Normal blood pressure is defined when the top number (systolic) is less than 120 mm Hg and the bottom number is under 80 mm Hg. Anything less than 90 over 60 is considered low, but low doesn’t always mean unhealthy or dangerous.
Sometimes there are reasons for low blood pressure. Factors that contribute to this are dehydration, medication, and certain medical conditions. Improper blood pressure measurement may also show a false reading. If none of these factors applies to you, then low blood pressure may be normal for you. Your doctor can help you determine this.
If low blood pressure causes you to feel dizzy or faint, be sure that you get up slowly, especially if you have been lying down. Before and while you are getting up, pump the muscles in your legs and thighs a few times to help get the blood to your upper body. The CDC has more information on blood pressure, including tips on how to measure it correctly.