The bottom number on your blood pressure reading is called diastolic pressure. It measures the pressure inside your arteries during the brief moment when your heart relaxes between beats. If your reading is 120/80, the 80 is your diastolic number, measured in millimeters of mercury (mmHg). While the top number (systolic) captures the force when your heart pumps, the bottom number reflects what’s happening in your blood vessels when the heart is at rest.
What the Bottom Number Tells You
Every time your heart beats, it squeezes blood out into your arteries, creating a spike in pressure. Between those beats, your heart fills back up with blood and the pressure in your arteries drops to its lowest point. That lowest point is your diastolic pressure.
The diastolic number reflects the baseline tension in your arterial walls. It’s influenced by how flexible or stiff your blood vessels are, how much resistance your smaller arteries create, and how much blood volume is circulating in your system. A consistently high bottom number means your arteries never fully relax between heartbeats, which puts ongoing strain on your heart and blood vessels.
Normal, High, and Low Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology define the following categories for diastolic blood pressure:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Hypertensive crisis: above 120 mmHg
On the low end, blood pressure below 90/60 mmHg is generally considered low. A diastolic number under 60 isn’t automatically a problem if you feel fine, but it can cause dizziness, fatigue, blurred vision, trouble concentrating, and fainting in some people. Severely low diastolic pressure can reduce oxygen delivery to the heart and brain.
A diastolic reading above 120 mmHg is a medical emergency. At that level, blood vessel walls can sustain acute damage, and organs like the kidneys, brain, and heart are at immediate risk.
When Only the Bottom Number Is High
It’s possible for your diastolic number to be elevated while your systolic number stays normal. This is called isolated diastolic hypertension: a bottom number of 80 mmHg or higher with a top number below 130. It tends to be more common in younger and middle-aged adults.
Researchers don’t fully understand why this pattern occurs, but three of the strongest risk factors are carrying excess body weight, sleep apnea, and smoking. If your doctor flags a high bottom number with a normal top number, those are the first areas worth addressing.
How the Bottom Number Changes With Age
Your diastolic pressure doesn’t stay the same throughout your life. It tends to rise gradually from early adulthood through about age 50, driven by increasing resistance in your smaller arteries. After that, it typically starts to decline.
This drop happens because the large arteries stiffen with age. Stiff arteries don’t absorb as much of the heart’s pumping force, so the systolic (top) number keeps climbing while the diastolic (bottom) number falls. That’s why older adults often have a wide gap between their two numbers. A reading like 150/65 is common in someone over 70, even though a younger person with the same reading would have a very different clinical picture.
Getting an Accurate Reading
The diastolic number is particularly sensitive to measurement errors. When blood pressure is taken manually with a cuff and stethoscope, the person reading it has to listen for the precise moment when the tapping sounds in the artery disappear. That moment marks the diastolic pressure. Research shows that both novice and experienced clinicians tend to slightly over-read the diastolic value compared to the true number.
There’s also a well-documented bias in manual readings: some clinicians unconsciously round a diastolic reading of 90 down to 88, because 90 mmHg crosses the threshold into stage 2 hypertension and may trigger treatment decisions. Automated home monitors avoid this kind of human bias, which is one reason home readings taken over several days often give a more reliable picture than a single office visit.
For the most accurate diastolic reading at home, sit with your back supported and feet flat on the floor for five minutes before measuring. Keep the cuff at heart level on bare skin, and don’t talk during the reading. Taking two or three readings a minute apart and averaging them will smooth out the natural beat-to-beat variation in your diastolic pressure.