Dangerous blood pressure readings fall into three ranges, according to Healthline. A blood pressure reading of 140 to 159 over 90 to 99 indicates Stage 1 hypertension. Stage 2 is a reading of 160 or more over 100 or more. Hypertensive crisis is indicated by a reading higher than 180 over 110.
The first number of a blood pressure reading is the systolic pressure, which measures pressure during contraction of the heart muscle. The second number is the diastolic pressure, which measures pressure when the heart is between beats. A normal reading is a first number below 120 and a second number below 80. The two numbers are written as a fraction, such as 120/80.
A blood pressure of 120 to 139 over 80 to 89 is classified as pre-hypertension, according to Healthline. Pre-hypertension may develop to Stage 1 hypertension with an increased risk of heart disease. A single reading of elevated blood pressure doesn't indicate a diagnosis of hypertension. But blood pressure readings that remain high over a period of time may indicate hypertension.
Normal blood pressures for infants and children are lower than those for adults, states Healthline. A diagnosis of pediatric high blood pressure depends upon the child's age, gender and height. For example, a 3-year-old would have high blood pressure with a top number above 104 to 110 for a girl or 104 to 113 for a boy. A high systolic pressure for a 17-year-old is 125 to 132 for a girl or 131 to 140 for a boy.