Poundsmass

Pounds-mass, abbreviated lbm, is a unit of mass used in the British imperial and United States customary systems. It is part of the avoirdupois system, which is the common mass unit system in these traditions. One pound-mass is defined as 0.45359237 kilograms exactly, and thus 1 kilogram equals approximately 2.2046226218 pounds-mass.

The symbol lbm distinguishes mass from the closely related unit of force, the pound-force (lbf). In many engineering and physics contexts that use imperial or US customary units, mass and force are linked through a gravitational constant, commonly denoted g_c. A typical relation is F(lbf) = m(lbm) × a(ft/s^2) / g_c, where g_c is about 32.174 lbm-ft/(lbf-s^2). This framework preserves consistency between mass, acceleration, and force in non-SI unit systems.

Usage and notation notes: lbm is standard for specifying mass in technical contexts, while the plain symbol lb is sometimes used informally for either mass or force, which can cause confusion. To maintain clarity, lbm should be used when referring to mass and lbf when referring to force.

Historical and context notes: The pound-mass derives from the avoirdupois pound, historically used in the British Empire and widely adopted in the United States. In modern practice, SI units (kilograms) are the preferred standard in science and most international settings, but lbm remains common in some engineering, construction, and older documentation within non-SI contexts.