What’s a good blood pressure?  It depends on the age and size of the individual. Little kids don’t have as much gravity to deal with, so they don’t require as high of a blood pressure. For adults, the normal range for systolic blood pressure (which is the first number in the reading) is between 100 and 130 millimeters of mercury.

Millimeters mercury expresses the amount of pressure in terms of how high it can push a column of mercury – a really heavy liquid.  You probably noticed that silver stuff in some blood pressure measuring devices?  That’s mercury. 

Systolic pressures in the 90’s are OK as long as there is no fainting or dizziness, but below that is usually a sign of trouble.

The diastolic pressure (or the second number) should ideally be below 80. There is no official number for a low diastolic blood pressure and there are few conditions that cause it to be low without also bringing down the systolic.