Blood pressure — mmHg, kPa, and a 17th-century glass tube
My grandfather was a country doctor and he had one of the old mercury sphygmomanometers on his desk — a glass column of silver liquid that literally rose to 120 mm when he inflated the cuff. I used to think it was magic. It's just physics.
Blood pressure is quoted as two numbers — systolic over diastolic — in millimeters of mercury (mmHg). The number is the height a column of mercury would rise in a vertical tube if that column were balanced against the pressure. 120 mmHg means the pressure would push a mercury column 120 mm (about 4.7 inches) up a tube. It's a direct physical measurement, not a converted one — old mercury manometers didn't need calibration because the units were baked into the apparatus.
Modern cuffs are aneroid (spring-based) or oscillometric (electronic), and neither contains any mercury. But the scale stuck because every doctor, every reference range, every study, and every drug dosing guideline is written in mmHg. Switching now would be a decades-long re-education problem for a marginal SI benefit.
The SI-approved alternative
The strictly-correct SI unit for pressure is the pascal (Pa), or kilopascal (kPa) at reasonable magnitudes. 1 mmHg = 0.1333 kPa, so 120/80 mmHg is 16.0/10.7 kPa. A few countries — notably Russia and parts of continental Europe in academic contexts — do report blood pressure in kPa. Almost everywhere else, including hospitals, kPa is used for arterial blood gas partial pressures (pO2, pCO2) but blood pressure stays in mmHg. You end up with a chart mixing units on the same patient, which is medically fine but historically ridiculous.
Why systolic and diastolic
Systolic (the higher number) is peak arterial pressure at the moment the heart's left ventricle contracts. Diastolic is the pressure between beats when the heart is refilling. Normal adult resting is around 120/80. Hypertension starts at 130/80 by current US guidelines (140/90 by older European ones — same physiology, different agreed thresholds).
The pulse pressure — the difference between the two — matters too. A wide pulse pressure (say 160/70, giving 90 mmHg difference) suggests stiff arteries. A narrow one (110/95) suggests heart-output problems. Neither number in isolation tells the whole story.
Home cuffs and honest measurement
An upper-arm oscillometric cuff is accurate to within about ±3 mmHg of a properly-done mercury reading, if used correctly. Correctly means: sit for 5 minutes first, feet flat, back supported, arm at heart level on a table, no talking, no caffeine or exercise in the last 30 minutes. Take three readings a minute apart and use the average of the last two — the first reading is always high because your body reacts to the cuff inflating.
Wrist and finger cuffs are less accurate because peripheral pressure differs from central pressure by a variable amount. If you're tracking hypertension seriously, get an upper-arm cuff and validate it against your doctor's device once.
Try the converters mentioned in this article
FAQ
Why doesn't the medical world switch to kPa?+
Institutional momentum plus safety. Every published guideline, every trial endpoint, every drug dosing decision uses mmHg. Changing units would introduce transcription errors during the transition.
Is 'white coat hypertension' real?+
Yes — being at the doctor's office reliably raises blood pressure in some people. It's why 24-hour ambulatory monitoring is the gold standard for diagnosing borderline hypertension.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-22 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.