mmHg to Pascals Converter
Common Conversions
| mmHg | Pa |
|---|---|
| 1 | 133.322 |
| 5 | 666.612 |
| 10 | 1333.22 |
| 25 | 3333.06 |
| 50 | 6666.12 |
| 100 | 13332.2 |
| 200 | 26664.5 |
| 400 | 53328.9 |
| 500 | 66661.2 |
| 760 | 101325 |
| 1000 | 133322 |
Why this conversion matters in chemistry
The multiplier of 133.322 Pa per mmHg comes straight out of the physics of a mercury column: the density of mercury (13,595.1 kg/m³) times standard gravity (9.80665 m/s²) times one millimeter (10⁻³ m). Standard atmospheric pressure works out to 760 mmHg = 101,325 Pa exactly. The conversion is the standard step from a manometer reading or a vapor-pressure table value into the units PV = nRT actually wants when R is in J/(mol·K). Water's vapor pressure at 25 °C, 23.8 mmHg, becomes 3173 Pa for the SI calculation; the mercury-column number and the SI number describe the same pressure.
Formula
Where the factor comes from
Weigh the column and the factor falls out. Static fluid pressure is ρgh, and the units run kg/m³ × m/s² × m = kg·m⁻¹·s⁻², which is the pascal written in base units — no conversion constant is smuggled in anywhere along the way. The conventional millimeter of mercury assigns ρ = 13595.1 kg/m³, mercury's density at the ice point, measured once and then frozen as a defined number, together with standard gravity 9.80665 m/s². Multiply those by h = 10⁻³ m and 1 mmHg = 133.322387415 Pa, exact in the sense that both inputs are assigned rather than remeasured for each job. The torr arrives at nearly the same place down a different road, 101325/760 = 133.3223684 Pa, disagreeing only in the seventh figure.
Precision and significant figures
Six figures, 133.322, exhaust anything a pressure measurement can use; the trailing 387415 earns its place only when you are propagating the definition symbolically or checking it against the torr. Digits in the answer belong to the reading, not to the factor. A mercury column read by eye against a millimeter scale is good to perhaps half a millimeter, which is 67 Pa, or seven hundredths of a percent at atmospheric — and that sets the floor however many decimals the arithmetic returns. Electronic transducers labeled in mmHg do better, but such a display is almost always a computed conversion of an SI-referenced internal reading, so it carries the sensor's uncertainty rather than the definition's.
Worked Examples
Standard atmospheric pressure expressed in SI base units — the calibration anchor for any sea-level pressure conversion.
The factor itself, written out — useful as a sanity check on a manometer reading.
The vapor pressure of water at 25 °C — the value that lands on the right-hand side of any calculation involving gas collected over water.
A typical reduced-pressure setting on a rotary evaporator running a moderate-boiling solvent.
Common mistakes
Truncating the factor to 133
Dropping the decimals costs 0.24 percent. On a barometric reading that turns 760 mmHg into 101,080 Pa rather than 101,325 — a 245 Pa shift, equivalent to nearly two millimeters of column height. It is worse than a sloppy meniscus reading, and unlike a sloppy reading it pushes every value in the set the same direction instead of scattering them.
The defined density is ice-point mercury
The assigned 13595.1 kg/m³ describes mercury at 0 °C. The mercury in a bench manometer at 22 °C is roughly 0.4 percent less dense, so a physically measured height needs a temperature correction before the defined factor applies to it. The conversion constant is a property of the unit; it makes no claim about the fluid actually sitting in your tube.
A "100 mmHg" setting means two pressures
A procedure that says reduce to 100 mmHg usually means 100 absolute, but compound gauges and older notes sometimes mean 100 below ambient. Those convert to 13.3 kPa and about 88 kPa — a factor of six and a half apart, and both are plausible rotary-evaporator settings. Settle which reference the number carries before multiplying anything by 133.322.