The bottom number in a blood pressure reading is called diastolic pressure, and it measures the force of blood pushing against your artery walls while your heart rests between beats. If your reading is 120/80, the 80 is your diastolic pressure, measured in millimeters of mercury (mm Hg). While the top number (systolic) captures the peak force when your heart contracts, the bottom number reflects what’s happening during the pause, and it plays a surprisingly important role in keeping your heart muscle healthy.
Why the Bottom Number Matters for Your Heart
Your heart muscle is unusual compared to other organs. Most of your body receives fresh, oxygenated blood when the heart contracts, but the heart itself gets most of its blood supply during the resting phase between beats. That’s the exact moment diastolic pressure is measuring. The pressure in your arteries during this rest period is what drives blood through the small coronary vessels that feed the heart muscle itself.
This is why a healthy diastolic pressure is critical. If it drops too low, your coronary arteries may not get enough blood flow to keep the heart adequately supplied with oxygen. If it stays too high, the constant extra force damages artery walls over time. Either extreme creates real problems, which is why doctors pay attention to both numbers on your reading, not just the top one.
Normal, High, and Low Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology define diastolic blood pressure categories as follows:
- Normal: below 80 mm Hg
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
On the low end, a reading below 60 mm Hg (as part of an overall reading below 90/60) is generally considered low blood pressure. If your top and bottom numbers fall into two different categories, the higher category applies. So a reading of 118/85 would be classified as stage 1 hypertension because of the bottom number, even though the top number is normal.
What a High Bottom Number Means
When the bottom number is elevated but the top number stays in the normal range, the condition is called isolated diastolic hypertension. Experts don’t fully understand why this happens, but obesity, sleep apnea, and smoking are three of the biggest risk factors. Higher body weight has a particularly strong link: research from the American Heart Association found that for every one-unit increase in BMI, diastolic pressure rises by about half a mm Hg on average. Alcohol intake also nudges the number up, while physical activity pushes it down.
Isolated diastolic hypertension may not cause obvious symptoms right away, but it raises your lifetime risk of a heart attack and makes death from cardiovascular disease more likely. It also increases the risk of heart failure. These risks are greatest for women and people under 60, which is notable because younger adults are more likely to have this particular pattern of high blood pressure.
What a Low Bottom Number Means
A diastolic reading that dips too low can cause symptoms like dizziness, lightheadedness, blurred vision, fatigue, and trouble concentrating. A sudden drop of just 20 mm Hg can be enough to make you feel faint. That matters because falls from fainting are a real injury risk, especially for older adults.
There’s also a less obvious danger. Research published in the Journal of the American College of Cardiology has shown that overly aggressive blood pressure treatment that pushes diastolic pressure too low can actually starve the heart muscle of blood, triggering cardiac events. This is sometimes called the “J-curve” problem: both high and low extremes are harmful. If you’re on blood pressure medication and consistently getting diastolic readings in the low 60s or below, that’s worth discussing with whoever manages your prescriptions.
How the Bottom Number Changes With Age
Your diastolic pressure doesn’t stay fixed throughout your life. CDC data shows a clear pattern: it rises gradually from early adulthood, peaks around age 60, and then starts to decline. This means a 35-year-old and a 70-year-old can have very different “typical” bottom numbers even when both are healthy. The decline after 60 happens because arteries stiffen with age, which tends to push the top number higher while the bottom number drops. That widening gap between the two numbers is itself a marker of arterial stiffness.
Getting an Accurate Reading
Blood pressure readings are sensitive to surprisingly small details, and an inaccurate diastolic number can lead to unnecessary worry or missed problems. Common errors that throw off your reading include skipping a five-minute rest period before measurement, having your arm positioned below heart level (which can inflate the reading by up to 23 mm Hg), using the wrong cuff size, and crossing your legs during the measurement.
A full bladder alone can raise your systolic reading by up to 33 mm Hg. Caffeine, nicotine, food, and physical activity within 30 minutes of a reading all affect accuracy. Even talking during the measurement introduces error. For the most reliable number, sit quietly for five minutes beforehand with your feet flat on the floor, your arm supported at heart level, and an appropriately sized cuff on bare skin. Multiple readings taken a minute or two apart give a better picture than a single measurement.
White-coat syndrome, the anxiety spike that happens in a medical setting, can push readings up by as much as 26 mm Hg. If your numbers are consistently higher at the doctor’s office than at home, a home blood pressure monitor with a validated device can give you and your provider a more accurate baseline to work from.