pound per square foot

Pounds per square foot is the pressure unit of surfaces. An aircraft's wing loading, the snow a roof must carry, the live load a floor is rated for and the wind pressure on a wall are all quoted this way, because the areas involved are measured in feet and the resulting numbers stay conveniently sized.

The conversion is exact: $1\ \mathrm{lb/ft^2} = 47.880259\ \mathrm{Pa}$, which follows from the pound-force and the foot without approximation. Going the other way, a kilopascal is about 20.9 lb/ft².

The trap is the neighbouring unit. Pounds per square inch is 144 times larger, and both are casually called "pounds". A light aircraft at 14.4 lb/ft² is at 0.1 psi — the same pressure, written two ways, and confusing them moves a number by two orders of magnitude. Structural and aeronautical work almost always means per square foot; hydraulic and pneumatic work almost always means per square inch.

Wing loading is the clearest illustration of why the unit earns its place. It sets stall speed, which scales as the square root of wing loading, so a wing loaded four times as heavily stalls at twice the speed. Quoted in psi the numbers become uselessly small decimals; quoted per square foot they stay in the range a pilot recognises at a glance.