Convert mmHg to kilopascals. Multiply by 0.133322, with a blood pressure table and why mmHg is not exactly 1/760 of an atmosphere.

The formula

kPa = mmHg × 0.133322387415

A millimetre of mercury is the pressure exerted by a one-millimetre column of mercury at its standard density under standard gravity: 13,595.1 kg/m³ × 9.80665 m/s² × 0.001 m = 133.322387415 Pa exactly.

mmHg is not exactly one 760th of an atmosphere

That is the torr, and the two are not the same unit:

1 torr  = 101,325 / 760 = 133.322368… Pa   (defined from the atmosphere)
1 mmHg  =                 133.322387415 Pa (defined from mercury)
760 mmHg = 101,325.014 Pa, not 101,325

The gap is 14 parts per million. It is invisible in medicine and real in precision vacuum metrology, which is why both units survive with almost identical values. This converter uses mmHg.

Blood pressure

The reason this conversion is looked up. Blood pressure is reported in mmHg essentially everywhere, but physiology papers and some European hospitals use kPa:

mmHg	kPa
90	12.0	low systolic
100	13.3
110	14.7
120	16.0	normal systolic
130	17.3	elevated
140	18.7	stage 1 hypertension threshold
160	21.3	stage 2
180	24.0	urgent
60	8.0	normal diastolic
80	10.7	normal diastolic, upper
90	12.0	diastolic hypertension threshold

A reading of 120/80 mmHg is 16.0/10.7 kPa.

Mercury sphygmomanometers are being withdrawn for environmental reasons, but the unit has outlived the instrument: digital monitors report mmHg because that is what the thresholds are written in, not because anything inside them contains mercury.

Quick reference

mmHg	kPa	psi	bar
1 mmHg	0.133	0.019	0.0013
10 mmHg	1.33	0.193	0.0133
100 mmHg	13.33	1.934	0.1333
500 mmHg	66.66	9.669	0.6666
760 mmHg	101.33	14.696	1.0133
1000 mmHg	133.32	19.337	1.3332

Vacuum

Below atmospheric, mmHg and torr remain the working units, alongside the millitorr:

	mmHg / torr	kPa
Atmosphere	760	101.325
Rough vacuum	25	3.33
Medium vacuum	0.1	0.0133
High vacuum	10⁻⁶	1.33 × 10⁻⁷

Freeze-drying, rotary evaporation and vacuum distillation all quote pressures in mbar or torr, and a rotovap running at “20 mbar” is at 15 torr.

Estimating in your head

Divide by 7.5. 120 ÷ 7.5 is exactly 16.0 kPa, which happens to be the exact answer. The factor is 0.1333, so a third of the number, shifted two places, is always close.