Article — Torr to Atm Converter
Torr to atm: exactly 760 torr to one atmosphere
One atmosphere equals exactly 760 torr, by international definition set at the 10th General Conference on Weights and Measures (CGPM) in 1954. To convert torr to atm, divide by 760. The conversion is exact — not a measurement, but a defined ratio.
The torr is the working unit of vacuum technology and many branches of laboratory chemistry. It gives convenient whole-number values across the range from rough vacuum (1 torr) to ultra-high vacuum (10⁻⁹ torr). The atmosphere, by contrast, is a reference unit — used as the natural one-unit baseline against which all laboratory pressures are compared.
How many torr in one atmosphere
760 torr to one atm, exactly. The number comes from the historic barometer: at sea level on Earth, atmospheric pressure supports a 760-mm column of mercury. Evangelista Torricelli built the first barometer in 1643 and observed that the column always settled near this height. Three centuries later, BIPM formalised the relationship by definition rather than measurement.
The standard atmosphere itself is defined as 101,325 Pascals exactly — also a defined value, not a measured one. Dividing 101,325 by 760 gives the Pa-per-torr factor: 133.322368 Pa per torr. Both numbers are exact at the level of the SI brochure; rounding only ever happens in user-facing displays.
How to convert torr to atm
Divide torr by 760 to get atm. Reverse: multiply atm by 760 to get torr. So 500 torr ÷ 760 = 0.6579 atm; 0.5 atm × 760 = 380 torr. The factor is rational and exact, so spreadsheet conversions terminate cleanly without precision loss.
For mental arithmetic, treat 760 as 750 plus 1.3%. So torr ÷ 750 ≈ atm, plus a 1.3% correction. 1,000 torr ÷ 750 = 1.333 atm; subtract 1.3% gives 1.316 atm (true: 1.3158). The shortcut is accurate to better than 0.1% with the correction, and to about 1.3% without.
torr ÷ 760 = atmatm × 760 = torrtorr × 133.322 = Patorr × 0.001333 = barTorr vs. mmHg: a subtle distinction
Torr and millimeters of mercury are nearly identical but not equal. 1 torr = 0.999999857 mmHg — different by about 1 part in 7 million. The discrepancy comes from how each unit is defined. Torr is fixed at 1/760 atm by international agreement; mmHg is defined by the height of a mercury column under standard gravity and temperature.
For practical work the difference is invisible. Blood pressure cuffs report in mmHg; vacuum gauges report in torr; the two are interchangeable in every clinical and engineering setting. The distinction matters only at the level of NIST-traceable metrology, where a few parts per million can shift the calibration of a national standard. Most scientific publications now prefer torr for vacuum work because its definition is independent of physical mercury.
Mercury is a liquid metal with a density of 13,595 kg/m³ — so dense that a 760 mm column weighs the same per unit area as the entire atmosphere above sea level. A water barometer requires a column of 10.3 meters; Robert Hooke built one in 1672 inside a converted well.
Torr in vacuum technology
Vacuum systems are classified in torr ranges, and the boundaries are conventional rather than physical. Rough vacuum (1 to 760 torr) is what mechanical pumps reach — household vacuum cleaners reach the upper part of this range. Medium vacuum (10⁻³ to 1 torr) is achieved by rotary-vane pumps or roots blowers and is used for vacuum drying, distillation and freeze-drying.
High vacuum (10⁻⁶ to 10⁻³ torr) is the regime of electron microscopes, vacuum coating systems and most semiconductor processing. Ultra-high vacuum (10⁻⁹ to 10⁻⁶ torr) is needed for surface science, particle physics and space simulation. Extreme-high vacuum below 10⁻¹² torr is reached only with cryopumps and specialty equipment; the inside of a CERN beampipe operates in this regime.
Torr in chemistry and physics
Many gas-phase reactions are reported at standard pressure of 760 torr (1 atm) and standard temperature of 273.15 K or 298 K. The ideal-gas law, PV = nRT, can be used directly with torr as the pressure unit if R is taken as 62.364 L·torr / (mol·K). Working in torr saves a unit-conversion step that would otherwise be needed when starting from Pascal-based R values.
Vapor pressure curves are most often published in torr. Antoine equation coefficients for thousands of compounds use torr (or its near-cousin mmHg) as the output unit. Distillation column design, evaporator sizing and reaction-equilibrium work all benefit from staying in torr because lab and industrial gauges report in torr or mmHg.
When using the ideal-gas law in torr-based units, R = 62.364 L·torr / (mol·K). If you mix units — pressure in torr, volume in liters, but R in Pa-based units — you will get answers off by a factor of 133. Always make R consistent with the pressure unit being used.
Torr to Pascal, bar and psi
The Pascal (Pa) is the SI unit of pressure: 1 Pa = 1 N/m². 1 torr = 101,325/760 = 133.322368 Pa. The bar is 100,000 Pa exactly, so 1 atm = 1.01325 bar, and 1 torr = 0.001333 bar. The psi (pounds per square inch) is an engineering unit equal to 6,894.757 Pa, giving 1 atm = 14.6959 psi and 1 torr ≈ 0.01934 psi.
For weather, meteorologists use hectopascals (hPa, equal to millibars). 1 hPa = 0.7501 torr. A weather report of "1013 hPa" is sea-level standard pressure, corresponding to 760 torr or 1 atm. Hurricane lows of 950 hPa correspond to about 712 torr — a 6% drop from standard, but enough to drive 200 km/h winds.
Common torr-to-atm mistakes
The first common error is rounding 760 to 750 in head-math. The 1.3% gap compounds in multi-step calculations. Use the exact 760 in any stored constant.
The second is mixing absolute and gauge pressure. Torr is almost always absolute. Engineering psi values can be either psia (absolute) or psig (gauge); psig adds 14.696 psi to reach absolute. A vacuum-gauge reading "300 torr below atmospheric" is 460 torr absolute, not 300 torr.
The third is treating torr and mmHg as truly identical in metrology-grade work. They differ by 0.00001%, which is negligible clinically but enough to shift NIST-traceable barometer calibrations.
- 1 atm = 760 torr (exact, BIPM 1954)
- 1 atm = 101,325 Pa = 1.01325 bar = 14.696 psi
- 1 torr = 133.322 Pa = 0.001333 bar
- 1 torr ≈ 0.999999857 mmHg
- Sea-level pressure ≈ 760 torr ± 30 torr (weather)
- Mt. Everest summit ≈ 250 torr (one-third atm)
- Mars surface ≈ 4.5 torr (0.006 atm)
- CERN beampipe < 10⁻¹² torr
A short history of the torr
Evangelista Torricelli, a student of Galileo, built the first barometer in 1643. He filled a sealed glass tube with mercury, inverted it into a mercury dish, and observed the column always settled near 760 mm — held aloft by atmospheric pressure. The experiment was the first direct proof that air has weight.
For three centuries, pressure was reported in mmHg without any agreed definition. The 1954 CGPM standardised "1 standard atmosphere = 101,325 Pa exactly" and defined the torr as 1/760 atm, named in Torricelli's honor.