lbft3

lbft3 is the shorthand commonly used for pound-force per cubic foot, written as lbf/ft^3, a unit of weight density (also called specific weight) in English engineering units. It describes the amount of gravitational force per unit volume that a material weighs. In contrast, lbm/ft^3 denotes mass density, which does not include gravity explicitly. In practical use, weight density gamma = rho * g, where rho is mass density and g is local gravitational acceleration. Because lbf already represents a force, lbf/ft^3 is the density-like quantity that encodes weight rather than mass.

Converting to SI units, 1 lbf/ft^3 is approximately 157.473 N/m^3. A familiar reference is water, which at standard gravity has a weight density of about 62.4 lbf/ft^3, equivalent to roughly 9.81 kN/m^3. This relationship allows engineers to switch between English and metric units in hydrostatics and fluid mechanics problems.

Common uses include calculating hydrostatic pressures p = gamma h, designing foundations and earth pressures in geotechnical engineering, and specifying materials’ weight loads in building and mechanical systems. The phrase lbft3 may appear in older texts or informal notes, but the standard modern notation is lbf/ft^3 for weight density and lbm/ft^3 for mass density to avoid confusion.

Notes on notation: because lbf denotes a force, it is important to specify whether the context refers to weight density (lbf/ft^3) or mass density (lbm/ft^3). In many contexts, local gravity is assumed constant, which fixes gamma accordingly.