Pound-force

The pound-force is a non-SI unit of force or weight (properly abbreviated "lb_f" or "lbf"). The pound-force is equal to a mass of one avoirdupois pound multiplied by the standard acceleration due to gravity on Earth (which is defined as exactly 9.806 65 m/s, or approximately 32.174 05 ft/s). (The pound-force is thus roughly the force exerted due to gravity by a mass of one pound at the surface of the Earth.)

Though pounds-force had been used in low-precision measurements since the 18th century, they were never well-defined units until the 20th century. It was in 1901 when the CGPM first adopted a standard acceleration of gravity for the purpose of defining grams-force and kilograms-force, a value often borrowed to define pounds-force, though other values such as 32.16 ft/s (9.80237 m/s) have been used as well.

In SI units, a pound-force is equal to exactly 4.448 221 615 260 5 newtons, if the metric standard acceleration of gravity is borrowed for this purpose.