poundsforce

Pound-force, abbreviated lbf, is a unit of force used in the English engineering system and in US customary units. It is defined as the force required to accelerate a mass of one avoirdupois pound by standard gravity. In exact terms, one pound-force is defined as 4.4482216152605 newtons. The symbol lbf is commonly used to distinguish this unit of force from the pound-mass (lbm).

In these systems, the pound-mass is the unit of mass, while the pound-force is the corresponding unit of force. The relationship between mass, force, and gravity is tied to standard gravity, g0. Under standard gravity, the weight of 1 lbm is defined to be 1 lbf, which is why calculations in this system often use the conversion factor g_c to maintain consistency. In customary engineering units based on feet and pounds, 1 lbf is also equal to approximately 4.44822 newtons. Conversely, 1 newton is about 0.2248089 lbf.

Usage of the pound-force is common in mechanical, aerospace, and civil engineering in the United States and in some other contexts that employ US customary units. It is not part of the International System of Units (SI), where the newton is the standard unit of force. When converting to SI, lbf is converted to newtons, and to avoid confusion, the mass unit lbm is distinguished from the force unit lbf, often by context or explicit notation such as lbm or lb_f in technical documents.