What the two numbers mean
A blood pressure reading has a top number and a bottom number, both in millimeters of mercury. Systolic pressure is the peak pressure in your arteries while the heart contracts; diastolic pressure is the resting pressure between beats. A reading of 118/76 means the pressure peaks at 118 mm Hg and falls to 76 between beats. Systolic pressure climbs steadily with age as arteries stiffen, which is why it predicts cardiovascular events better than diastolic in adults over 50.
In 2017 the American College of Cardiology and the American Heart Association lowered the thresholds that had stood since 2003. Normal is now under 120 and under 80. Elevated is 120-129 systolic and under 80 diastolic. Stage 1 hypertension is 130-139 or 80-89, and stage 2 is 140 or higher or 90 or higher. Above 180 systolic and/or 120 diastolic is a hypertensive crisis that needs care the same day. Note the difference between "and" and "or": one high number is enough to put you in a stage, so 135/74 is stage 1.
Pulse pressure and mean arterial pressure
Pulse pressure is simply systolic minus diastolic, 118 - 76 = 42 mm Hg in the example above, and a value of 40 to 60 is typical. A wide pulse pressure in an older adult usually reflects stiff large arteries and is an independent risk marker; a narrow one can appear with heart failure or blood loss.
Mean arterial pressure estimates the average pressure driving blood through the circulation over a whole cardiac cycle. Because the heart spends roughly twice as long in diastole as in systole, the standard estimate weights them accordingly: MAP = diastolic + (systolic - diastolic) / 3. For 118/76 that is 76 + 42/3 = 90 mm Hg. Clinicians treat about 65 mm Hg as the minimum needed to perfuse the kidneys and brain, and 70 to 100 as the normal range.
Measuring it properly
Most surprising readings are measurement errors. Sit quietly for five minutes with your back supported and both feet flat on the floor, rest your bare arm on a table so the cuff is level with your heart, and do not talk during the measurement. Avoid caffeine, exercise and nicotine for 30 minutes beforehand and empty your bladder first. A cuff that is too small can read 10 to 40 mm Hg high, so check the size markings against your arm circumference. Take two readings a minute apart and average them.
One high reading is not a diagnosis. Guidelines call for elevated readings on at least two separate occasions, and home or 24-hour ambulatory monitoring is often used to rule out white-coat hypertension, which affects a substantial minority of patients. Losing excess weight, cutting sodium toward 1,500 mg a day, following the DASH eating pattern, 150 minutes a week of aerobic exercise and limiting alcohol each lower systolic pressure by roughly 4 to 11 mm Hg, and they stack.