Torr to mmHg

One Gauge Says Torr, the Next Says mmHg

You have a process written in Torr, a replacement gauge whose dial is graduated in mmHg, and a purchasing form that will not accept "about the same". For working purposes they are the same: this converter treats both units as 133.3223684211 Pa, so whatever you type on one side appears unchanged on the other. That is not a shortcut — it is how nearly every instrument catalogue, process recipe and service report on the market treats the pair.

Conversion factor: 1 Torr = 1 mmHg here, a straight 1:1. A vacuum oven programmed to hold 30 Torr is holding 30 mmHg, and a 500-micron target on a digital gauge is 0.5 Torr, which is 0.5 mmHg.

Where the Two Labels Came From

A height became a pressure

The older unit was literal: the millimetre of mercury a column stood at. Torricelli's barometer gave the idea its name centuries before either unit was tied to a fixed number of pascals.

Vacuum hardware chose Torr

Pumps, gauges, deposition tools and semiconductor process recipes settled on Torr and its thousandth, the millitorr, so American-built controllers almost always print that word.

Mercury instruments kept mmHg

Anything descended from a real mercury column — barometers, manometers, clinical and physiological instruments — carried the older label forward, and standards bodies kept publishing a definition for it.

Certificates name the definition

A calibration report usually states which definition it worked from and the pascal value behind it, which is the one place where the two are not simply interchangeable words.

Checking a Value Across Two Instruments

The useful work on this page is rarely the Torr-to-mmHg step itself. It is what you do on either side of it.

Enter the value your instrument shows

Either box accepts input and the other follows immediately. A comma is accepted in place of the decimal point and spaces inside the figure are dropped, so 1 013,25 is understood exactly as typed.

Change one side to the unit you are really comparing against

Both dropdowns are searchable and carry all 26 pressure units in eight groups. Set the right-hand side to mbar, inHg, kPa or Pa and the page answers the question a mixed-unit spec sheet is actually posing.

Copy the digits into the record

The copy button on each field puts the plain number on the clipboard, with no unit and no thousands spacing, ready for a purchase requisition, a calibration sheet or a batch record. Ctrl + C in a field behaves identically.

Swap when the paperwork runs the other way

The ↔ button reverses the pair to mmHg → Torr. The multiplier in that direction is also 1, so the digits stay put; what swapping buys you is that the labels above the fields now match the document you are filling in.

Check the reference before the unit: when two instruments disagree it is far more often absolute versus below-atmosphere, or a head being read outside its usable range, than anything to do with the word printed on the dial.

Which Unit Each Gauge Family Prints on Its Scale

A tour of the instruments that put one word or the other on the display, with a representative reading in both columns. The last two rows are gauges that print neither, to show what the same pressure looks like when it arrives in a third unit.

Instrument family	Unit on the scale	Reading in Torr	Reading in mmHg
Laboratory mercury barometer at standard sea-level pressure	mmHg	760	760
Capacitance manometer, 100-unit head at half scale	Torr	50	50
Vacuum drying oven controller, typical set point	Torr	30	30
Pirani or thermocouple gauge controller, mid-range	Torr and mTorr	0.05 (50 mTorr)	0.05
Digital micron gauge at a 500-micron target	micron of mercury	0.5	0.5
Cold-cathode controller on a high-vacuum rig	Torr	1 × 10⁻⁷	1 × 10⁻⁷
Diaphragm vacuum controller set to 20 mbar	mbar	15.00	15.00
Compound Bourdon gauge showing 25 inHg of vacuum	inHg below atmosphere	635 (of vacuum)	635

The pattern is a habit, not a physical difference: hardware descended from vacuum technology says Torr, hardware descended from a mercury column says mmHg, and the number under either word is the same. Only the last two rows need arithmetic — 20 mbar works out at 15.00 Torr, and 25 inHg is 635 mmHg because an inch holds 25.4 millimetres.

Useful When Comparing Instruments

Bring in a third unit to settle it

The real disagreement is usually with a millibar or inHg instrument; change one selector and that comparison is on screen without leaving the page.

Micron-range digits survive

Output runs to eight decimal places and drops into scientific notation below one millionth, so millitorr and micron figures are not rounded away.

Start from whichever gauge you are holding

Neither field is fixed as the input, so the reading goes in on the side whose label matches the instrument in front of you.

Clipboard gives the bare digits

Copying returns the number with no unit attached, which keeps a pasted value from being treated as text in a form field or a spreadsheet column.

Label and Definition Questions

If both scales show the same number, why do two names exist at all?

History rather than physics. The millimetre of mercury described an apparatus, and it was in daily use long before pressure had an agreed SI value. Once the standard atmosphere was fixed at 101 325 Pa, the vacuum community defined the Torr as exactly one 760th of it — deliberately landing on the old mercury figure — and named it after Torricelli. Fields already printing mmHg on their instruments simply carried on. The outcome is two words for one quantity, split along industry lines instead of numerical ones.

How far apart are the strict definitions?

One Torr is 101 325 ÷ 760 = 133.322 368 421 Pa exactly. The conventional millimetre of mercury is built instead from a mercury density of 13 595.1 kg/m³ and standard gravity of 9.806 65 m/s², which over one millimetre gives 133.322 387 415 Pa. The gap is 0.000 019 Pa — about 1.4 parts in ten million, or 0.000 014 %. Spread over a full atmosphere of 760 units that is a discrepancy of roughly 0.000 1 mmHg. This page applies the Torr value to both, which is why the two fields track each other digit for digit.

Could that sub-part-per-million gap ever change a result?

Only inside a metrology laboratory. Set it beside the instruments: a good capacitance manometer is specified at something like 0.15–0.25 % of reading, a Pirani head is often quoted at 5–10 %, and even a carefully kept reference gauge is orders of magnitude coarser than 0.000 014 %. The definitional difference is roughly ten thousand times smaller than the best working gauge's own uncertainty, so it vanishes into the noise of any process measurement. It counts when a national standards institute compares pressure balances, and when a certificate must state exactly which definition its numbers rest on — not when you are setting a vacuum oven.

What is a micron of mercury on a vacuum gauge?

A micron is one thousandth of a Torr — equally, one thousandth of a millimetre of mercury, or 0.133 322 Pa. It is the same quantity as a millitorr, just wearing the mercury-column name, and it survives because writing 0.000 5 Torr on a small display is awkward while 500 microns is not. Digital gauges built for evacuation work read in microns from a few thousand down to single digits, so an instruction such as "pump below 500 microns and hold" is asking for 0.5 Torr, which is 0.5 mmHg.

Is a reading in Torr or mmHg absolute, or measured from atmosphere?

On vacuum instruments it is nearly always absolute: zero on the scale means an ideal vacuum, and ambient air reads about 760 near sea level. Compound dial gauges are the exception — they indicate how far below the surrounding air the system has been drawn, so their scale runs the opposite way. Reconciling the two views is subtraction, not a unit change: 25 inHg of vacuum is 635 mmHg below ambient, which with ambient at 760 leaves 125 Torr absolute inside the vessel. Mixing an absolute figure with a below-atmosphere one is the most common reason two gauges on one rig look wildly inconsistent.