The bottom number of your blood pressure reading is called diastolic pressure. It measures the force your blood exerts against your artery walls during the brief pause between heartbeats, when your heart muscle is relaxing and refilling with blood. A healthy diastolic reading is less than 80 mmHg.
What Diastolic Pressure Actually Tells You
When your heart beats, it squeezes blood out into your arteries. That’s the top number (systolic). Between beats, your heart relaxes for a fraction of a second, and the pressure in your arteries drops. The lowest point it reaches before the next beat is your diastolic pressure.
This resting-phase pressure matters more than people realize. It reflects how much resistance your smaller blood vessels are creating against blood flow. When those vessels are tight or stiff, the pressure between beats stays higher than it should. A healthy diastolic pressure is also critical for your coronary arteries to adequately supply oxygen to the heart muscle itself, since most of that blood flow happens during the resting phase, not during the squeeze.
Normal, High, and Low Ranges
The American Heart Association categorizes blood pressure using both numbers together, but here’s what the bottom number means on its own:
- Normal: below 80 mmHg (with a top number below 120)
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Low (hypotension): generally below 60 mmHg (with a top number below 90)
You only need one of the two numbers to be in an elevated range for a reading to count as high blood pressure. So a reading of 118/92 would still be considered hypertensive even though the top number looks fine.
When Only the Bottom Number Is High
Some people have a normal top number but a high bottom number. This is called isolated diastolic hypertension, and it typically affects people younger than 55. It’s diagnosed when your diastolic reading is 80 mmHg or higher at two or more office visits while the systolic stays below 130.
The reason this pattern skews younger relates to what’s happening inside your blood vessels. In younger adults, a high diastolic reading usually signals increased resistance in smaller arteries. The big arteries are still flexible enough to keep the top number in check, but the smaller vessels are working harder than they should. Excess weight, sleep apnea, and smoking are three of the biggest risk factors identified so far, though researchers don’t fully understand why the bottom number rises independently in some people.
How the Bottom Number Changes With Age
Diastolic pressure follows a different trajectory over your lifetime than systolic pressure does. For most people, the bottom number gradually rises through early and middle adulthood, then starts to fall after about age 55. The top number, meanwhile, tends to keep climbing.
This divergence happens because large arteries stiffen with age. Stiff arteries create a bigger surge when the heart pumps (raising the top number) but can’t hold pressure as well between beats (lowering the bottom number). That’s why isolated diastolic hypertension becomes less common in older adults, while a wide gap between the two numbers becomes more typical. A reading like 155/68 in a 70-year-old reflects this pattern of arterial stiffening.
When the Bottom Number Is Too Low
A diastolic reading below 60 mmHg, especially paired with a low systolic number, can mean your organs aren’t getting enough blood flow. The heart itself is particularly vulnerable because its own blood supply depends on adequate pressure during that resting phase. Symptoms of low blood pressure include dizziness, fainting, blurred vision, and fatigue. Severely low blood pressure can reduce oxygen delivery to the brain and heart enough to cause lasting damage, and in extreme cases can lead to shock.
Some people naturally run on the lower side without any symptoms at all, and that’s generally not a concern. The reading matters most when it’s paired with how you feel. A consistent diastolic reading in the 50s with no lightheadedness is very different from a sudden drop into the 40s with confusion.
Getting an Accurate Reading
Diastolic pressure is more sensitive to measurement errors than you might expect. Research from the American Heart Association found that common mistakes, like using the wrong cuff size or not supporting your arm at heart level, can throw off readings significantly. In patients with atrial fibrillation (an irregular heartbeat), automated monitors have been found to be accurate for systolic readings but not for diastolic ones.
For the most reliable bottom number, your arm should be supported at the level of your heart with your palm facing up. The cuff should fit properly: too small inflates the reading, too large deflates it. Wrist monitors add another layer of potential error, since even slight differences in arm position relative to the heart can skew results. If you’re tracking your blood pressure at home and the bottom number seems inconsistent, double-check your technique before worrying about the numbers themselves.