One kilogram per cubic metre is 0.0624279606 pounds per cubic foot. The factor combines a mass conversion and a cubed length conversion, which is why it lands on an untidy number that has to be looked up rather than remembered.
About the units
Kilograms per cubic metre is the SI density unit. Pounds per cubic foot is its American counterpart, standard in construction, HVAC and bulk materials handling. The conversion multiplies the pound-to-kilogram factor by the cubic-foot-to-cubic-metre factor, and because the second is a cube, the result is not intuitive from the linear conversions. Anyone who tries to derive it mentally from 2.2 pounds per kilogram and 3.28 feet per metre will get it wrong unless they remember to cube the second.
The exact factor
The derivation is 0.45359237 kg per lb divided by 0.028316846592 m³ per ft³, giving 16.018463374 kg/m³ per lb/ft³. Inverted, that is 0.0624279605761446. Reference values that make it concrete: water is 62.43 lb/ft³ against 1,000 kg/m³; concrete about 150 lb/ft³ or 2,400 kg/m³; structural steel 490 lb/ft³ or 7,850 kg/m³; and dry air 0.0765 lb/ft³ or 1.225 kg/m³. The figure 62.43 for water is worth memorising, because it anchors the whole scale in American practice the way 1,000 does in metric.
Where you meet this conversion
Construction and civil engineering across markets. Concrete mix designs, soil unit weights, insulation density and aggregate specifications are all in pounds per cubic foot in American practice and kilograms per cubic metre elsewhere. HVAC uses air density in both. Bulk material handling — grain, coal, wood chips — quotes bulk density in one unit or the other, and shipping calculations that mix them produce load errors. Timber grading uses density as a strength proxy and reports in both systems.