Atmospheres to Torr Converter
Common Conversions
| atm | torr |
|---|---|
| 0.0001 | 0.076 |
| 0.001 | 0.76 |
| 0.01 | 7.6 |
| 0.05 | 38 |
| 0.1 | 76 |
| 0.25 | 190 |
| 0.5 | 380 |
| 1 | 760 |
| 1.5 | 1140 |
| 2 | 1520 |
| 5 | 3800 |
| 10 | 7600 |
Why this conversion matters in chemistry
Atmospheres are how pressure gets written in a gas-law problem; torr is how it reads on a rotary-evaporator gauge or a Schlenk line. A textbook value of 1 atm is 760 torr by definition — the same number a mercury manometer would show at sea level, which is exactly where the unit came from. The conversion matters when a solvent table recommends running an evaporation at around 58 torr (a low vacuum for toluene at 40 °C) and the method section was written in atm, or when a Clausius-Clapeyron fit wants pressures in atm and the instrument only knows torr.
Formula
Where the factor comes from
There is no derivation chain here, because the factor is the definition. The torr was fixed as exactly 1/760 of a standard atmosphere, so 1 atm = 760 torr holds by construction rather than by measurement, and the 760 carries neither uncertainty nor rounding. The untidiness moves to the other side of the relation: 1 torr = 101325/760 Pa = 133.32236842… Pa, a repeating decimal. The choice of 760 is inherited from the barometer — a mercury column at sea level stands near 760 mm — but the modern definition deliberately cut the tie to the fluid, so a torr no longer depends on mercury's density or on local gravity. That independence from a physical liquid is the entire point of the redefinition.
Precision and significant figures
The arithmetic contributes no uncertainty of its own, so a result inherits precisely the figures the gauge supplied. Those figures are usually few. Thermocouple and Pirani gauges covering the 10⁻³ to 1 torr range are calibrated against air or nitrogen and read differently for solvent vapor or helium, sometimes by tens of percent. A capacitance manometer is what you reach for when the torr value itself has to be trusted, and it typically specifies a fraction of a percent of reading. Quoting a rotary-vane pump's ultimate as 0.001 torr to three decimals claims a confidence the manifold gauge does not have.
Worked Examples
Standard atmospheric pressure at sea level — the anchor point of the conversion.
Low vacuum — well below what a water aspirator can reach and into the territory of a membrane or rotary-vane pump.
Medium vacuum, where Schlenk-line chemistry with a diffusion or rotary-vane pump tends to operate.
Roughly atmospheric pressure at the summit of a 5500 m peak — half an atmosphere still supports chemistry, just not comfortably.
Common mistakes
Microns and torr on the same manifold
Vacuum gauges frequently read in microns, where one micron is 10⁻³ torr. A display showing 50 means 0.05 torr, not 50 torr. The two scales sit on neighboring instruments in most vacuum lines, and reading the wrong one puts the pressure out by a factor of a thousand in the direction that looks reassuring.
Converting where the ratio already cancels
The two-point Clausius–Clapeyron form contains ln(P₂/P₁), so torr values go in untouched — the units cancel. Relations carrying a standard state, ΔG° = −RT ln K among them, do not cancel, and there the pressures have to be referenced to that standard state before anything else in the calculation happens.
Assuming your building sits at 760 torr
A Schlenk line vented to atmosphere is at whatever the room is, and a lab 300 m above sea level runs nearer 733 torr on an ordinary day, lower still inside a weather system. For distillation head pressures and boiling-point corrections that 3.5 percent is the difference between a clean cut and a smeared one.