The formula
kg/m³ = lb/ft³ × 16.01846337
Where the factor comes from — each unit by its definition, then its size in SI:
1 lb/ft³ = lb / ft³ = 16.018463374 kg/m³1 kg/m³ is the SI unit of density.
Dividing the two SI sizes gives the factor: 16.018463374 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 0.0624279605761.
Density is mass per volume. 1 g/cm³ is exactly 1000 kg/m³ — water is close to that — and the buoyancy and pressure formulas take kg/m³.
Worked examples
- Water at room temperature (approximately) (1 g/cm³): 62.428 lb/ft³ = 1,000 kg/m³ — 62.428 × 16.01846337 = 1,000
- Typical structural steel: 490.059 lb/ft³ = 7,850 kg/m³ — 490.059 × 16.01846337 = 7,850
- Air at sea level (ISA): 0.0764743 lb/ft³ = 1.225 kg/m³ — 0.0764743 × 16.01846337 = 1.225
Frequently asked questions
How many kilograms per cubic metre is 1 lb/ft³?
1 lb/ft³ = 16.018463374 kg/m³.
How do I convert kilograms per cubic metre back to pounds per cubic foot?
Divide by 16.018463374 — or multiply by 0.0624279605761: 1 kg/m³ = 0.0624279605761 lb/ft³.