, blurred vision, fatigue, trouble concentrating, fainting, and nausea. Even a drop of just 20 mmHg from your usual reading can trigger symptoms.
Diastolic pressure also naturally changes with age. Both numbers tend to rise through middle age, but after about age 60 to 70, diastolic pressure plateaus and then gradually declines while systolic pressure keeps climbing. This is why older adults sometimes have a wide gap between the two numbers, like 150/65. That widening gap reflects stiffening of the large arteries, which is a cardiovascular risk factor in its own right.
How Diastolic Readings Get Measured
A diagnosis of high diastolic pressure requires at least two readings taken at two separate visits. At each visit, your provider should take at least two measurements. The one exception: if your reading is 160/100 or higher, treatment can begin right away without waiting for a follow-up visit.
Several common factors can throw off your diastolic reading. Eating, drinking caffeine, smoking, or exercising within 30 minutes of the measurement can all skew results. Having a full bladder raises the reading. If your arm is positioned below heart level during the test, the error can be anywhere from 4 to 23 mmHg, which is enough to push a normal reading into the hypertension range.
Crossing your legs at the knees, talking during the measurement, or using a blood pressure cuff that’s the wrong size for your arm also produces inaccurate numbers. For the most reliable reading, you should sit quietly in a chair for five minutes beforehand with your feet flat on the floor and your arm supported at heart level. If your numbers seem unexpectedly high, it’s worth asking whether any of these factors may have been at play before assuming the worst.
What Your Diastolic Number Tells You
The diastolic number is a window into the health of your smaller blood vessels. While the systolic number gets more attention, especially in older adults, the diastolic number carries its own weight, particularly if you’re younger. A consistently elevated diastolic reading signals that your vascular system is under strain even when your heart is resting, and that strain compounds over years into real organ damage.
If your diastolic number has been creeping up, the factors that improve it are the same ones that improve overall cardiovascular health: regular physical activity, reducing sodium intake, maintaining a healthy weight, and managing stress. Even modest changes in these areas can bring the number down by several points, which at the margins between normal and stage 1 hypertension makes a meaningful difference.