Poundforce

Pound-force (lbf) is a unit of force used in the United States customary and related British gravitational systems. It is defined as the force required to accelerate a mass of one pound‑mass (lbm) at standard gravity, g0, which is 32.174 ft/s^2. Equivalently, it is the weight of a one-pound mass under standard gravity. In engineering practice, the relationship among force, mass, and acceleration in these units is F(lbf) = m(lbm) × a(ft/s^2) / g_c, where g_c = 32.174 lbm·ft/(lbf·s^2).

One pound-force is exactly equal to 4.448221615 N in SI units (approximately). The unit is commonly used for thrust, loads, and other force specifications, and appears in expressions such as pounds per square inch (psi) when describing pressure in terms of force per area.

Pound-force is distinct from the pound-mass (lbm). The avoirdupois pound is a unit of mass, whereas the lbf is a unit of force; their conversion depends on gravity and the g_c constant. The symbol lbf is widely used, with variants such as lb_f sometimes seen in practice.

The pound-force forms part of the English engineering units system and remains in use in some engineering and manufacturing contexts in the United States, though many disciplines convert to SI units for standardization. See also Newton, pound-mass, g0, g_c, and psi for related concepts.