The bottom number in a blood pressure reading is called diastolic pressure, and it measures the force your blood exerts against your artery walls between heartbeats, when your heart is resting. If your reading is 120/80, that “80” tells you how much pressure remains in your arteries during the brief pause between pumps. A healthy diastolic pressure is below 80 mm Hg.
What Happens Between Heartbeats
Your heart works in a two-phase cycle: squeeze and relax. The top number (systolic) captures the peak pressure when the heart contracts and pushes blood out. The bottom number captures the baseline pressure that stays in your arteries while the heart refills with blood before its next beat.
That baseline pressure matters more than it might sound. During this resting phase, your coronary arteries, the vessels that feed your heart muscle itself, receive most of their blood supply. If diastolic pressure is too high, your arteries are under constant strain even when the heart is supposed to be resting. If it’s too low, your heart may not get enough oxygen-rich blood between beats. Think of it as the minimum load your blood vessels never get a break from.
What determines that number is largely the resistance in your smaller blood vessels. When those vessels are tight or stiff, blood has a harder time flowing through, and the pressure between beats stays elevated. Factors like stress, excess sodium, and lack of physical activity can all increase that resistance.
Diastolic Pressure Categories
The most recent guidelines from the American Heart Association and American College of Cardiology define these diastolic ranges:
- Normal: below 80 mm Hg
- Elevated blood pressure: below 80 mm Hg (this category is defined by the top number being 120 to 129, with a normal bottom number)
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
If your top and bottom numbers fall into two different categories, the higher category applies. So a reading of 118/86 would be classified as Stage 1 hypertension because of that diastolic number, even though the systolic looks normal.
When the Bottom Number Is Too High
A consistently elevated diastolic reading, sometimes called isolated diastolic hypertension, means the pressure in your arteries between beats is higher than it should be. This is more common in younger adults because of how arteries change with age.
It raises your lifetime risk of heart attack and makes death from cardiovascular disease more likely. It also increases your risk of congestive heart failure. These risks are greatest for women and people under 60. The tricky part is that high diastolic pressure rarely causes noticeable symptoms on its own. You won’t feel it. That’s why regular blood pressure checks matter, even when you feel fine.
When the Bottom Number Is Too Low
A very low diastolic reading (generally below 60 mm Hg) can also be a concern, particularly for older adults or people with existing heart disease. Because the heart muscle receives its own blood supply primarily during the diastolic phase, a very low bottom number can mean reduced blood flow to the heart itself. This creates a paradox: the heart needs adequate resting pressure to keep itself nourished.
Symptoms of low diastolic pressure can include dizziness, fatigue, and feeling faint, especially when standing up. That said, some people naturally run low without any problems. Context matters. A diastolic reading of 58 in an otherwise healthy 25-year-old is very different from the same number in a 72-year-old on blood pressure medication.
How Diastolic Pressure Changes With Age
Your bottom number doesn’t follow the same trajectory as your top number over a lifetime. Systolic pressure tends to climb steadily as you age. Diastolic pressure follows a different curve: it rises from early adulthood until about age 60, then starts to decline. CDC data shows this pattern holds for both men and women.
This is why isolated diastolic hypertension is primarily a concern for younger and middle-aged adults. After 60, the gap between the top and bottom numbers tends to widen. In older adults, doctors typically focus more on systolic pressure and on that widening gap (called pulse pressure) as markers of arterial stiffness.
Getting an Accurate Reading
Diastolic readings can be thrown off by the same things that affect any blood pressure measurement. A cuff that’s too small will give you an artificially high number. Talking during the reading, crossing your legs, or having a full bladder can all bump both numbers up. Caffeine and stress in the 30 minutes before a check can inflate your results temporarily.
For a reliable picture, take your reading while sitting with your back supported, feet flat on the floor, and your arm resting at heart level. Multiple readings on different days give a much clearer picture than any single measurement. Home monitors are a practical way to track trends, since many people read higher in a clinical setting simply from the anxiety of being in a medical office.
Lowering a High Diastolic Number
Lifestyle changes can meaningfully bring down your diastolic pressure. A diet rich in whole grains, fruits, vegetables, and low-fat dairy while cutting back on saturated fat can lower blood pressure by up to 11 mm Hg. Limiting sodium to 1,500 mg per day, roughly the amount in two-thirds of a teaspoon of table salt, can drop it another 5 to 6 mm Hg. Increasing potassium through foods like bananas, potatoes, and leafy greens can lower it by 4 to 5 mm Hg.
Regular aerobic exercise, things like brisk walking, cycling, or swimming, lowers blood pressure by about 5 to 8 mm Hg. You don’t need to train for a marathon. Consistent moderate activity, around 150 minutes per week, is what drives the benefit. These changes are additive, meaning stacking several together can bring a diastolic number in the mid-80s back into the normal range without medication for many people.