Blood pressure measurement is a common practice for monitoring cardiovascular health, yet users often encounter confusion when their home device displays readings in a unit other than the familiar millimeters of mercury (mmHg). While mmHg is the accepted standard in clinical medicine, some digital blood pressure monitors may default to kilopascals (kPa). Most modern devices allow the user to switch the display unit to the preferred clinical standard.
Understanding the Conversion Rate
The conversion factor for changing kilopascals to millimeters of mercury is 7.5006. This value is derived from the physical properties of mercury at a standard temperature. To manually convert a blood pressure reading from kPa to the clinically used mmHg, the kPa value must be multiplied by 7.5006. For example, a systolic pressure reading of 16 kPa translates to 120.01 mmHg (16 x 7.5006). Conversely, to convert an mmHg reading back to kPa, the value is divided by 7.5006.
Locating and Changing the Unit Setting
Changing the unit display requires accessing a hidden settings menu, typically by pressing a specific button while the device is powered off. Although the exact steps vary by manufacturer, common buttons used to initiate the setup mode include ‘SET’, ‘S’, ‘Memory’ (M), or ‘Start/Stop’. Press and hold one of these buttons for a few seconds until a setting, such as the year or a user profile, begins to flash on the screen.
Once in the setup mode, the user must often cycle through initial settings like the date and time by briefly pressing the same button used to enter the menu. The unit selection option is often located near the end of the configuration sequence, sometimes labeled ‘Unit’ or ‘Mode’. On some models, this setting may appear after navigating past screens labeled ‘P1’, ‘P2’, or ‘SP’.
To toggle the unit from kPa to mmHg, a secondary button, such as the ‘Memory’ or M button, is pressed. After the desired mmHg unit is displayed, press the main power or ‘Start/Stop’ button to confirm the selection. If the initial button combination does not work, consult the specific device’s instruction manual, as some models may have a unique button sequence or may not offer the ability to change the unit at all.
Why Units Differ in Blood Pressure Measurement
The existence of two different units stems from a blend of scientific standards and medical tradition. The kilopascal (kPa) is the standard metric unit for pressure within the International System of Units (SI). Many countries that strictly adhere to SI standards may default their medical devices to display pressure in kPa.
The unit millimeters of mercury (mmHg) has a historical origin that dates back to the invention of the sphygmomanometer. This original device used a column of mercury, and the pressure was measured by the height of the mercury column in millimeters. Because of this long-standing history, mmHg became the established unit in medicine, and medical professionals worldwide are familiar with its reference values, such as 120/80 mmHg.
Despite the scientific preference for the SI unit, mmHg remains the global standard in clinical practice. Switching the standard unit would require a massive and potentially confusing change in medical textbooks, research papers, and established treatment guidelines. Therefore, while kPa is the official metric unit, mmHg persists as the practical and universally understood language of blood pressure measurement in healthcare.