H ello Anon, are you asking about a blood pressure reading for an adult? The Mayo Clinic states that most doctors consider blood pressure too low only if it causes symptoms. Some experts define low blood pressure as readings lower than 90 mm Hg systolic or 60 mm Hg diastolic. If either number is below that, pressure is lower than normal. A sudden fall in blood pressure can be dangerous and it would be best to get immediate medical attention.