The pound or pound-mass is a unit of mass,used in the imperial, United States customary and other systems of measurement. MKS, or SI units are based on meters (m), kilograms (kg) , and seconds (s). lbm represents the mass that weighs one pound-force (lbf) on earth. In SI units, the force unit is the newton (N), and it is defined as the force required to accelerate a mass of 1⋅kg at a rate of 1⋅ms2. The pound mass (abbreviated as lbm or just lb) is also a fundamental unit within the Imperial. This video desribes the difference between pounds-force (LBf) and pounds-mass (LBm) as.