Why mercury? Why not water?
Torricelli tried water first. It worked — but the column needed to be over 10 meters tall. Mercury is 13.6 times denser than water, so the column only needs to be about 760 mm. That fits in a room. A water barometer would need a three-story building. The choice was purely practical, and it locked the millimeter of mercury into the language of pressure for the next four centuries.
The conversion factor — 1 mmHg = 133.322 Pa — is not arbitrary. Take the density of mercury at 0°C (13,595.1 kg/m³), multiply by standard gravity (9.80665 m/s²), and multiply by a column height of 0.001 m. You get 133.322 pascals. It's a definition anchored to physical properties, not a committee vote. The pascal arrived two centuries later as the SI unit, named after Blaise Pascal, who had independently confirmed Torricelli's results by carrying a barometer up a mountain in 1648.
Three places mmHg still rules
Blood pressure. 120/80 mmHg. Systolic over diastolic. The most-performed medical measurement on Earth uses a unit from 1643. Normal systolic (120 mmHg) converts to 16,000 Pa — but no doctor would recognize that number. The clinical world is stubbornly non-SI on this one point, and for good reason: a century of diagnostic thresholds are keyed to mmHg integers. Moving the decimal place adds no clinical value and risks misreading.
Weather barometry. Standard sea-level pressure is 760 mmHg = 101,325 Pa = 1013.25 hPa. Barometers in many countries still report in mmHg, though the trend is toward hectopascals. A typhoon's central pressure dropping to 920 mmHg (122,656 Pa) is a warning sign — the mercury literally falls as the storm deepens. The visual image of a falling mercury column predates any digital sensor.
Vacuum systems. Rough vacuum pumps are often specified in mmHg of ultimate pressure. A pump that reaches 0.01 mmHg pulls down to about 1.33 Pa. In semiconductor fab, you'll hear both mmHg and Pa depending on whether the engineer is old-school (mercury-era training) or SI-native. The conversion is the bridge between two generations of vacuum practice.
mmHg-to-Pa reference values
| mmHg | Pa | Real-world equivalent |
|---|---|---|
| 0.0075 mmHg | 1 Pa | 1 pascal — barely measurable |
| 1 mmHg | 133.3 Pa | 1 mm column of mercury |
| 7.5 mmHg | 1,000 Pa (1 kPa) | Roughly 1% of atmospheric pressure |
| 80 mmHg | 10,666 Pa | Normal diastolic blood pressure |
| 120 mmHg | 16,000 Pa | Normal systolic blood pressure |
| 760 mmHg | 101,325 Pa | 1 standard atmosphere |
| 1,520 mmHg | 202,650 Pa | 2 atmospheres — SCUBA at 10 m depth |
Frequently asked questions
What is the conversion factor from mmHg to pascals?
1 mmHg = 133.322 Pa. The number comes from mercury density at 0°C × standard gravity × 1 mm column height. For everyday engineering use, 133.3 Pa per mmHg is close enough — the fraction only matters in metrology labs.
Why hasn't medicine switched from mmHg to pascals?
Riva-Rocci's 1896 sphygmomanometer used a mercury column, and every hypertension study since then has reported results in mmHg. Drug dosing, surgical thresholds, and emergency protocols are all written around mmHg integers. A switch to pascals would add four digits to every reading and introduce conversion errors into life-critical decisions. The cost-benefit is terrible — so mmHg stays.
Is mmHg the same as Torr?
For all practical purposes, yes. 1 Torr = 1/760 atm = 133.322368 Pa. The mmHg definition depends on mercury density at 0°C and yields the same number to six significant figures. You can treat them as interchangeable unless you work in a national metrology institute.
Engineering context
mmHg-to-Pa is a fixed-factor conversion. There is no temperature correction in standard use — the factor 133.322 presumes mercury at 0°C. If you need temperature-compensated values (for precision barometry), apply the thermal expansion correction: the mercury column lengthens by about 0.000181 per °C. At room temperature, the error is around 0.4%, negligible for clinical use but noticeable in a calibrated reference standard. For the reverse direction, see Pa to mmHg. Adjacent conversions: atm to Pa, kPa to psi, bar to psi. Full topic in the Pressure conversion guide.