A blood pressure reading below 120/80 is considered normal. In general, lower is better. However, very low blood pressures can sometimes be a cause for concern and should be checked out by a doctor.

Doctors classify blood pressures under 140/90 as either “normal,” or “prehypertension.”

 
• “Normal” blood pressures are lower than 120/80.  
• “Prehypertension” is blood pressure between 120 and 139 for the top number, or between 80 and 89 for the bottom number. For example, blood pressure readings of 138/82, 128/89, or 130/86 are all in the “prehypertension” range. If your blood pressure is in the prehypertension range, it is more likely that you will end up with high blood pressure unless you take action to prevent it.

See Also:

What Is High Blood Pressure?
What is Hypertension?
What is blood pressure?