kg/m³ vs kgf/m³
Understand Density (mass) vs Unit Weight (force) in construction & structural design.
Concept
Quick understanding1) kg/m³ (mass density)
kg/m³ means kilogram per cubic meter. It tells you the mass inside 1 m³ of material.
- Type: Mass density (mass / volume)
- Used in: material property tables (density)
- Example: water ≈ 1000 kg/m³
2) kgf/m³ (weight density using kgf)
kgf/m³ means kilogram-force per cubic meter. Here kgf is a force, not a mass.
- Type: Force density (force / volume)
- 1 kgf = weight of 1 kg under standard gravity
- Relation:
1 kgf = 9.80665 N
In structural engineering, we usually use kN/m³ (SI) instead of kgf/m³.
3) Why numbers sometimes look “the same”
If someone uses standard gravity, the numeric value can look the same:
2400 kg/m³(mass density) → can be stated as2400 kgf/m³(weight density in kgf)- But that is only because kgf is defined from gravity.
Best practice: for weight/unit weight, write it in N/m³ or kN/m³.
Conversions
Use g = 9.80665| From | To | Formula |
|---|---|---|
| kg/m³ | kgf/m³ | ρ(kgf/m³) ≈ ρ(kg/m³) (standard gravity convention) |
| kg/m³ | N/m³ | γ(N/m³) = ρ × 9.80665 |
| kg/m³ | kN/m³ | γ(kN/m³) = ρ × 9.80665 / 1000 |
| kgf/m³ | N/m³ | γ(N/m³) = (kgf/m³) × 9.80665 |
Quick calculator (enter density in kg/m³)
ρ in kgf/m³
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Unit weight γ in N/m³
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Unit weight γ in kN/m³
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Tip: In design tables, concrete is often taken as about 23.5–24.0 kN/m³.
Example (Concrete)
If concrete density ≈ 2400 kg/m³:
- ρ ≈ 2400 kgf/m³ (kgf convention)
- γ = 2400 × 9.80665 = 23536 N/m³
- γ = 23536 / 1000 = 23.54 kN/m³
Engineering note: Prefer SI force units (N/m³ or kN/m³) for unit weight. Use kg/m³ for density.