How to distinguish between high blood pressure grades

There are three main grades of hypertension. The first grade is systolic blood pressure greater than or equal to 140mmHg and diastolic blood pressure greater than or equal to 90mmHg. The second grade is systolic blood pressure maintained between 140mmHg and 159mmHg and diastolic blood pressure maintained between 90mmHg and 109mmHg. The third grade is systolic blood pressure greater than or equal to 180mmHg and diastolic blood pressure greater than or equal to 110mmHg.