The bottom number in a blood pressure reading is called diastolic pressure. It measures the force of blood pushing against your artery walls while your heart is resting between beats. In a reading like 120/80, the 80 is your diastolic pressure, measured in millimeters of mercury (mm Hg).
What Happens During the Bottom Number
Every heartbeat has two phases. During the first phase, your heart contracts and pumps blood out into your arteries. That’s the top number, systolic pressure. During the second phase, your heart’s lower chambers relax and refill with blood. The pressure in your arteries during that resting moment is the bottom number.
This resting phase matters more than it might seem. Your heart muscle gets most of its own blood supply during this relaxation period. The pressure that remains in your arteries between beats is what pushes oxygen-rich blood into the small vessels feeding the heart itself. If diastolic pressure drops too low, the heart may not receive enough fuel. If it stays too high, it signals that your arteries are under constant strain even when your heart isn’t actively pumping.
Normal Ranges for the Bottom Number
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories based on both numbers. For diastolic pressure specifically:
- Normal: below 80 mm Hg
- Elevated blood pressure: below 80 mm Hg (defined mainly by a systolic reading of 120 to 129)
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
Notice that “elevated blood pressure” is really about the top number creeping up while the bottom number stays normal. Once the bottom number reaches 80 on its own, you’ve crossed into hypertension territory regardless of what the top number reads.
When Only the Bottom Number Is High
Sometimes the top number sits in a healthy range while the bottom number is elevated. This is called isolated diastolic hypertension, and it tends to show up more often in younger adults. It usually doesn’t cause immediate symptoms or complications, but it isn’t harmless either.
Over time, isolated diastolic hypertension raises the risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are highest for women and people under 60. Because you typically feel fine, this pattern often goes unnoticed unless you’re checking your blood pressure regularly.
When the Bottom Number Is Too Low
There’s no single cutoff that defines “too low” for diastolic pressure. Most clinicians focus less on a specific number and more on whether you’re experiencing symptoms like dizziness, lightheadedness, blurred vision, or fainting. A diastolic reading in the 50s or below may warrant attention, especially if those symptoms are present.
Common causes of a low bottom number include dehydration, prolonged bed rest, pregnancy, and certain medications, particularly those prescribed for high blood pressure that may overcorrect. A very low diastolic pressure can reduce blood flow to the heart muscle during its resting phase, which is a concern for people who already have narrowed coronary arteries. If you notice your bottom number dropping significantly after starting a new medication, that’s worth bringing up at your next appointment.
Getting an Accurate Reading
Diastolic pressure is sensitive to how the measurement is taken. Small technical errors can shift the number enough to change your category. According to the American Medical Association, having your arm positioned below heart level can inflate the reading by anywhere from 4 to 23 mm Hg. Using the wrong cuff size, one that’s too small or too large for your arm, also introduces errors.
For the most reliable reading, sit with your back supported, feet flat on the floor, and your arm resting on a surface at heart height. Avoid caffeine, exercise, and smoking for at least 30 minutes before measuring. Take two or three readings a minute apart and average them, since the first reading tends to run higher. Home monitors are generally reliable, but it helps to bring yours to an appointment occasionally so you can compare it against the office equipment.
Why Both Numbers Matter
For years, the top number got most of the clinical attention, especially in older adults where systolic pressure tends to climb while diastolic pressure may actually drop. But the bottom number carries its own weight. In people under 50, a high diastolic reading is a stronger predictor of cardiovascular problems than the systolic number alone. In older adults, a diastolic reading that falls too low while the top number stays high (creating a wide gap between the two) can signal stiffened arteries and reduced blood flow to the heart.
The two numbers together tell a more complete story than either one alone. A reading of 135/70 tells you something different from 135/95, even though the top number is identical. Paying attention to both gives you a clearer picture of how much stress your cardiovascular system is under, both when your heart is working and when it’s at rest.