The bottom number on 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. When you see a reading like 120/80, the 80 is your diastolic pressure. A normal diastolic reading is below 80 mm Hg.
What the Bottom Number Actually Measures
Your heart works in a pump-and-rest cycle. The top number (systolic) captures the pressure when your heart squeezes and pushes blood out. The bottom number captures the pressure during the pause between beats, when the heart fills back up with blood. Think of it as the baseline pressure your arteries never drop below, even when your heart is resting.
This resting pressure matters more than it might seem. A healthy diastolic pressure is critical for the coronary arteries, the small vessels that feed the heart muscle itself, to deliver enough oxygen. Those arteries fill with blood primarily during the resting phase, so if diastolic pressure is too low or too high, the heart’s own blood supply can suffer.
Blood Pressure Categories by the Numbers
The 2025 guidelines from the American Heart Association and American College of Cardiology classify blood pressure into four levels. If your top and bottom numbers fall into different categories, the higher category applies.
- Normal: below 120 systolic and below 80 diastolic
- Elevated: 120 to 129 systolic and below 80 diastolic
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic or 90 or higher diastolic
Notice that a diastolic reading of 80 to 89 on its own qualifies as Stage 1 hypertension, even if the top number looks fine. A diastolic reading of 90 or above puts you into Stage 2.
When Only the Bottom Number Is High
Some people have a normal top number but an elevated bottom number, a condition called isolated diastolic hypertension. The exact cause isn’t fully understood, but the biggest risk factors include carrying excess weight, sleep apnea, and smoking.
This pattern tends to show up in younger adults. While it may not cause symptoms right away, it raises your lifetime risk of heart attack and makes death from cardiovascular disease more likely. It also increases the chance of heart failure. These risks are greatest for women and people under 60.
Which Number Matters More
For most people, the top number is the stronger predictor of future heart attacks, strokes, and death, regardless of age. That’s why doctors often focus on systolic pressure. But the bottom number isn’t irrelevant. A large study reported by the American Heart Association found that in people under 50, diastolic readings provided additional information about cardiovascular risk that systolic pressure alone didn’t capture. In practical terms: if you’re younger, both numbers deserve attention.
How the Bottom Number Changes With Age
Diastolic pressure follows a predictable arc over a lifetime. CDC data shows it rises gradually from early adulthood until around age 60, then starts to decline. This happens because arteries stiffen with age. Stiffer arteries don’t expand as easily when the heart pumps, which drives the top number up, but they also spring back less during the resting phase, which lets the bottom number drop. That’s why older adults often have a large gap between their two numbers, sometimes called a wide pulse pressure.
When the Bottom Number Is Too Low
Low blood pressure is generally defined as a reading below 90/60, so a diastolic number under 60 can be part of that picture. Some people naturally run low without any problems. But when diastolic pressure drops enough to reduce blood flow to organs, you’ll notice symptoms: dizziness, lightheadedness, blurred vision, fatigue, nausea, difficulty concentrating, or even fainting. In severe cases, very low pressure can reduce blood flow enough to damage organs.
A diastolic reading that’s too low can also starve the heart itself of blood, since the coronary arteries depend on that resting-phase pressure. This is one reason doctors are cautious about pushing blood pressure medications too aggressively, particularly in older adults whose diastolic pressure may already be declining naturally.
How to Get an Accurate Reading
Diastolic readings are sensitive to how you sit and prepare. Small errors in technique can shift your bottom number by several points, enough to change which category you fall into.
Sit quietly for three to five minutes before the measurement. Avoid caffeine, exercise, and smoking for at least 30 minutes beforehand, and make sure your bladder is empty. During the reading, don’t talk or move. Rest your arm on a flat surface like a desk at chest height. If your arm hangs unsupported at your side, the reading will come in artificially high. Sitting on an exam table without back support can inflate the diastolic number by as much as 6 mm Hg, and crossing your legs can add another 3 to 5 points.
Cuff size also matters. A cuff that’s too small will give a falsely high reading, and one that’s too large will read low. The cuff should go on bare skin, not over a rolled-up sleeve, which can act like a tourniquet and skew results. When it’s snug, you should be able to slide one finger under the top and bottom edges easily, but two fingers should feel tight.
Lowering a High Diastolic Reading
The same lifestyle changes that lower overall blood pressure work on the bottom number. Regular aerobic exercise, at least 30 minutes most days, can lower blood pressure by about 5 to 8 mm Hg. Adding strength training at least two days a week helps further.
Diet has an even larger effect. Eating more whole grains, fruits, vegetables, and low-fat dairy while cutting back on saturated fat can reduce blood pressure by up to 11 mm Hg. Increasing potassium intake to 3,500 to 5,000 mg per day (from foods like bananas, potatoes, and beans) can lower it by another 4 to 5 points. Cutting sodium to 1,500 mg per day, roughly two-thirds of a teaspoon of table salt, can bring an additional 5 to 6 mm Hg reduction. Combined, these changes can be as effective as medication for many people with Stage 1 numbers.