The bottom number of your blood pressure, called diastolic pressure, should be below 80 mm Hg. A reading between 60 and 80 is considered healthy for most adults. This number represents the pressure in your arteries when your heart is resting between beats, and it matters more than many people realize because your heart muscle actually receives its own blood supply during this resting phase.
What the Bottom Number Means
Each time your heart beats, it squeezes blood out into your arteries, then relaxes briefly before the next beat. The bottom number captures the pressure during that relaxation. While it’s always lower than the top number (systolic pressure), it plays a unique biological role: your coronary arteries, the vessels that feed the heart itself, fill with blood during this resting phase. A healthy diastolic pressure ensures your heart gets enough oxygen between beats to keep functioning well.
The Target Ranges
The 2025 guidelines from the American Heart Association and American College of Cardiology break blood pressure into four categories. Here’s where the bottom number falls in each:
- Normal: below 80 mm Hg (with a top number below 120)
- Elevated: still below 80 mm Hg (the top number, 120 to 129, is what pushes you into this category)
- Stage 1 hypertension: 80 to 89 mm Hg
- Stage 2 hypertension: 90 mm Hg or higher
If your top and bottom numbers fall into different categories, the higher category is the one that applies. So a reading of 118/86 would be classified as stage 1 hypertension because of the bottom number alone, even though the top number looks perfectly normal.
European guidelines from 2024 are largely consistent, recommending a diastolic target of 70 to 79 mm Hg for people being treated for high blood pressure. They also specifically warn against pushing diastolic pressure below 70 mm Hg during treatment, citing evidence that going too low can cause harm.
When the Bottom Number Is Too High
Having a high bottom number while the top number stays normal is called isolated diastolic hypertension. It accounts for up to 20% of hypertension cases and is especially common in younger adults. A large Korean study of over 6 million adults aged 20 to 39 found that even mildly elevated diastolic pressure increased the risk of cardiovascular events by 32%, while more significantly elevated readings nearly doubled the risk.
Age matters here in an interesting way. In people under 50, isolated diastolic hypertension carried nearly triple the risk of cardiovascular events compared to normal blood pressure. In people over 50, that same pattern didn’t hold. This likely reflects differences in arterial stiffness: younger arteries are more elastic, so a high diastolic reading in a young person signals genuinely increased resistance in the blood vessels rather than just the normal stiffening that comes with age.
When the Bottom Number Is Too Low
A diastolic reading below 60 mm Hg raises a different set of concerns. Researchers at the University of Alabama at Birmingham coined the term “isolated diastolic hypotension” to describe readings below 60 with a normal top number. Because the coronary arteries fill during the diastolic phase, low pressure during that window means less blood and oxygen reaching the heart muscle. Over time, this chronic, low-level oxygen deprivation can weaken the heart and potentially lead to heart failure.
Data from the ARIC study, a long-running cardiovascular research project, backs this up. Participants with diastolic pressure below 60 had a 50% higher risk of coronary heart disease and a 30% higher risk of death compared to those with readings in the 80 to 89 range. Even readings in the 60 to 69 range showed signs of subtle heart stress, with elevated levels of a protein that indicates heart muscle damage.
The practical takeaway: a diastolic reading somewhere between 60 and 80 is the sweet spot for most people. Below 60, your heart may not be getting the blood flow it needs. Above 80, you’re entering territory that increases cardiovascular risk over time.
Why Your Bottom Number Might Read Wrong
Before worrying about a single reading, it’s worth knowing that cuff size alone can throw off your diastolic measurement. A study highlighted by the American Academy of Family Physicians found that using a regular cuff on someone who needs a large cuff overestimated diastolic pressure by about 1.8 mm Hg. For someone who needs an extra-large cuff, that error jumped to 7.4 mm Hg. That’s enough to push a normal reading into the hypertension range on paper.
Other common sources of error include crossing your legs during the reading, resting the cuff below heart level, or talking while the measurement is being taken. If your bottom number seems surprisingly high or low, ask your provider whether the cuff was the right size for your arm and whether the reading should be repeated under better conditions. Blood pressure is best assessed as an average of multiple readings taken on different days, not from a single snapshot.
Age and the Bottom Number
Current U.S. guidelines apply the same thresholds to all adults regardless of age, with no separate targets for people over 65. This was a deliberate departure from older guidelines, which had allowed a higher threshold of 150/80 for adults 65 and older. European guidelines take a slightly different approach, stratifying targets by age: below 130/80 for adults under 65, below 140/80 for those 65 to 79, and a top number of 140 to 150 with diastolic below 80 for adults 80 and older.
In practice, diastolic pressure tends to rise through middle age and then gradually decline after about age 60 as arteries stiffen. This is why isolated diastolic hypertension is more of a concern for younger adults, while older adults are more likely to see the top number creep up while the bottom number drifts down. Both patterns deserve attention, just for different reasons.