Specific Strength Calculator
Specific strength is the ratio of a material's tensile (or yield) strength to its mass density. It represents the load-carrying capacity per unit mass and is the primary criterion for selecting materials in weight-critical designs such as aircraft structures, race car chassis, sporting equipment, and lightweight automotive components. Also calculated here is specific stiffness (E/rho), which governs deflection-limited designs. Compare multiple materials by entering their data to find the best strength-to-weight option.
Specific strength formula
Specific strength = sigma (Pa) / rho (kg/m^3) = J/kg = N*m/kg
In kN*m/kg: = sigma (MPa) / rho (kg/m^3) * 1000
Specific stiffness = E (GPa) / rho (kg/m^3) * 10^6 [N*m/kg]
Numerically, if sigma is in MPa and rho is in kg/m^3: specific strength in kN*m/kg = sigma * 1000 / rho. For Ti-6Al-4V: 900 * 1000 / 4430 = 203 kN*m/kg.
Material comparison guide
Structural steel A36: 250 MPa / 7,850 kg/m^3 = 32 kN*m/kg. Aluminum 7075-T6: 500 MPa / 2,810 kg/m^3 = 178 kN*m/kg. Ti-6Al-4V: 900 MPa / 4,430 kg/m^3 = 203 kN*m/kg. CFRP unidirectional: 1,500 MPa / 1,580 kg/m^3 = 949 kN*m/kg. These values show why CFRP dominates in aerospace primary structure despite its high cost.
Specific strength: frequently asked questions
What is specific strength?
Specific strength is the ratio of a material's strength to its density: sigma / rho. It tells you how much load a material can carry per unit of mass. It is the key selection criterion when weight is a primary design constraint, as in aerospace, automotive, and sporting equipment applications.
What are the units of specific strength?
Specific strength = (Pa) / (kg/m^3) = m^2/s^2 = N*m/kg = J/kg. It is also commonly expressed in kN*m/kg or MN*m/kg. Another common unit is MPa/(g/cm^3) = kN*m/kg * 0.001, numerically equal when strength is in MPa and density is in g/cm^3.
Which material has the highest specific strength?
Carbon fiber composites (unidirectional) have among the highest specific strengths at around 2,000 to 3,000 kN*m/kg. Spider silk achieves approximately 1,000 kN*m/kg. Ultra-high-strength steel AHSS: about 120 kN*m/kg. Grade 5 titanium (Ti-6Al-4V): about 250 kN*m/kg. Aluminum alloy 7075-T6: about 195 kN*m/kg.
What is specific stiffness?
Specific stiffness (specific modulus) = E / rho, where E is Young's modulus. It governs deflection-limited designs where the goal is to minimize mass while meeting a stiffness target. Beryllium, carbon fiber, and silicon carbide have very high specific stiffness. Aluminum alloys have similar specific stiffness to steels despite their lower modulus.
Can I use specific strength to compare different material forms?
Specific strength is most useful for comparing materials that carry the same type of load (e.g., all under tension). The comparison changes for bending-dominated designs: the efficiency figure of merit becomes strength^(2/3)/density for beams or strength^(1/2)/density for plates, depending on which dimension is free to vary.
Official sources
- NIST, Materials Properties Reference Data: nist.gov.
- ASM International, "ASM Handbook Vol. 21: Composites": asminternational.org.
- ASTM E8/E8M, "Standard Test Methods for Tension Testing of Metallic Materials": astm.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.