mmHg, kPa and Blood Pressure Units Explained

Medicine kept an old unit for a good reason: the original instrument really was a tube of mercury.

What mmHg means

One millimetre of mercury is the pressure exerted by a 1 mm column of mercury under standard gravity — 133.322 pascals. A sphygmomanometer literally measured how far the blood pushed that column.

1 mmHg = 0.133322 kPa = 0.0193368 PSI 1 kPa = 7.50062 mmHg

Blood pressure in other units

Reading	mmHg	kPa
Low	90/60	12.0 / 8.0
Normal	120/80	16.0 / 10.7
Elevated	130/85	17.3 / 11.3
Stage 1 hypertension	140/90	18.7 / 12.0
Stage 2 hypertension	160/100	21.3 / 13.3

The two numbers are systolic (heart contracting) over diastolic (heart relaxed). Some countries — notably Russia and parts of Scandinavia — chart in kPa; the clinical thresholds are the same pressures under a different label. Convert with the mmHg to kPa tool.

This article is general information, not medical advice. Discuss any reading with a clinician.

Torr and atmospheres

Torr is defined as 1/760 of a standard atmosphere = 133.3224 Pa. It differs from mmHg only in the eighth significant figure, so vacuum work treats them as interchangeable.

One standard atmosphere is 760 mmHg = 101.325 kPa = 14.696 PSI — which is also why weather reports quoting "1013 hPa" are describing normal sea-level pressure.

Why mercury persists

Mercury manometers are self-calibrating: the physics is defined by gravity and the density of mercury, with nothing to drift. Digital monitors must be validated against them. Mercury devices are being phased out for environmental reasons, but the unit outlived the instrument — the same way we still measure engines in horsepower.

Related conversions

• kPa to mmHg
• Atmospheres to PSI
• All pressure units