Three families that share a vocabulary
Pressure, torque and force all get written with a force unit and a length unit, and that is exactly why they are confused. The distinction is what the length is doing.
Force is a push or a pull: newtons, pounds-force, kilograms-force. Pressure is force divided by an area: pounds per square inch, newtons per square metre. Torque is force multiplied by a distance from an axis: pounds-force times feet, newtons times metres. A foot-pound and a pound per square foot both contain a pound and a foot, and they are not related quantities at all — one is a lever, the other is a spread.
The tool refuses to convert across families rather than producing a number, because a number would be worse than an error message. If you selected two units and got a refusal, one of them is not the quantity you thought it was.
Which factors are exact and which are conventional
Most of the conversions here are exact by definition, which is a stronger statement than it sounds. The inch is defined as exactly 25.4 millimetres. The pound-force is defined from the pound, exactly 0.45359237 kg, times standard gravity, exactly 9.80665 m/s². Multiply those and the pound-force is exactly 4.4482216152605 newtons, with no measurement uncertainty anywhere in the chain. From there:
| Conversion | Value | Status |
|---|---|---|
| 1 psi | 6894.757293168 Pa | Exact, from the pound-force over a square inch |
| 1 bar | 100 000 Pa | Exact by definition |
| 1 standard atmosphere | 101 325 Pa | Exact by definition, which is 14.696 psi |
| 1 lbf-ft | 1.3558179483314 Nm | Exact |
| 1 lbf-in | 0.11298482902762 Nm | Exact, one twelfth of the above |
| 1 kgf | 9.80665 N | Exact, from standard gravity |
| 1 inch of mercury | 3386.389 Pa | Conventional \u2014 depends on a stated fluid density and temperature |
| 1 inch of water | 249.0889 Pa | Conventional, and reference temperature varies between sources |
The last two are the ones to be careful with. A column of fluid has a pressure that depends on its density, and density depends on temperature, so inches of mercury and inches of water are only defined once someone states the reference conditions. The values used here are the conventional ones that instrument scales are built against. Different handbooks quote inches of water at 4 °C, at 15.6 °C or at 20 °C, and the spread between them is a few tenths of a percent — irrelevant for a duct static pressure reading, relevant if you are calibrating something.
Gauge and absolute, and the fourteen and a half psi nobody mentions
A tyre gauge reading 32 psi is telling you the pressure inside is 32 psi above the air outside. The absolute pressure in the tyre is nearer 46.7 psi. Nearly every dial gauge you meet — compressors, hoses, tyres, boilers — reads that way, because it works by measuring a difference against its own surroundings, and the notation psig makes it explicit when anyone bothers. Absolute pressure, sometimes written psia, is measured from a vacuum, and it is what thermodynamic tables, weather reports and vacuum work use.
At normal working pressures the distinction is a bookkeeping detail. At low pressures it dominates: a gauge reading 2 psi is at 16.7 psi absolute, so treating the two as the same is a factor of eight error. The checkbox on the tool adds one standard atmosphere so you can see both readings side by side.
Torque, where the unit error is expensive
The most common unit accident in the shop is between foot-pounds and inch-pounds. They differ by exactly twelve, they are written almost identically, and both appear in the same document more often than they should. A small fastener specified at 45 inch-pounds, tightened to 45 foot-pounds because that was what the wrench was set to, receives twelve times the intended torque, and the thread is gone before anyone notices. The failure in the other direction is quieter and worse: a large fastener at a twelfth of its intended torque is loose, and it stays loose until something moves.
Kilogram-force metres turn up on older equipment and on machinery from markets that used the metric technical system, and they convert cleanly — one kgf-m is 9.80665 Nm, so roughly 7.2 foot-pounds. Newton metres and metre-newtons are the same thing written two ways. Anything written as pound-feet rather than foot-pounds is also the same thing; the ordering is a stylistic preference and carries no information.
What none of that tells you is how tight a given fastener should be. That figure belongs to the assembly, not to the fastener alone: grade, lubrication, washer, clamped material and the manufacturer specification all move it. Convert the number you were handed. Do not derive one. If you are chasing what a fastener even is before you can look up a figure, the screw thread identifier is the starting point, and the wrench size chart covers what turns it.
Questions people ask
Is 1 bar the same as 1 atmosphere?
Close but not equal, and the difference has bitten people. One bar is exactly 100,000 pascals. One standard atmosphere is exactly 101,325 pascals, so an atmosphere is 1.325% larger. In everyday tyre and compressor work the difference is inside the accuracy of the gauge. In anything where a percent matters — instrument calibration, gas law arithmetic, altitude work — it is not, and the two must be kept apart.
Why will it not convert psi to newton metres?
Because they are not the same quantity. Psi is a pressure, a force spread over an area. A newton metre is a torque, a force applied at a lever arm. No conversion factor exists between them, and any tool that produced one would be inventing it. The confusion is understandable — both combine a force unit with a length unit — but one divides by length squared and the other multiplies by length.
My compressor says 8 bar and the tool says 116 psi. Is that right?
Yes. One bar is 14.5038 psi, so 8 bar is 116.03 psi, and that is why 8 bar and 115 psi appear as the same machine in two markets. The rounding runs both ways: 100 psi is 6.89 bar, which most equipment rounds to 7 bar. Where the rounding was done in the direction of a higher figure, treat the lower of the two numbers as the working value.
What is the difference between lb-ft and ft-lb?
Nothing. Both mean a pound-force acting at a one-foot lever arm, and the ordering is a house style rather than a distinction. Some writers prefer lb-ft for torque and reserve ft-lb for energy, since a foot-pound of work is dimensionally the same product, but that convention is not universally followed and you cannot rely on it to tell you which quantity is meant. Context does that: a tightening figure is torque.
Should I use gauge or absolute pressure?
Match whatever produced the number. A reading taken off a dial on a tyre, a hose or a compressor is a gauge reading, and comparing it to a specification also written as gauge is straightforward. Weather pressures, vacuum figures and anything entering a gas law calculation are absolute, and mixing a gauge reading into that arithmetic is off by one atmosphere. The tick box on the tool shows both so the difference is visible rather than assumed.