Blood pressure is recorded as two numbers measured in millimeters of mercury (mmHg) and expressed as a ratio, such as 120/80 mmHg. The top number is Systolic Blood Pressure (SBP), which measures the peak force exerted by blood against arterial walls when the heart's left ventricle contracts. The bottom number is Diastolic Blood Pressure (DBP), which measures the minimum pressure in the arteries while the heart muscle rests between beats.
The American Heart Association (AHA) categorizes blood pressure into five clinical zones: Normal (SBP <120 AND DBP <80), Elevated (SBP 120-129 AND DBP <80), Stage 1 Hypertension (SBP 130-139 OR DBP 80-89), Stage 2 Hypertension (SBP ≥140 OR DBP ≥90), and Hypertensive Crisis (SBP >180 AND/OR DBP >120).
Mean Arterial Pressure (MAP) and Pulse Pressure formulas
Two additional clinical calculations derived from your reading offer deeper insight into vascular health: Pulse Pressure and Mean Arterial Pressure (MAP). Pulse Pressure is the difference between systolic and diastolic pressure: Pulse Pressure = SBP - DBP. A reading of 120/80 mmHg yields a pulse pressure of 120 - 80 = 40 mmHg. High pulse pressure (>60 mmHg) indicates arterial stiffness.
Mean Arterial Pressure (MAP) represents the average perfusion pressure flowing through organs during a complete cardiac cycle. Because diastole lasts twice as long as systole in normal heart rhythms, MAP is calculated using the formula: MAP = DBP + 1/3(SBP - DBP) or MAP = (2 × DBP + SBP) / 3. For a reading of 130/85 mmHg, MAP is (2(85) + 130) / 3 = 300 / 3 = 100.0 mmHg. A MAP between 70 and 100 mmHg is optimal for tissue perfusion.
Measurement protocols and common error sources
Blood pressure fluctuates throughout the day due to circadian rhythms, stress, caffeine, and activity. Accurate measurement requires following clinical protocols: rest quietly for 5 minutes before reading, sit with back supported and feet flat on the floor, and position the arm cuff at heart level. Taking readings over clothing can artificially distort readings by 5 to 50 mmHg.
Other common protocol errors include using a cuff that is too small (+2 to +10 mmHg error), having a full bladder (+10 to +15 mmHg error), or talking during the reading (+10 mmHg error). Evaluating your blood pressure trends alongside body composition metrics from a BMI calculator provides a more complete cardiovascular risk profile.