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To convert a pressure from millimeters of mercury (mmHg) to pascals (Pa), multiply by 133.322.
The factor is the pressure exerted by a 1 mm column of mercury at 0 °C under standard gravity — mercury's density (13,595.1 kg/m³) times 9.80665 m/s² times 0.001 m. The conventional millimeter of mercury is defined as exactly 133.322387415 Pa; the rounded 133.322 used here differs by three parts per million, far below what any instrument resolves.
The anchor that fixes the scale:
A blood-pressure reading of 120 mmHg systolic needs to be expressed in SI for a physiology model:
That is 16.0 kPa — the figure a European clinical paper would print for the same patient. A normal systolic pressure is, in SI terms, about a sixth of an atmosphere above ambient.
| mmHg | Pa |
|---|---|
| 1 | 133.3 |
| 10 | 1,333 |
| 40 | 5,333 |
| 80 | 10,666 |
| 120 | 15,999 |
| 760 | 101,325 |
Nearly every other pressure conversion modernizes an old unit for convenience; this one is usually compulsory. The pascal is the only pressure unit that cancels correctly inside SI formulas, so any mmHg value entering a calculation — flow through an airway resistance, gas solubility from a partial pressure, force on a diaphragm — must be converted first. Medicine supplies most of the input: blood pressure, intraocular pressure, intracranial pressure, and blood-gas partial pressures are all recorded in mmHg in North America and much of the world, while the journals, ISO standards and sensor datasheets those readings feed into are written in Pa or kPa. Vacuum technology supplies the rest, since rough-vacuum work quotes torr, practically identical to mmHg, and pump curves increasingly arrive in pascals.
Two habits keep the results trustworthy. First, convert once, at the boundary, rather than hopping between units mid-calculation, so the truncated factor cannot accumulate. Second, watch the reference frame: clinical pressures are gauge values relative to atmosphere, so 120 mmHg is 16 kPa above ambient, or about 117 kPa absolute — a distinction that matters the moment a calculation involves gas laws, where only absolute pressure is physically meaningful.
To convert a pressure from millimeters of mercury (mmHg) to kilopascals (kPa), multiply by 0.13332.
The conventional millimeter of mercury is defined as 133.322 Pa, so dividing by 1,000 gives the kPa factor. Two anchors cover most uses: 760 mmHg = 101.32 kPa (one atmosphere), and 7.5 mmHg ≈ 1 kPa for quick mental division.
A blood-pressure reading of 120 mmHg (systolic) needs to be recorded in SI units for a research protocol:
The familiar "120 over 80" therefore becomes "16.0 over 10.7" in kilopascals. For the reverse conversion multiply kPa by 7.5006.
| mmHg | kPa |
|---|---|
| 40 | 5.33 |
| 80 | 10.67 |
| 120 | 16.00 |
| 250 | 33.33 |
| 760 | 101.32 |
mmHg survives almost entirely because of medicine: blood pressure, intraocular pressure, ventilator settings, blood-gas partial pressures (PaO₂, PaCO₂) and central venous pressure are all traditionally quoted in millimeters of mercury. But the scientific literature, SI-based hospital systems in some countries, and physiology calculations (gas laws, alveolar equations) want kPa — several European countries report blood gases in kPa as standard, so clinicians moving between systems convert constantly: a PaCO₂ of 40 mmHg is 5.3 kPa. Outside medicine the pairing appears in vacuum measurement and in older engineering texts. The practical habit is to memorize one clean crossover in your own field — 7.5 mmHg per kPa, or "normal PaO₂ ≈ 100 mmHg ≈ 13.3 kPa" — and let it flag any converted value that lands an order of magnitude off.
To convert from millimeter of mercury (mmHg) to megapascal (MPa), use the following formula:
megapascal (MPa)
= 0.133322 × 11000× millimeter of mercury (mmHg)
= 0.000133322× millimeter of mercury (mmHg)
To convert from millimeter of mercury (mmHg) to hectopascal (hPa), use the following formula:
hectopascal (hPa)
= 0.133322 × 10× millimeter of mercury (mmHg)
= 1.33322× millimeter of mercury (mmHg)
To convert a pressure from millimeters of mercury (mmHg) to millibars (mbar), multiply by 1.3332.
The factor comes from the definition of the conventional millimeter of mercury as 133.322 Pa, while a millibar is exactly 100 Pa; dividing the two gives 1.33322. The classic anchor holds in this direction too: 760 mmHg — one standard atmosphere — equals 1,013.25 mbar.
