Torr to Atmospheres Converter
Common Conversions
| torr | atm |
|---|---|
| 0.001 | 0.00000132 |
| 0.01 | 0.0000132 |
| 0.1 | 0.000132 |
| 1 | 0.001316 |
| 10 | 0.01316 |
| 50 | 0.06579 |
| 100 | 0.1316 |
| 200 | 0.2632 |
| 380 | 0.5 |
| 500 | 0.6579 |
| 760 | 1 |
| 1520 | 2 |
Why this conversion matters in chemistry
Vacuum work almost always lives in torr — rotary evaporators at 10 to 100 torr, Schlenk lines at 10⁻² to 10⁻³ torr, high-vacuum systems several decades lower. But any calculation that plugs a pressure into the ideal gas law with R = 0.08206 L·atm/(mol·K), or into a Clausius–Clapeyron fit for sublimation enthalpy, needs atm. Dividing by 760 is the bridge. 380 torr is half an atmosphere; 23.8 torr is the vapor pressure of water at 25°C, or 0.0313 atm. The conversion is exact — no precision trade-off — but forgetting it silently drops a pressure reading by a factor of 760.
Formula
Where the factor comes from
The 760 in the denominator began as a measurement and ended as a decree. Early barometry established that the atmosphere would support roughly 760 millimeters of mercury, and for a long stretch that column was the definition. Once the standard atmosphere was fixed at exactly 101325 Pa the relationship inverted: the torr was redefined as exactly one seven-hundred-sixtieth of that pascal value, and mercury dropped out of the definition altogether. So dividing by 760 is exact rather than approximate, and it is exact because an integer was chosen, not because a column was ever measured carefully enough. The quotient itself never terminates — 1/760 is 0.001315789473684… recurring — but the exactness lives in the relation, not in how many digits anyone writes down.
Precision and significant figures
Because the factor is exact, every figure lost is one the gauge never supplied, and vacuum instruments differ enormously in what they can supply. A capacitance manometer measures absolute pressure independently of the gas species and is commonly specified to a fraction of a percent of reading, which supports three or four figures comfortably. A thermal-conductivity gauge infers pressure from heat loss and is calibrated against one gas, so on an organic vapor its reading can be wrong by a factor rather than a percentage — careful division rescues none of that. Note also how fast the atmosphere becomes an awkward container: 10⁻³ torr is 1.32 × 10⁻⁶ atm, and anywhere below that scientific notation stops being optional.
Worked Examples
The defining equivalence. One atmosphere is 760 torr by historical convention.
Water's vapor pressure at 25°C. The correction you subtract when collecting a gas over water to get the dry-gas partial pressure.
A low to moderate vacuum. Achievable on a rotary pump in reasonable condition — below where a water aspirator can reach.
Half an atmosphere — roughly the ambient air pressure at around 5500 m elevation.
Common mistakes
One atmosphere is a constant, not the weather
The standard atmosphere is fixed at 101325 Pa and does not move with conditions. A barometer reading 745 torr converts to 0.980 atm, not 1.000 atm, and using the round number instead puts a two percent bias into anything downstream. Gas collected over water is the usual casualty, since the total pressure there has to be the measured ambient value, converted.
Atmospheres commit you to R = 0.08206
The only reason to reach for atmospheres is the L·atm gas constant. Convert a pressure to atm, leave the volume in liters, then pair both with R = 8.314 J/(mol·K), and the result is off by a factor of about 101 — that constant expects pascals and cubic meters, and the two substitutions miss canceling by exactly that much. Fix the gas constant first, then convert to match.
Leak rates hide the pressure unit inside
Vacuum leak rates are quoted as torr·L/s, while standard leak rates arrive as standard cubic centimeters per second, which is atm·cm³/s. The two are not interchangeable numbers: 1 atm·cm³/s is 0.76 torr·L/s, because the volume unit shifted alongside the pressure unit. Convert both parts of a composite unit or the rate comes out wrong by more than the 760 you were watching for.