The bottom number in a blood pressure reading is your diastolic pressure, measured in millimeters of mercury (mm Hg). It reflects the pressure inside your arteries during the brief moment between heartbeats, when your heart is relaxed and filling with blood. A normal diastolic reading is below 80 mm Hg.
What Happens Between Heartbeats
Each heartbeat has two phases. During the first phase, called diastole, your lower heart chambers (ventricles) relax and fill with blood from the upper chambers. During the second phase, called systole, the ventricles contract and push blood out into your arteries. The top number in your reading captures the peak pressure during that contraction. The bottom number captures the lowest pressure, when your heart is resting between pumps.
Think of it like a garden hose with a pulsing nozzle. The top number is the pressure when the nozzle opens fully. The bottom number is the residual pressure still in the hose between pulses. That residual pressure depends on how tight or relaxed your blood vessels are and how elastic your artery walls remain.
Diastolic Pressure Ranges
The American Heart Association breaks diastolic pressure into clear categories:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
- Severe hypertension: above 120 mm Hg
The “elevated” blood pressure category (systolic 120 to 129) still requires a diastolic reading below 80. So you can have an elevated top number while your bottom number looks perfectly fine.
When Only the Bottom Number Is High
Some people have a normal top number but a diastolic reading of 80 or above, a pattern called isolated diastolic hypertension. This is more common in younger adults and is strongly linked to carrying excess weight, sleep apnea, and smoking.
It often causes no symptoms at all, which is part of what makes it easy to miss. But it raises your lifetime risk of heart attack and congestive heart failure, and increases the likelihood of dying from cardiovascular disease. These risks are greatest for women and people under 60. If your bottom number has been creeping up while your top number stays normal, it’s worth paying attention to rather than dismissing.
When the Bottom Number Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mm Hg, so a diastolic number under 60 may warrant attention, especially if you’re experiencing dizziness, lightheadedness, or fainting. Common causes include dehydration (from illness, intense exercise, or certain medications that increase urination), heart valve problems, and a slow heart rate.
In people who already have high systolic pressure, a very low diastolic number can actually signal trouble. Research published in the Journal of the American College of Cardiology found that among men with high systolic readings, those with a diastolic pressure below 90 had 73% higher cardiovascular death rates compared to those in the 90 to 99 range. The reason: when large arteries stiffen (which raises the top number) and smaller blood vessels also tighten (which raises the bottom number), the two forces can cancel each other out in the diastolic reading, making it look deceptively normal or low even though real vascular damage is occurring.
How Diastolic Pressure Changes With Age
Your diastolic and systolic numbers don’t age the same way. Both tend to rise together through middle age. But after about age 60, something shifts. Systolic pressure keeps climbing while diastolic pressure plateaus and often starts to drop. By the time people reach their 70s, it’s common to see a wide gap between the two numbers.
This happens because arteries lose their elasticity over time. Stiff arteries can’t absorb the force of each heartbeat as well, so peak pressure (systolic) goes up. But that same stiffness also means the arteries recoil less between beats, which pulls diastolic pressure down. The growing gap between the two numbers, called pulse pressure, is itself a marker of cardiovascular risk in older adults. For this reason, diastolic pressure alone becomes a less reliable indicator of heart health as you age, and doctors tend to focus more on the top number and the spread between the two.
Which Number Matters More
For decades, doctors treated diastolic pressure as the primary number to worry about. That thinking shifted in the 1990s and 2000s as large studies showed systolic pressure was a stronger predictor of stroke and heart disease, particularly in older adults. Today, most guidelines treat both numbers as important, but in different contexts.
In younger adults, a rising diastolic number can be an early signal that blood vessels are tightening, often due to lifestyle factors that are still reversible. In older adults, the systolic number and the gap between the two readings tend to carry more predictive weight. The relationship also differs by sex: research on middle-aged adults found that women with both numbers elevated face higher cardiovascular risk than women with only a high systolic reading, while in men the pattern is more complex, with a U-shaped curve where both very low and very high diastolic readings increase risk.
The bottom line: neither number tells the full story on its own. Your diastolic pressure reveals how much strain your arteries are under even when your heart is at rest, and tracking it over time gives you a meaningful window into how your cardiovascular system is holding up.