The bottom number in a blood pressure reading is called diastolic pressure, and it measures the force of blood pushing against your artery walls between heartbeats, when your heart is resting and refilling with blood. If your reading is 120/80, for example, that 80 is your diastolic pressure, measured in millimeters of mercury (mmHg). It tells you something important about how much pressure your arteries are under even when your heart isn’t actively pumping.
What Happens in Your Body During Diastole
Your heart works in two phases. During the pumping phase (systole), your heart contracts and pushes blood out into your arteries, creating the top number. During the resting phase (diastole), both the upper and lower chambers of your heart relax. Blood flows in from your veins, steadily refilling the chambers until pressure equalizes.
While your heart rests, your arteries don’t go slack. Healthy arteries are elastic, like a rubber band. They stretch when blood surges through during a heartbeat, then spring back between beats to keep blood moving forward. That elastic recoil is what maintains diastolic pressure. A normal diastolic reading (below 80 mmHg) reflects arteries that are flexible and resilient. The reading also matters because your heart muscle itself receives most of its own blood supply during this resting phase, so adequate diastolic pressure helps keep the heart nourished with oxygen.
Diastolic Pressure Categories
Current guidelines break diastolic blood pressure into clear ranges:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
Notably, the “elevated blood pressure” category (when your top number is between 120 and 129) still requires a diastolic reading below 80. So the bottom number can push you into a hypertension diagnosis even when the top number looks fine.
When Only the Bottom Number Is High
Some people have a normal top number (below 130 mmHg) but a diastolic reading of 80 or higher. This is called isolated diastolic hypertension. It’s typically diagnosed after two or more office visits show the same pattern. While it sounds less alarming than having both numbers elevated, it still carries real consequences: a higher lifetime risk of heart attack, a greater chance of developing heart failure, and increased risk of dying from cardiovascular disease overall.
Isolated diastolic hypertension is more common in younger adults. The causes overlap with general high blood pressure: excess weight, high sodium intake, lack of physical activity, and heavy alcohol use. Because the top number looks reassuring, this pattern sometimes gets overlooked, which makes it worth paying attention to both numbers in your reading.
When the Bottom Number Is Too Low
A diastolic number that drops significantly can also be a concern, though most doctors focus on whether it’s causing symptoms rather than on a specific cutoff. Signs of low diastolic pressure include dizziness, lightheadedness, blurred vision, fainting, and fatigue.
Common causes of low diastolic pressure include dehydration, pregnancy (which causes blood vessels to expand), certain heart conditions like heart valve disease or a slow heart rate, and medications used to treat high blood pressure. Parkinson’s disease and diabetes can also contribute.
In older adults, a low bottom number paired with a high top number is especially common, and it points to a specific problem: stiff arteries. Healthy, elastic arteries absorb the surge of each heartbeat and then recoil to maintain pressure between beats. When arteries stiffen with age or disease, they lose that recoil. The result is higher systolic pressure (because rigid arteries can’t absorb the surge) and lower diastolic pressure (because there’s less spring-back to sustain it). Research published in The Lancet found that this combination reflects diseased, stiff arteries rather than simply a blood pressure number that dipped too low.
How the Bottom Number Changes With Age
Your diastolic pressure doesn’t stay constant over your lifetime, and the pattern is somewhat counterintuitive. Data from a study of more than 10,000 people published in the American Heart Association’s journal Hypertension found that both the top and bottom numbers rise steadily from young adulthood through the 50s. After that, the two numbers diverge. Systolic pressure keeps climbing, but diastolic pressure plateaus around the 60s and then actually decreases slightly after the 70s.
This is why doctors treating older adults tend to focus more on the top number and on the gap between the two numbers (called pulse pressure). A widening gap, with a rising top number and falling bottom number, generally signals increasing arterial stiffness. In younger adults, though, the bottom number often carries more diagnostic weight because it can be the first sign of developing hypertension.
What Your Bottom Number Tells You
The diastolic number is essentially a report card on how your arteries perform between heartbeats. A reading below 80 means your blood vessels are maintaining healthy, low-level pressure while your heart rests. A reading of 80 to 89 is an early warning. A reading of 90 or above signals your arteries are under significant strain even during the “easy” part of the cardiac cycle.
If you’re younger than 50, pay close attention to the bottom number. It tends to be the first value to creep up with lifestyle factors like sodium, stress, and inactivity. If you’re over 60, a dropping bottom number alongside a rising top number is worth discussing with your doctor, since it often reflects changes in your artery walls rather than an improvement in blood pressure. Both numbers together give the full picture of your cardiovascular health.