The bottom number on a blood pressure reading is called diastolic pressure. It measures the force of blood pushing against your artery walls between heartbeats, during the brief moment when your heart relaxes and refills with blood. A normal diastolic pressure is below 80 mmHg.
What Diastolic Pressure Actually Measures
When you see a blood pressure reading like 120/80, the top number (systolic) captures the peak pressure as your heart contracts and pushes blood out. The bottom number captures the lowest pressure in your arteries while your heart rests between beats. That resting phase matters more than you might think: it’s when your heart muscle itself receives its own blood supply. The coronary arteries, the small vessels that feed oxygen to the heart, fill primarily during this relaxation phase. So diastolic pressure is essentially the driving force behind your heart’s own oxygen delivery.
If diastolic pressure drops too low, your heart muscle may not get enough oxygen. If it stays too high, it signals that your arteries are under constant excess strain, even when the heart isn’t actively pumping.
What the Numbers Mean
The American Heart Association’s 2025 guidelines classify diastolic blood pressure into clear categories:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
The “elevated” blood pressure category (systolic 120 to 129) only applies when diastolic remains below 80. In other words, a high bottom number on its own is enough to qualify as hypertension, even if your top number looks fine.
When Only the Bottom Number Is High
Having a diastolic reading of 80 or above while the systolic stays under 130 is called isolated diastolic hypertension. It’s more common in younger adults, and it often gets overlooked because the top number appears normal. But it carries real consequences. Isolated diastolic hypertension raises the lifetime risk of heart attack and increases the likelihood of dying from cardiovascular disease. It also raises the risk of heart failure. These risks are greatest for women and people under 60.
The condition often reflects increased stiffness or resistance in smaller arteries. Being overweight, physically inactive, or consuming excess sodium are common contributing factors.
When the Bottom Number Is Too Low
There’s no single cutoff for “too low,” but diastolic readings that consistently fall well below 60 mmHg can cause problems, especially in older adults. Because the heart muscle relies on diastolic pressure for its blood supply, a very low bottom number can starve the heart of oxygen.
Symptoms of low diastolic pressure overlap with general low blood pressure: dizziness, lightheadedness, blurred vision, fatigue, and fainting. Common causes include dehydration, certain medications (particularly those used to treat high blood pressure), and prolonged bed rest. A sudden drop of even 20 mmHg can trigger noticeable symptoms.
How the Bottom Number Changes With Age
Diastolic pressure follows a different pattern than systolic pressure over a lifetime. Systolic pressure tends to climb steadily as you age. Diastolic pressure, however, rises from early adulthood until roughly age 60, then begins to decline. This happens because arteries stiffen with age, which pushes systolic pressure up while allowing diastolic pressure to fall. CDC data confirms this curvilinear pattern in both men and women.
This is why older adults sometimes develop a wide gap between their two numbers, like 160/65. That combination, high systolic with low diastolic, can be particularly tricky to manage because lowering the top number with medication risks pushing the bottom number dangerously low.
How to Lower a High Diastolic Reading
The lifestyle changes that lower systolic pressure also bring diastolic pressure down, though typically by a smaller amount. Reducing sodium intake is one of the best-studied strategies. A modest reduction of about 4.4 grams of salt per day lowers diastolic pressure by roughly 3 mmHg in people with hypertension and about 1 mmHg in people with normal blood pressure. Those numbers may sound small, but across an entire population, even a 2 to 3 mmHg shift reduces the rate of strokes and heart attacks significantly.
Regular aerobic exercise, maintaining a healthy weight, limiting alcohol, and eating a diet rich in fruits, vegetables, and whole grains all contribute. For people whose diastolic pressure stays in the hypertension range despite these changes, medication is the next step.
Getting an Accurate Reading
Your diastolic number is particularly sensitive to measurement conditions. Using a blood pressure cuff that’s the wrong size for your arm can throw off the bottom number. A cuff that’s too small tends to inflate the reading, while one that’s too large can underestimate it. Other common sources of error include crossing your legs during the reading, resting your arm below heart level, talking, or measuring right after exercise or caffeine.
For the most reliable picture of your diastolic pressure, take readings at the same time each day, sit quietly for five minutes beforehand, and keep your arm supported at heart level. If your bottom number seems unexpectedly high or low on a single reading, repeating it a minute later often gives a more accurate result.