pound per square inch
Pressureexact by definition
The pound per square inch is one pound-force distributed over one square inch, which works out to 6894.757... pascals. The definition is exact, since the pound-force, the inch and standard gravity are all exact by definition, but the resulting decimal does not terminate so any tabulated value is truncated. Standard atmospheric pressure is about 14.696 psi.
Watch out: Almost every psi reading in the field is gauge pressure, psig, meaning pressure above ambient. Absolute pressure is psia. A tyre at 32 psig is at 46.7 psia. Gas law calculations, compressor work and boiling point predictions all need absolute pressure, and feeding a gauge reading into them is one of the most common errors in process work.
| 1 psi | 6.8947573 kPa |
About the pound per square inch
Psi is the customary system's most-used pressure unit and it inherits both of the pound-force's properties: it is exactly defined, and it is easy to misuse. The exactness comes from stacking exact definitions. The pound is exactly 0.45359237 kg, standard gravity is exactly 9.80665 m/s², and the inch is exactly 0.0254 m, so one lbf/in² is exactly \(\frac{0.45359237 \times 9.80665}{0.0254^2}\) pascals. The decimal expansion of that fraction runs forever, which is why tables show 6894.757293168 and stop.
The gauge-versus-absolute distinction is where the real errors live. A pressure gauge has atmosphere on the back of its sensing element, so it reads zero at atmospheric pressure and reports the difference. Anything derived from the ideal gas law, including compressor power, gas cylinder contents and altitude effects, needs absolute pressure, which means adding about 14.7 psi to the gauge reading at sea level. A compressor "delivering 100 psi" is compressing by a ratio of 114.7/14.7, not 100/1, and sizing it as though the ratio were 100 would be wildly wrong.
Useful anchors: sea-level atmosphere 14.696 psi, car tyre 30 to 35 psig, domestic water supply 40 to 80 psig, and one psi of water column is about 2.31 feet of head, the conversion that pump curves quietly rely on.