The bottom number on a blood pressure reading is called diastolic pressure, and it measures the lowest pressure inside your arteries between heartbeats. When your heart relaxes and refills with blood, your arteries still maintain some pressure, and that’s what the bottom number captures. A normal diastolic reading is below 80 mmHg.
What Happens Between Heartbeats
Your heart works in two phases: squeezing blood out and then relaxing to refill. The top number (systolic) reflects the force during the squeeze. The bottom number reflects what’s left over during the rest period. Think of it as the baseline tension in your blood vessels when your heart isn’t actively pumping.
That baseline matters because it tells you something about the condition of your arteries. Healthy, elastic arteries absorb the surge of blood from each heartbeat and then gently recoil, maintaining steady pressure. When arteries become stiff or narrowed, the dynamics of both numbers change. In younger adults, a high bottom number often signals that blood vessels are resisting blood flow. In older adults, the bottom number can actually drop as arteries stiffen and lose their ability to recoil, which is why the two numbers don’t always move in the same direction.
What the Ranges Mean
The 2025 guidelines from the American Heart Association and American College of Cardiology classify diastolic blood pressure into clear categories:
- Normal: below 80 mmHg
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
Notice that the “elevated” blood pressure category (120 to 129 systolic) still requires a diastolic reading below 80. In other words, your bottom number alone can push you into a hypertension diagnosis even if your top number looks fine. A reading of 118/86, for example, would count as stage 1 hypertension because of that bottom number.
When Only the Bottom Number Is High
Having a high diastolic reading while your systolic stays normal is called isolated diastolic hypertension, and it affects roughly 6 in 100 adults in the United States. Among all people diagnosed with high blood pressure, about 1 in 5 have this pattern. It typically shows up in people younger than 55.
This condition may not cause noticeable symptoms day to day, but the long-term picture is more serious. It raises your lifetime risk of heart attack and makes death from cardiovascular disease more likely. It also increases your chance of developing heart failure. These risks are greatest for women and people under 60. If your top number is normal but your bottom consistently lands in the 80s or 90s, it’s worth paying attention to rather than dismissing the reading as “close enough.”
When the Bottom Number Is Too Low
A diastolic reading below 60 mmHg also raises concerns. Your coronary arteries, the vessels feeding blood to the heart muscle itself, receive most of their blood flow during the relaxation phase between beats. When diastolic pressure drops too low, the heart may not get enough of its own blood supply. Researchers describe this as a J-shaped risk curve: both high and low diastolic readings carry danger, with the sweet spot sitting between 70 and 80 mmHg.
A large study of U.S. adults found that people with diastolic pressure below 60 had a 30% higher risk of death from any cause and a 34% higher risk of cardiovascular death compared to those in the 70 to 80 range. Interestingly, this increased risk applied mainly to people who weren’t taking blood pressure medication. Those whose diastolic dropped below 60 as a result of medication did not show the same elevated risk, suggesting that treatment-related dips are less dangerous than naturally low readings, which can signal underlying problems like heart failure or prior heart attacks.
Very low diastolic pressure in older adults can also be an early sign that both the heart and arteries have stiffened, a combination that makes the heart less efficient at filling and pumping.
Why It Matters More at Younger Ages
The bottom number doesn’t carry the same weight across your lifespan. In younger adults, diastolic pressure is a strong predictor of stroke risk. A large European study found that both systolic and diastolic readings significantly predicted stroke until about age 62. But starting around age 47, the top number began outperforming the bottom number as a risk indicator, and after 62 the bottom number lost its statistical significance for stroke prediction altogether.
This is why doctors treating older patients tend to focus more on systolic pressure. It doesn’t mean the bottom number becomes irrelevant, especially since very low diastolic readings in older adults carry their own dangers, but systolic pressure becomes the stronger signal of cardiovascular risk as you age.
Getting an Accurate Reading
Blood pressure is surprisingly sensitive to how and when you measure it. On a beat-to-beat basis, readings can swing by more than 30 mmHg in either direction depending on what you’re doing. Several common factors can throw off your numbers:
- Full bladder: can raise your reading by up to 33 mmHg
- Talking during measurement: adds 4 to 19 mmHg
- Crossing your legs: adds 3 to 15 mmHg
- Recent caffeine: adds 3 to 14 mmHg
- Arm positioned below heart level: adds 4 to 23 mmHg
- Not resting before the reading: adds 4 to 12 mmHg
- Taking only one reading: can inflate the result by 3 to 10 mmHg
A difference of just 5 mmHg can change whether you fall into a normal or hypertensive category. If your bottom number is borderline, say 78 or 82, these measurement errors could easily push you across a threshold in either direction. Sit quietly for five minutes before measuring, keep your arm supported at heart level, don’t talk, and take at least two readings a minute apart. Use the average of the two.
Home blood pressure monitors are widely available, but many devices on the market haven’t been properly validated for accuracy, and using the wrong cuff size is a common source of error. If your wrist or arm circumference doesn’t match the cuff that came with your device, the readings may not be reliable. Most pharmacies carry replacement cuffs in different sizes.