DIA mmHg is the diastolic blood pressure reading displayed on a blood pressure monitor, measured in millimeters of mercury. It’s the bottom number in a blood pressure reading like 120/80. A normal diastolic reading is below 80 mmHg. If you’ve just taken your blood pressure at home or at a pharmacy and saw “DIA” on the screen, that number tells you the pressure inside your arteries when your heart is resting between beats.
What DIA and mmHg Stand For
DIA is short for diastolic. Every blood pressure reading has two numbers: systolic (SYS) on top and diastolic (DIA) on the bottom. The systolic number captures the peak pressure when your heart contracts and pushes blood out. The diastolic number captures the lowest pressure, measured in the brief pause while your heart relaxes and refills with blood.
MmHg stands for “millimeters of mercury.” Early blood pressure devices used a column of liquid mercury that rose and fell with arterial pressure, and the height of that column became the standard unit. Modern digital monitors no longer contain mercury, but the unit stuck. So when your monitor reads DIA 75 mmHg, it means the resting pressure in your arteries is equivalent to the force needed to push a column of mercury up 75 millimeters.
What Happens in Your Body During Diastole
Between heartbeats, your heart muscle relaxes and the pressure inside the ventricles drops to nearly zero. Blood flows from the upper chambers (atria) into the lower chambers (ventricles), refilling them for the next contraction. During this phase, the valves leading to the aorta and pulmonary artery are closed, so the pressure your arteries maintain on their own is what the monitor picks up as the diastolic reading. That residual pressure depends on how elastic your artery walls are and how much resistance your smaller blood vessels create.
This is also when the heart muscle itself receives most of its own blood supply. The coronary arteries that feed the heart fill primarily during diastole, which is one reason an abnormally low diastolic pressure can be a concern for heart health.
Diastolic Blood Pressure Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology classify blood pressure into four categories. Here’s where diastolic fits:
- Normal: below 80 mmHg (with systolic below 120)
- Elevated: below 80 mmHg (with systolic 120 to 129)
- Stage 1 hypertension: 80 to 89 mmHg
- Stage 2 hypertension: 90 mmHg or higher
Notice that the “elevated” category only applies to systolic pressure. Your diastolic number starts raising concern at 80 mmHg, and a reading of 90 or above puts you in stage 2 hypertension regardless of what the top number says.
When Diastolic Is Too High
A diastolic reading of 80 mmHg or higher with a normal systolic number is called isolated diastolic hypertension. The biggest risk factors include carrying excess weight, sleep apnea, and smoking. It tends to be more common in younger adults, typically under 60.
High diastolic pressure doesn’t usually cause symptoms you can feel, which is why it often goes unnoticed. Over time, though, it raises the risk of heart attack, heart failure, and death from cardiovascular disease. These risks are especially pronounced in women and people under 60, making it worth paying attention to even when you feel fine.
When Diastolic Is Too Low
Low blood pressure is generally defined as a reading below 90/60 mmHg. A diastolic number that drops below 60 can reduce blood flow to organs, including the heart itself, since the coronary arteries depend on adequate diastolic pressure to deliver oxygen to the heart muscle.
Symptoms of low diastolic pressure include dizziness, blurred vision, fatigue, trouble concentrating, and fainting. Severely low readings can starve the heart and brain of oxygen, and in extreme cases lead to shock, marked by confusion, cold and clammy skin, rapid shallow breathing, and a weak pulse. A diastolic reading below 70 mmHg has been linked to increased mortality risk in people who already have signs of heart muscle stress.
Getting an Accurate Diastolic Reading
Diastolic readings are sensitive to the same measurement errors that affect systolic pressure, and small mistakes can shift your numbers significantly. Research from the American College of Cardiology identifies several common pitfalls worth knowing about if you check your blood pressure at home.
A cuff that’s too small for your arm can inflate your reading by 5 to 20 mmHg. Taking a measurement without resting for at least five minutes first can add 10 to 20 mmHg. Talking or texting during the reading adds 10 to 15 mmHg. A full bladder can push the number up by 10 to 15 mmHg. Crossing your legs or letting them dangle off a table adds another 5 to 8 mmHg. Even coffee within 30 minutes of measuring can raise the reading by 5 to 8 mmHg.
For the most reliable reading, sit in a chair with your feet flat on the floor, rest your arm on a table at heart level, use the correct cuff size on bare skin, and sit quietly for five minutes beforehand. Taking two or three readings a minute apart and averaging them gives a better picture than relying on a single measurement.
Why Both Numbers Matter
For years, systolic pressure got most of the attention because it’s one of the strongest predictors of cardiovascular death. But research published in the Journal of the American College of Cardiology shows that systolic and diastolic pressures affect the heart in distinct ways. Elevated systolic pressure is consistently tied to cardiovascular mortality, while low diastolic pressure carries its own risk, particularly in people with early, undetected heart damage. Focusing on just one number misses part of the picture. Both the SYS and DIA readings on your monitor are telling you something different about the health of your heart and arteries.