Diastolic blood pressure 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. A normal diastolic reading is below 80 mmHg, and the number matters more than many people realize because it reflects how much pressure your blood vessels are under even at rest.
What Happens During Diastole
Your heart cycles between squeezing (systole) and relaxing (diastole) with every beat. During diastole, the heart’s pumping chambers relax, valves between the upper and lower chambers open, and blood flows in to refill the ventricles. About 75% of that filling happens passively as blood rushes in from the veins. The remaining 25% gets pushed in by a final contraction of the upper chambers (the atria) right before the next heartbeat.
While all this is happening, your arteries still contain pressurized blood from the previous heartbeat. Large arteries like the aorta are elastic, so they stretch when blood is pumped into them and then slowly recoil, keeping pressure from dropping to zero. The lowest point that pressure reaches before the next heartbeat is your diastolic blood pressure. In a healthy person, that’s around 80 mmHg.
What the Numbers Mean
Blood pressure is written as two numbers, such as 120/80. The top number (systolic) captures peak pressure during a heartbeat. The bottom number (diastolic) captures the lowest pressure between beats. Both numbers matter, but they can tell you different things about your cardiovascular health.
The 2025 guidelines from the American Heart Association and American College of Cardiology define diastolic ranges like this:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Hypertensive crisis: 120 mmHg or higher
A reading of 120/80 or higher is no longer considered “normal.” If your diastolic sits between 80 and 89 on multiple readings, you’re in Stage 1 hypertension territory even if your top number looks fine.
When Only the Bottom Number Is High
Some people have a normal systolic reading (under 130) but a diastolic reading of 80 or above. This is called isolated diastolic hypertension, and it’s more common in younger adults. It may not cause symptoms, but it’s not harmless. It raises your lifetime risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are highest for women and people under 60.
Isolated diastolic hypertension often signals that smaller blood vessels throughout the body are tightening or stiffening. Because it rarely causes noticeable symptoms, it tends to go undetected unless you’re checking your blood pressure regularly.
When the Bottom Number Is Too Low
A diastolic number that’s too low can also be a problem, especially for people with heart disease. Your heart muscle gets most of its own blood supply during diastole, so when diastolic pressure drops too far, the heart may not receive enough oxygen. Research in patients with coronary artery disease has shown a J-shaped risk curve: cardiovascular events start increasing when diastolic pressure falls below 70 mmHg. In patients with readings below 50 mmHg, the risk of long-term mortality rises by roughly 55%.
Low diastolic pressure is more common in older adults, people on multiple blood pressure medications, and those who are dehydrated. If your bottom number consistently reads in the 50s or lower and you feel dizzy, fatigued, or lightheaded, that’s worth discussing with your doctor.
How Diastolic Pressure Changes With Age
Your diastolic number doesn’t stay the same throughout life. CDC data on U.S. adults shows a clear pattern: diastolic pressure rises from early adulthood through roughly age 60, then starts to decline. This happens because arteries gradually stiffen with age. Stiff arteries push the systolic number up but allow the diastolic number to fall, which is why older adults often have a wide gap between their two numbers.
This means a high diastolic reading in a 35-year-old and a low diastolic reading in a 75-year-old can both signal cardiovascular risk, just through different mechanisms. The context of your age and overall health shapes what your diastolic number actually means for you.
Getting an Accurate Diastolic Reading
Diastolic readings are sensitive to how you position your body during measurement. A 2024 Johns Hopkins study found that resting your arm on your lap instead of a desk overestimated diastolic pressure by 4.0 mmHg. Letting your arm hang unsupported at your side was even worse, inflating the diastolic reading by 4.4 mmHg. Those errors are large enough to push a normal reading into the hypertension range.
For the most accurate reading, sit with your back supported, feet flat on the floor, legs uncrossed, and your arm resting on a table so the middle of the cuff sits at heart level. Use the right cuff size (too small a cuff inflates readings). Take at least two readings a minute or two apart and average them. Avoid caffeine, exercise, and a full bladder for 30 minutes beforehand.
Lifestyle Changes That Lower Diastolic Pressure
If your diastolic number is creeping up, several changes can bring it down without medication. The biggest single impact comes from diet. Eating a pattern rich in fruits, vegetables, whole grains, and low-fat dairy while cutting saturated fat can lower blood pressure by up to 11 mmHg. Reducing sodium to 1,500 mg per day (about two-thirds of a teaspoon of table salt) can drop it another 5 to 6 mmHg.
Regular aerobic exercise, such as brisk walking, cycling, or swimming for 150 minutes a week, lowers blood pressure by about 5 to 8 mmHg. Losing excess weight helps too: each kilogram lost (about 2.2 pounds) brings pressure down by roughly 1 mmHg. Increasing potassium intake to 3,500 to 5,000 mg per day through foods like bananas, potatoes, beans, and leafy greens can add another 4 to 5 mmHg reduction. These effects stack, so combining several changes often produces results comparable to a first-line medication.