lbf

Pound-force, abbreviated lbf, is a unit of force used in the English engineering (United States customary) system. It is defined as the force required to accelerate a mass of one pound-mass at Earth's standard gravitational acceleration, historically taken as 32.174 ft/s^2.

Because pounds-mass (lbm) and pounds-force (lbf) are part of different unit families, the relationship between them is governed by the constant g_c, where F_lbf = m_lbm × a / g_c and g_c = 32.174 lbm·ft/(lbf·s^2). In practical terms, 1 lbf is equal to approximately 4.448221615 newtons, and conversely, 1 newton is about 0.2248089431 lbf.

In everyday engineering usage, lbf is used to express forces such as weight, thrust, or loads, often appearing alongside other US customary units like psi (pounds per square inch) for pressure. It is distinct from the SI unit of force, the newton (N), and from the mass unit pound-mass (lbm). When mass is expressed in lbm and force in lbf, gravity is implicitly accounted for via g_c in calculations.

The term pound-force is sometimes abbreviated as lbf or simply lbf, with the latter used to emphasize a force rather than a mass. While still common in aerospace, automotive, and civil engineering in the United States, lbf is less used in international contexts, where the newton and other SI units dominate.