PSI to Torr Converter
Common Conversions
| psi | torr |
|---|---|
| 0.01 | 0.517 |
| 0.1 | 5.171 |
| 0.25 | 12.929 |
| 0.5 | 25.857 |
| 1 | 51.715 |
| 2 | 103.43 |
| 5 | 258.575 |
| 10 | 517.149 |
| 25 | 1292.873 |
| 50 | 2585.747 |
| 100 | 5171.493 |
| 1000 | 51714.93 |
Why this conversion matters in chemistry
Leak-checking a mass flow controller is one of the routine spots. A 0.5 psi low-pressure threshold lands at 25.9 torr on the test-stand pressure-decay trace, which is the figure that goes head-to-head against the controller's max-allowable-leak spec when qualifying it. The 51.7149 torr per psi factor falls right out of 1 atm = 14.696 psi = 760 torr. So the conversion is just the bookkeeping that lines up an industrial regulator gauge with the torr scale most lab vacuum gear runs in.
Formula
Where the factor comes from
The torr keeps a mercury name and has no mercury left in it. Since the standard atmosphere was fixed at exactly 101325 Pa, the torr has been defined as exactly one seven-hundred-sixtieth of that figure — 101325/760 Pa, or 133.32236842105263… Pa, a quotient that recurs forever because 760 does not divide any power of ten. The psi comes from a different tradition entirely: a pound-force of exactly 4.4482216152605 N acting over exactly 6.4516 × 10⁻⁴ m², which works out to 6894.757293… Pa. Divide one by the other for 51.71493257… torr per psi. Both units are exact by definition and the ratio inherits that exactness, but the 760 in the denominator also guarantees that no finite decimal will ever state it, however many digits you choose to take.
Precision and significant figures
Four figures — 51.71 — already land within one part in ten thousand, and five settle the number permanently. The interesting question is which end of the scale you are on. Vacuum work sits low, where a psi reading has nothing to offer: 1 torr is 0.0193 psi, a value no ordinary dial can indicate at all, so any converted figure below a few torr is arithmetic rather than measurement. The pairing earns its keep going upward, from a psi-graduated regulator into torr, and there the regulator sets the ceiling. A dial read to the nearest half psi carries roughly 26 torr of uncertainty, which makes the third digit of the converted result decorative.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
The conversion anchor — useful for any quick mental scale check.
About a typical low-pressure reaction-vessel setting.
About an elevated pressure for autoclave reactions.
Common mistakes
Negative psig does not become negative torr
A compound gauge showing −10 psig is describing 4.696 psia, which is 243 torr absolute. Convert the −10 directly and you get −517 torr, a quantity that does not exist. Add the signed gauge value to 14.696 first, then multiply. Vacuum manifolds are the usual place this bites, because the psi instrument reports relative to the room while everything else on the rig reports absolute.
Micron and millitorr label the same decade
Thermocouple gauges on rough-vacuum lines are frequently graduated in microns, meaning microns of mercury, and one micron is one millitorr. A converted psi value that lands at 0.05 torr is 50 microns on that display. Both labels turn up on the same pumping station, so a torr figure read against a micron scale is off by a factor of a thousand in whichever direction you mistook.
A regulator dial cannot see below ambient
The delivery gauge on a two-stage cylinder regulator is graduated upward from zero and pins there once the line drops to room pressure. Converting a needle resting at 0 psi gives 0 torr, which describes nothing about what a pump has actually pulled. The vacuum side of a rig needs its own gauge; the psi dial is only a source of torr values above one atmosphere.