Torr to atm Conversion

The Torr to atm conversion calculator converts between Torr and standard atmospheres, with extra rows for mmHg, pascals, and psi. Torr is tightly connected to vacuum measurement and mercury-column pressure history. Atm is the standard atmosphere unit used in gas laws, reference conditions, and many chemistry and physics problems. The calculator works both ways: enter Torr when a vacuum gauge, manometer, or lab table gives a mercury-style pressure, or enter atm when a gas-law problem needs the equivalent Torr value.

This page is deliberately vacuum-aware. A pressure of 100 Torr is not “100 Torr below atmosphere”; it is usually an absolute pressure equal to about 0.131579 atm. A perfect vacuum would be 0 Torr, and standard atmosphere is 760 Torr. That scale makes Torr intuitive for rough vacuum ranges where atm becomes a small decimal. For gauge-offset questions involving psi, compare bar to PSIG converter. For the reverse atmosphere-to-psi family, see psi to atm conversion, and for a wider unit list use the pressure converter.

Unit definitions

One standard atmosphere is defined as 101,325 pascals. One Torr is exactly one seven-hundred-sixtieth of a standard atmosphere, so the calculator uses 760 Torr per atm. Millimeters of mercury, abbreviated mmHg, come from the height of a mercury column in a manometer or barometer. In everyday vacuum, medical, weather, and classroom use, 1 Torr is treated as 1 mmHg for numerical conversion, which is why the result panel can show equivalent mmHg from the same Torr value.

the calculator’s calculation uses three constants: 760 Torr per atm, 133.32236842105263 Pa per Torr, and 0.0193367747 psi per Torr. Those extra rows are cross-checks. The primary result depends on the selected direction, but the supporting rows always describe the same pressure in several units.

Formula

For Torr to atmospheres:

$\text{atm} = \frac{\text{Torr}}{760}$

For atmospheres to Torr:

$\text{Torr} = \text{atm} \times 760$

The supporting rows are calculated from the Torr value:

$\text{Pa} = \text{Torr} \times 133.32236842105263$

$\text{psi} = \text{Torr} \times 0.0193367747$

Example calculation

Suppose a vacuum chamber reads 120 Torr and you need atmospheres. The calculator divides by 760:

$\text{atm} = \frac{120}{760} = 0.1578947368$

The displayed primary result is 0.157895 atm. The item rows show 120 Torr, 0.157895 atm, 120 mmHg, 15998.68 Pa, and 2.320413 psi. If you switch to the atm-to-Torr direction and enter 0.25 atm, the calculator multiplies by 760 and returns 190 Torr, with equivalent mmHg also shown as 190 mmHg.

Reference table

Vacuum, barometry, and laboratory use

Torr is common because it maps naturally to vacuum practice. Atmospheric pressure is 760 Torr, so a chamber at 100 Torr is far below atmosphere but still easy to describe. Rough vacuum pumps may be specified in Torr or millitorr. Lab procedures may list vapor pressures in Torr. Older manometer readings and many chemistry tables use mmHg, and the near one-to-one relationship lets users move between the labels without changing the number in ordinary work.

Barometry is related but not identical to vacuum practice. A weather value near 760 mmHg indicates pressure close to a standard atmosphere, while a vacuum system reading of 760 Torr would mean the chamber is essentially at atmospheric pressure rather than evacuated. Context decides whether a large Torr number is normal air pressure, poor vacuum, or a starting condition before pump-down.

Precision and pitfalls

The first pitfall is reversing the direction. Torr to atm divides by 760; atm to Torr multiplies by 760. The second is treating Torr as a gauge unit. Vacuum instruments sometimes display relative vacuum scales, but Torr in gas-law and vacuum specifications is normally absolute. The third is rounding too early. Dividing 120 Torr by 760 gives 0.1578947368 atm; using 0.16 atm may be fine for a rough lab note but too coarse for a calculation chained into moles, volume, or temperature.

Remember that a unit conversion cannot repair a measurement. A thermocouple vacuum gauge, capacitance manometer, and mercury barometer have different accuracy limits and gas dependencies. Keep the instrument uncertainty in mind when deciding how many digits to keep.

Sources

• NIST, Standard atmosphere constant — standard atmosphere value used to connect atm, pascals, and Torr.
• NIST, Guide for the Use of the International System of Units — pressure-unit conversion guidance and accepted usage notes.
• BIPM, The International System of Units brochure — SI framework for pressure and derived units.