The bottom number on a blood pressure reading is your diastolic pressure, the lowest pressure inside your large arteries when your heart relaxes between beats. If your reading is 120/80, the 80 is your diastolic pressure, measured in millimeters of mercury (mmHg). It tells you how much force your blood is still exerting on artery walls during that brief pause between heartbeats.
What Happens During the Bottom Number
Your heart works in a two-part cycle: squeeze and relax. The top number (systolic) captures peak pressure during the squeeze. The bottom number captures the lowest point, when the heart is refilling with blood and not actively pumping. Even in this resting phase, your arteries maintain pressure because they’re elastic, they stretch when blood pushes through and then recoil to keep blood moving forward.
How much pressure remains during that resting phase depends largely on the resistance in your smaller blood vessels. If those vessels are narrowed or stiff, blood has a harder time flowing through, and the pressure between heartbeats stays higher. That’s why the bottom number is sometimes described as a window into the health of your blood vessel walls.
What the Numbers Mean
Current guidelines from the American Heart Association 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 (120 to 129 on top) only applies when the bottom number is still below 80. So you can have a normal bottom number and still be in an elevated category based on the top number alone, or vice versa. Whichever number falls into the higher category determines your overall classification.
When Only the Bottom Number Is High
Some people have a high diastolic reading while their systolic number stays normal. This is called isolated diastolic hypertension, diagnosed when the bottom number is 80 mmHg or higher at two or more office visits while the top number stays below 130. It typically affects people younger than 55.
This pattern makes sense when you consider what drives each number. In younger adults, the progressive rise in diastolic pressure reflects increased resistance in the smaller blood vessels. The arteries are still elastic enough to handle the systolic push, but the baseline tension is creeping up. After age 50 or so, the pattern often reverses: arteries stiffen, systolic pressure climbs, and diastolic pressure may actually drop.
The 2017 guidelines lowered the threshold for high diastolic pressure from 90 to 80 mmHg, which increased the prevalence of hypertension two to threefold among young adults. That shift created two tiers: stage 1 isolated diastolic hypertension (bottom number 80 to 89 with a top number below 130) and stage 2 (bottom number 90 or higher with a top number below 140).
When the Bottom Number Is Too Low
A diastolic reading below 60 mmHg can also be a concern. Your coronary arteries, the ones feeding your heart muscle, receive most of their blood supply during the relaxation phase. If diastolic pressure drops too low, blood flow to the heart itself can suffer.
Research published in The Journal of Clinical Hypertension found that people with a diastolic reading below 60 mmHg had a 30% higher risk of death from any cause compared to those in the 70 to 80 range, even when their top number was normal. The relationship follows a J-shaped curve: risk increases at both extremes of diastolic pressure, not just the high end. Low diastolic readings are more common in older adults and in people with heart failure or a history of heart attack.
How the Bottom Number Changes With Age
Diastolic pressure doesn’t follow the same lifelong trajectory as systolic pressure. It tends to rise gradually through early and middle adulthood, often peaking around age 50, and then begins to decline. This is essentially the opposite of what the top number does.
The reason comes down to arterial stiffness. Younger arteries are flexible. They expand when the heart pumps and spring back between beats, maintaining diastolic pressure. As arteries lose elasticity with age, they still stretch under the force of a heartbeat (driving systolic pressure up), but they have a harder time rebounding, so the pressure between beats drops. This is why many older adults develop a wide gap between their two numbers, a pattern called widened pulse pressure.
Another contributor to declining diastolic pressure in older adults is a condition where the small arteries on the heart’s surface constrict when they should be relaxing. This dysfunction further reduces blood flow during the resting phase of each heartbeat.
How It’s Measured
When a healthcare provider uses a manual blood pressure cuff, they inflate it to temporarily stop blood flow and then slowly release the pressure while listening with a stethoscope. As the cuff deflates, they hear a series of sounds created by blood flowing through the compressed artery. The bottom number is recorded at the moment those sounds completely disappear, which marks the point when the artery is no longer being compressed at all, even at its lowest pressure.
Automatic home monitors use sensors instead of sound, but they’re detecting the same thing: the lowest arterial pressure between heartbeats. For the most accurate reading at home, sit quietly for five minutes before measuring, keep your arm supported at heart level, and take two readings a minute apart. A single high or low reading doesn’t tell you much. The pattern across multiple readings over days or weeks is what matters.