poundmass

Pound-mass, abbreviated lbm, is a unit of mass used in the United States customary system and in some engineering practices. It serves as the mass counterpart to the pound-force (lbf), which is a unit of force. In contexts that use both, mass and force must be related by a gravitational conversion constant to maintain consistent units.

One pound-mass is defined as 0.45359237 kilograms. Conversely, 1 kilogram equals 2.2046226218 pounds-mass. The avoirdupois pound, from which lbm is derived, is equal to 16 avoirdupois ounces, so 1 lbm = 16 oz. In many engineering calculations, lbm is used alongside lbf and the slug, another unit of mass.

The relationship between mass and force in the English engineering system is governed by the equation F = m a / g_c, where F is force in pounds-force, m is mass in pounds-mass, a is acceleration in feet per second squared, and g_c is the conversion constant, approximately 32.174 lbm·ft/(lbf·s^2). This constant ensures that weight at Earth's surface is consistent with conventional gravity (for practical purposes, weight ≈ mass × g, with appropriate units).

Pound-mass is commonly used in mechanical engineering, aerospace, and some physics contexts within the United States. In SI-preferred environments, kilograms or grams are typically used instead. To avoid confusion with weight, lbm is distinguished from lbf, the latter being a unit of force rather than mass.