How this instrument works
In 1643 Evangelista Torricelli inverted a sealed glass tube of mercury into an open dish and watched the column fall until it settled near 760 millimetres, leaving the first deliberate vacuum in the space above it. The unit that carries his name was tidied up three centuries later: the General Conference on Weights and Measures fixed the standard atmosphere at 101325 pascals in 1954, and this smaller unit was then pinned to precisely one seven-hundred-and-sixtieth of it, or 133.322368421 Pa.
Defining the ratio arithmetically instead of through a mercury column is what makes the conversion clean, because mercury density and local gravity no longer enter into it. The number 760 factors as 2³ × 5 × 19, and it is the 19 that spoils the decimal: 1/760 = 0.001315789473684210526…, repeating forever in an eighteen-digit cycle inherited from one nineteenth. The engine here carries that expansion truncated at twelve places, 0.00131578947368.
Two working communities keep these units in daily use. Vacuum engineering in North America still specifies sputtering chambers, freeze-dryers and electron-microscope columns in the smaller unit and its thousandth part, with rough vacuum running from ambient down to roughly 1 and ultra-high vacuum sitting below 10⁻⁹. Atmospheres survive as a human-scale yardstick: dive tables and hyperbaric chambers count in atmospheres absolute, and a chemist quoting a reaction at 3 atm is describing a pressure you can picture.
- Type your barometer or gauge reading into the Torr (torr) field. It opens at 760, one standard atmosphere.
- Read the answer on the Atmospheres (atm) line, which recalculates on every keystroke.
- Working in millitorr? Shift the decimal three places first, so a 250 mTorr set point is entered as 0.25.
- To go the other way, multiply your atm figure by 760 — 0.5 atm is 380 — then type that back in to confirm.
Worked example — a chamber before pump-down
A technician logs the vessel with its door still open, the wall barometer steady at 760. Entering 760 into the Torr (torr) field returns 1.0 on the Atmospheres (atm) line: the chamber sits at one standard atmosphere, the baseline every later reading gets compared against.
That answer is 1.0 by construction rather than by luck — 760 multiplied by one seven-hundred-and-sixtieth can be nothing else. Pump down to 76 and the same sheet reads 0.1 atm; reach 7.6 × 10⁻³ and you are at ten millionths of an atmosphere, deep enough that the steel walls begin outgassing faster than the pump clears them.
Questions
Is one atmosphere exactly 760 torr?
Yes, and by definition rather than by measurement. The standard atmosphere was fixed at 101325 pascals in 1954, and the smaller unit was then defined as precisely 1/760 of that, which comes to 133.322368421 Pa. No experiment can shift the ratio, because it is an agreement between standards bodies rather than a physical quantity. Any uncertainty in your answer came from the gauge, not from the arithmetic.
Is a torr identical to a millimetre of mercury?
Almost, but the two have separate definitions. The millimetre of mercury is physical — the pressure under a 1 mm column of fluid at density 13595.1 kg/m³ and standard gravity, giving 133.322387415 Pa — while the older unit is now purely arithmetic at 133.322368421 Pa. They disagree by roughly one part in seven million, invisible on a blood-pressure cuff and irrelevant on a vacuum gauge, though metrology work should keep them apart.
Why show 0.00131578947368 instead of the fraction?
Because 1/760 has no finite decimal form and a numeric engine has to stop somewhere. The expansion repeats in an eighteen-digit cycle, so cutting it at twelve places leaves the factor low by around three parts in a trillion — about a third of a nanometre measured across a football pitch. Divide by 760 by hand whenever you want the exact rational value instead.
How do I handle millitorr, or microns of mercury?
Divide by 1000 before entering the figure. One millitorr is 0.001 torr, and older vacuum shops call it a micron because it corresponds to a micrometre of mercury. A 500 mTorr set point therefore goes in as 0.5 and returns 6.58 × 10⁻⁴ atm. Lyophilisation and magnetron sputtering are usually specified somewhere in this band, which is why the input field steps in thousandths.
Should I quote pressures in atm or in bar?
Bar, for most modern chemistry. IUPAC moved the thermodynamic standard state from 1 atm to exactly 1 bar (100000 Pa) in 1982, so tables published since then are referenced to bar unless they state otherwise. One atmosphere equals 1.01325 bar, about 1.3% higher, which is enough to move an equilibrium constant in the third digit. Atmospheres persist mainly in diving, hyperbaric medicine and informal description.
Is the air pressure where I live really 760 torr?
Probably not — 760 is a defined reference point, not a weather report. A calm day at sea level usually falls between 750 and 770. Denver sits near 630, Mexico City near 580, and the summit of Everest around 253, roughly a third of an atmosphere, which is why bottled oxygen is standard up there. Weather services also correct station readings back to sea level, so a published figure and your own barometer will rarely match.