The bottom number in a blood pressure reading is called the diastolic pressure, and it shows how much pressure your blood is putting on your artery walls between heartbeats, when the heart is resting.

Quick Scoop: What that bottom number means

When you see a reading like “120/80”:

  • The top number (120) is the systolic pressure – pressure when the heart contracts and pumps.
  • The bottom number (80) is the diastolic pressure – pressure in your arteries when the heart relaxes and refills with blood.

Think of it like this:

  • Systolic = “push” phase.
  • Diastolic = “rest” phase that still keeps steady pressure in your system.

Why the bottom number matters

Doctors look at both numbers because:

  • A normal blood pressure is generally below 120/80 mm Hg.
  • If the bottom number is consistently 80–89, that can be stage 1 hypertension ; 90 or higher is stage 2 hypertension , which raises your risk for heart attack and stroke over time.
  • Diastolic pressure is especially important for the heart’s own blood supply, which flows mainly while the heart is in that resting phase.

Simple example

If your reading is 135/92 :

  • 135 = systolic (top, heart squeezing).
  • 92 = diastolic (bottom, heart resting).

Because the bottom number is over 90, this would fall into stage 2 hypertension , and you should talk with a healthcare professional soon about what to do next.

If your question is about your own numbers, it’s always safest to ask a doctor or nurse, especially if the bottom number is often above 80.