What is the formula for Poisson’s ratio‘σ’?

Ratio of lateral contraction to linear elongation is called Poisson’s ratio. It can also be defined as the ratio of unit transverse strain to unit longitudinal strain.

The formula for Poisson’s ratio ‘σ’ is given as,

Poisson’s ratio = σ =Lateral contraction / Linear elongation.

or

Poisson’s ratio = σ = – ε_t / ε_{l     ——— where} ε_t = transverse strain and ε_l = longitudinal strain

The decrease in thickness in lateral direction is lateral contraction and extension in length is known as linear elongation.