Poisson's Ratio Calculator
Poisson's ratio (v) is a dimensionless material constant that quantifies the relationship between lateral and axial strains under uniaxial loading. When a material is stretched along one axis, it contracts in the perpendicular directions, and the ratio of those strains characterizes material compressibility. Poisson's ratio is essential in structural mechanics, geotechnics, and finite element analysis. Enter the lateral strain (the perpendicular contraction, as a positive value) and the axial strain (the elongation in the load direction, as a positive value) to compute v. Both inputs should be small dimensionless numbers: for example, an axial strain of 0.001 with lateral strain of 0.0003 gives v = 0.30.
Poisson's ratio formula
v = -(lateral strain) / (axial strain) = |epsilon_lateral| / |epsilon_axial|
The negative sign in the definition ensures a positive v for conventional materials, since lateral and axial strains have opposite signs. This calculator accepts the magnitudes of both strains and computes v directly.
How to use Poisson's ratio
Poisson's ratio, together with Young's modulus, completely defines the elastic behavior of an isotropic material. It appears in formulas for shear modulus (G = E / (2(1+v))), bulk modulus (K = E / (3(1-2v))), and in the full 3D stress-strain constitutive law. Values near 0.5 indicate near-incompressibility (rubber, gels), while values near 0 indicate materials that barely change their lateral dimensions (cork, certain foams). Negative values characterize auxetic materials that expand sideways when pulled.
Poisson's ratio: frequently asked questions
What is Poisson's ratio?
Poisson's ratio (v) describes how a material deforms in directions perpendicular to the applied load. When you stretch a material axially, it typically contracts laterally. Poisson's ratio is the negative of the lateral strain divided by the axial strain.
What is a typical Poisson's ratio for metals?
Most metals have Poisson's ratios between 0.25 and 0.35. Steel is approximately 0.30, aluminum is about 0.33, and copper is around 0.34. Cork has a near-zero value (0.0), while rubber approaches 0.50.
Can Poisson's ratio exceed 0.5?
For conventional materials, Poisson's ratio ranges between -1 and 0.5. A value of 0.5 means the material is incompressible (like rubber). Some engineered auxetic materials have negative Poisson's ratios, meaning they expand laterally when stretched.
How is Poisson's ratio measured experimentally?
Strain gauges are bonded to a specimen in two orientations: one aligned with the load axis and one perpendicular to it. Under tensile or compressive load, both strains are recorded and the ratio is computed.
Why does Poisson's ratio matter in finite element analysis?
FEA software requires Poisson's ratio along with Young's modulus to fully define isotropic elastic material behavior. Incorrect values lead to errors in predicted deflections, stresses, and stability analyses, particularly near notches and boundaries.
Official sources
- NIST, "NIST Recommended Practice Guide: Mechanical Testing": nist.gov.
- ASM International, "ASM Handbook Vol. 8: Mechanical Testing and Evaluation": asminternational.org.
- ASTM E132, "Standard Test Method for Poisson's Ratio at Room Temperature": astm.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.