Yes, diastolic blood pressure is the bottom number in a blood pressure reading. In a reading like 120/80, the 80 is your diastolic pressure. It measures the force your blood exerts against your artery walls while your heart rests between beats. That resting phase, called diastole, is when the heart muscle relaxes and refills with blood before the next contraction.
What the Two Numbers Mean
A blood pressure reading always has two numbers written as a fraction. The top number (systolic) captures the peak pressure in your arteries when your heart contracts and pushes blood out. The bottom number (diastolic) captures the lowest pressure, during the pause between beats. Both are measured in millimeters of mercury, abbreviated mmHg.
Think of it like a wave: systolic is the crest, diastolic is the trough. Neither number tells the full story on its own. A reading of 130/70 and a reading of 130/90 have the same top number but very different implications for your health.
Why the Bottom Number Matters
Diastolic pressure reflects the baseline tension in your arteries even when your heart isn’t actively pumping. It’s influenced by how flexible or stiff your blood vessels are. When diastolic pressure stays elevated, it means your arteries never fully relax between heartbeats, which puts constant strain on your cardiovascular system.
Your heart muscle itself depends on the diastolic phase. Most of the blood flow to the heart’s own tissue happens during diastole, when the muscle is relaxed enough to allow blood into the coronary arteries. Adequate diastolic pressure helps ensure oxygen and nutrients reach the heart between each beat.
Healthy Ranges for Diastolic Pressure
Current guidelines break diastolic pressure into clear categories:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
The “elevated blood pressure” category only applies to the top number (120 to 129 systolic). If your systolic is in that range but your diastolic stays below 80, it’s classified as elevated rather than full hypertension. Once diastolic hits 80, though, it crosses into Stage 1 regardless of the top number.
High Diastolic Pressure in Younger Adults
When only the bottom number is elevated while the top number remains normal, it’s called isolated diastolic hypertension. This pattern accounts for up to 20% of hypertension cases and tends to be underrecognized. Clinicians historically focused more on the top number when making a diagnosis, but the bottom number carries real risk, especially for younger people.
A large study of over 6.4 million Korean adults aged 20 to 39 found that those with Stage 1 isolated diastolic hypertension had a 32% higher risk of cardiovascular events compared to people with normal blood pressure. Stage 2 raised the risk by 82%. An international database tracking ambulatory blood pressure found that people under 50 with isolated diastolic hypertension had nearly three times the risk of cardiovascular events compared to those with normal readings.
In adults over 50, the picture gets murkier. Isolated diastolic hypertension becomes less common with age, particularly after 55, as arteries stiffen and systolic pressure tends to rise while diastolic may actually drop. For older adults, a low diastolic number can sometimes signal reduced heart function rather than good health.
When Diastolic Pressure Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mmHg, though most doctors only consider it a problem if it causes symptoms. A diastolic number that drops too low can mean your organs aren’t getting enough blood flow during the resting phase of each heartbeat.
Symptoms of low diastolic pressure overlap with general low blood pressure: dizziness, lightheadedness, blurred vision, fatigue, trouble concentrating, and fainting. A drop of just 20 mmHg from your usual reading can be enough to trigger these symptoms. Dehydration, blood loss, certain medications, and prolonged bed rest are common causes.
Getting an Accurate Reading
Small mistakes during measurement can skew your results by a surprising amount. According to the American College of Cardiology, these common errors can inflate your reading:
- Wrong cuff size: a cuff that’s too small can add 5 to 20 mmHg to your reading
- Skipping the rest period: not sitting quietly for five minutes beforehand can add 10 to 20 mmHg
- Talking or texting: adds 10 to 15 mmHg
- Full bladder: can raise your reading by 10 to 15 mmHg
- Unsupported back: sitting upright on an exam table instead of a chair with back support can add 5 to 15 mmHg
- Crossed legs or dangling feet: adds 5 to 8 mmHg
- Recent caffeine: consuming 200 to 300 mg of caffeine (roughly two cups of coffee) can raise systolic pressure by about 8 mmHg
- Measuring over clothing: can shift the reading by 10 to 50 mmHg in either direction
These numbers represent effects on the top number specifically, but both numbers are affected by the same measurement conditions. For the most accurate diastolic reading, sit in a chair with your feet flat on the floor, back supported, arm resting at heart level on a table, cuff on bare skin, and wait five minutes before measuring. Take two or three readings a minute apart and average them. A single reading in a doctor’s office, especially if you’re anxious or rushed, may not reflect your true baseline.