Blood Pressure Units — mmHg, and Nothing to Convert

If you came here to convert a blood pressure reading into some other unit, the useful answer is that there is nothing to convert. Blood pressure is reported in millimeters of mercury, mmHg, in every country, on every device and in every guideline. The SI unit for pressure is the pascal, and medicine simply did not adopt it.

Why a pressure is measured in millimeters

The unit is a fossil of the instrument. The original sphygmomanometer balanced the pressure in an arm cuff against a column of mercury in a glass tube, and you read the answer off a ruler beside the tube. Saying “120” meant the pressure had pushed the mercury 120 millimeters up.

Digital cuffs contain no mercury, and mercury manometers have largely been retired for environmental reasons. The scale stayed because everything written about blood pressure — every threshold, every trial, every chart in every clinic — is in it. Changing the unit would mean reissuing all of it simultaneously for no clinical gain.

NIST does define the conversion: one conventional millimeter of mercury is 133.3224 pascals, which makes 120 mmHg equal to 16.0 kPa. That figure appears in physiology teaching and on some ventilator and gas-analysis equipment. It does not appear on a blood pressure report.

The two numbers

Position	Name	What it is
Top	Systolic	Pressure while the heart contracts and pushes blood out
Bottom	Diastolic	Pressure between beats, while the heart refills

Both are in mmHg, both are needed, and either one on its own can be the number that puts a reading into a category.

The NHLBI puts a healthy systolic pressure under 120 mmHg and a healthy diastolic under 80, and describes blood pressure as high when systolic readings are consistently 130 mmHg or above, or diastolic readings 80 mmHg or above. MedlinePlus lays the bands out:

Category	Systolic	Diastolic
Normal	under 120	and under 80
Elevated	120 to 129	and under 80
Stage 1 high blood pressure	130 to 139	or 80 to 89
Stage 2 high blood pressure	140 or above	or 90 or above

The word “or” in the last two rows does real work: 138/76 and 124/92 are both stage 1, arriving there from different directions.

For completeness, those thresholds in the SI unit: 120 mmHg is 16.0 kPa, 130 is 17.3 kPa, 140 is 18.7 kPa, and 80 and 90 are 10.7 and 12.0 kPa. Nobody uses them, and the table above is the one to remember.

What does vary is the measurement, not the unit

Since the unit is fixed everywhere, the differences between two readings of the same person come from how the reading was taken. That is where attention is better spent:

• Cuff size. A cuff too small for the arm reads high, sometimes by 10 mmHg or more.
• Arm position. An unsupported arm, or one below heart level, raises the number.
• Posture and timing. Feet flat, back supported, five minutes of sitting still, no caffeine or cigarette in the previous half hour.
• Which arm. Left and right commonly differ; the higher arm is the one to keep using.
• The white coat. Readings in a clinic run higher than at home for many people, which is why home and ambulatory monitoring exist.

No single reading diagnoses anything. High blood pressure is established from repeated measurements over time, which is why a home log is worth more to a clinician than a one-off number.

For the units on the rest of a checkup that genuinely do differ between countries, see cholesterol mg/dL to mmol/L and blood sugar mg/dL to mmol/L. For the weight most cardiac medication doses reference, pounds to kg.

This content is informational and does not replace medical evaluation of your blood pressure.