The bottom number of a blood pressure reading is called diastolic pressure, and it measures the force of blood against your artery walls while your heart is resting between beats. A healthy diastolic reading is below 80 mmHg. This number matters because that resting phase is when your heart muscle gets its own blood supply, making adequate diastolic pressure critical for keeping the heart itself nourished with oxygen.
What the Bottom Number Tells You
Every time your heart beats, it squeezes blood out into your arteries, creating a surge of pressure. That surge is the top number (systolic). Between beats, your heart relaxes and refills with blood. The pressure in your arteries during that brief pause is the bottom number (diastolic).
This distinction matters more than it might seem. During that resting phase, the coronary arteries, the small vessels that feed your heart muscle, receive most of their blood flow. If diastolic pressure is too low, your heart may not get enough oxygen. If it’s too high, it means your blood vessels are under constant strain even when the heart isn’t actively pumping.
Normal, High, and Low Ranges
The American Heart Association breaks blood pressure into categories based on both numbers. Here’s where the bottom number falls in each:
- Normal: below 80 mmHg (with a top number below 120)
- Elevated: still below 80 mmHg, but the top number is 120 to 129
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
- Hypertensive emergency: higher than 120 mmHg
If your two numbers fall into different categories, the higher category applies. So a reading of 118/86 would be classified as stage 1 hypertension because of that bottom number, even though the top number is normal.
On the low end, a blood pressure reading below 90/60 mmHg is generally considered low. A diastolic drop of just 20 mmHg from your usual baseline can cause dizziness, lightheadedness, blurred vision, fatigue, or fainting.
When Only the Bottom Number Is High
Some people have a normal top number (below 130) but a diastolic reading of 80 or higher. This is called isolated diastolic hypertension, and it’s diagnosed when the pattern shows up at two or more office visits. It’s more common in younger adults and often flies under the radar because the top number looks fine.
Even without symptoms, isolated diastolic hypertension raises your lifetime risk of heart attack, congestive heart failure, and death from cardiovascular disease. These risks are greatest for women and people under 60. It’s not something to ignore just because one number looks normal.
Why Doctors Focus More on the Top Number With Age
For people over 50, the top number typically becomes the stronger predictor of heart disease risk. This happens because arteries stiffen with age and accumulate more plaque, which drives systolic pressure up while diastolic pressure may actually drop. In younger adults, the bottom number often carries more diagnostic weight.
The gap between the two numbers, called pulse pressure, also tells a useful story. You calculate it by subtracting the bottom from the top. A gap of around 40 mmHg is considered healthy. A gap greater than 60 mmHg suggests the large arteries have become stiff and damaged, which is an independent risk factor for heart attacks and strokes, particularly in older adults. So a reading of 150/70 (pulse pressure of 80) can signal more arterial damage than 140/90 (pulse pressure of 50), even though the second reading has a higher bottom number.
How the Bottom Number Gets Measured
When a healthcare provider uses a traditional blood pressure cuff and stethoscope, they inflate the cuff to temporarily block blood flow in your arm, then slowly release the pressure. As blood begins flowing again, they hear thumping sounds through the stethoscope. The point where those sounds disappear completely marks your diastolic pressure. Automatic cuffs use sensors to detect the same transition, which is why you feel the cuff slowly deflate before it gives you a reading.
For the most accurate result, sit with your feet flat on the floor, your arm supported at heart level, and avoid caffeine or exercise for at least 30 minutes beforehand. A single reading can be misleading. Patterns over multiple readings give a much clearer picture of where your diastolic pressure actually sits.