A vacuum drying oven is holding 5 mmHg absolute, and the process specification is written in mbar:
For the reverse conversion multiply by 0.75006 (or divide by 1.3332).
| mmHg | mbar |
|---|---|
| 1 | 1.33 |
| 5 | 6.67 |
| 100 | 133.32 |
| 250 | 333.31 |
| 760 | 1,013.25 |
| 1,000 | 1,333.22 |
Outside meteorology, the place these two units constantly meet is vacuum technology. Older vacuum practice — and the entire North American vacuum industry — measures in mmHg and its alias the Torr (for practical purposes 1 Torr = 1 mmHg; they differ only at the ninth decimal). European vacuum equipment, by contrast, is instrumented in mbar: pump curves, gauge displays and leak-detection specs. Freeze dryers, rotary evaporators, degassing chambers and semiconductor load-locks routinely carry documentation in one unit and gauges in the other, making ×1.3332 a daily operation in lab and plant work. A convenient sanity check when working near rough vacuum: values in mbar should always be about a third larger than in mmHg — if your converted number got smaller, you multiplied the wrong way. Medical readers note: blood-pressure values stay in mmHg by convention worldwide; converting them to mbar is practically never done clinically.
To convert from millimeter of mercury (mmHg) to bar (bar), use the following formula:
bar (bar)
= 0.133322 × 1100× millimeter of mercury (mmHg)
= 0.00133322× millimeter of mercury (mmHg)
Performing a unit conversion between these two measurements is one of the simplest tasks in physics.
Because the Torr is a unit of pressure historically defined to be identical to the millimeter of mercury, the relationship between these two unit symbols (mmHg and Torr) is essentially one-to-one.
The unit is named in honor of Evangelista Torricelli, a renowned Italian physicist and mathematician who invented the barometer. While modern re-definitions have introduced a microscopic difference (where 1 mmHg is technically 0.999997 Torr), for all general applications, you can treat them as equal when you convert mmhg to torr.
For most practical purposes, the mmhg to torr conversion formula is:
Let's say you have a standard atmospheric pressure reading of 760 mmHg and need to express it in Torr.
The value remains the same, making the conversion instant and easy.
To convert from millimeter of mercury (mmHg) to meter of water @ 4°C (mH2O), use the following formula:
meter of water @ 4°C (mH2O)
= 0.133322 × 19.80665× millimeter of mercury (mmHg)
= 0.013595060494664334916× millimeter of mercury (mmHg)
To convert from millimeter of mercury (mmHg) to pound per square inch (psi), use the following formula:
pound per square inch (psi)
= 0.133322 × 0.00014503768078 × 1000× millimeter of mercury (mmHg)
= 0.01933671367695116× millimeter of mercury (mmHg)
With 0.00014503768078 is the ratio between the base units kilopound per square inch (ksi) and kilopascal (kPa).
To convert from millimeter of mercury (mmHg) to kilopound per square inch (ksi), use the following formula:
kilopound per square inch (ksi)
= 0.133322 × 0.00014503768078× millimeter of mercury (mmHg)
= 0.00001933671367695116× millimeter of mercury (mmHg)
With 0.00014503768078 is the ratio between the base units kilopound per square inch (ksi) and kilopascal (kPa).
To convert from millimeter of mercury (mmHg) to Inch of mercury (inHg), use the following formula:
Inch of mercury (inHg)
= 0.133322 × 0.00014503768078 × 10.000491154× millimeter of mercury (mmHg)
= 0.039369960698581626129× millimeter of mercury (mmHg)
With 0.00014503768078 is the ratio between the base units kilopound per square inch (ksi) and kilopascal (kPa).
A millimeter of mercury (mmHg) is a manometric unit of pressure.
It was historically defined as the pressure generated by a column of mercury exactly one millimeter high.
Today, it is most famous as the unit used in medicine for measuring blood pressure. It is also commonly used in meteorology, aviation, and physics to measure atmospheric pressure or vacuum pressure.
The mmHg unit originates from the invention of the barometer by Evangelista Torricelli in 1643.
Torricelli's device measured atmospheric pressure by seeing how high it could support a column of mercury in a tube.
Standard atmospheric pressure was found to support a column approximately 760 mm high, which established 760 mmHg as a standard reference point.
The most well-known application of mmHg is in medicine.
Blood Pressure: Readings are given as two numbers: systolic pressure (when the heart beats) over diastolic pressure (when the heart rests), such as 120/80 mmHg. This is a critical vital sign for human health.
Meteorology: Used for reporting barometric (atmospheric) pressure, which helps in weather forecasting.
Physics & Engineering: Used for measuring vacuum pressures and other low-pressure systems where precision is needed.
While mmHg is common, other pressure units are standard in different fields. Here are the most common conversions: