Blood Pressure Converter

Blood pressure is traditionally measured in millimeters of mercury (mmHg), a unit left over from the mercury-column gauges doctors relied on for more than a century. The number literally meant how far the pressure could push mercury up a tube. Most monitors are digital now, but the unit stuck.

Conversion factors:

• 1 mmHg = 0.1333 kPa
• 1 kPa = 7.5006 mmHg
• 1 mmHg = 1.3595 cmH₂O

A reading always comes as two numbers, written as systolic over diastolic. Systolic is the higher figure, the pressure as the heart beats; diastolic is the lower, the pressure while the heart rests between beats.

Typical ranges (systolic/diastolic):

• Normal: under 120 and under 80 mmHg (under about 16.0/10.7 kPa)
• Elevated: 120–129 systolic with diastolic still under 80
• High, stage 1: 130–139 or 80–89 mmHg
• High, stage 2: 140+ or 90+ mmHg

“120 over 80” is the edge of normal, not the middle of it, and that trips people up because the phrase is so familiar. Under the current ACC/AHA thresholds normal means below both numbers, so a reading of exactly 120/80 sits just outside it. The older JNC7 guidance called that range “prehypertension” and the 2017 revision renamed and lowered the bands, which is why an old chart and a new one can label the same reading differently. Note also that the categories use or, not and: one number in a higher band puts the whole reading there.

Most of the world reports blood pressure in mmHg, but a handful of countries and many research settings use kilopascals, which is the usual reason to convert. The figures look alarmingly different at first: a normal 120/80 in mmHg is roughly 16/11 in kPa, so the smaller numbers aren’t a cause for worry, they describe the same pressure. This is general information, not medical advice.