Measure the object's original length and width.
Stretch the object. Measure the length and width of the stretched object.
Calculate the strain on the object's physical dimensions by using the formula "ε = dl / L." The strain "ε" is equal to the change in length, or "dl," divided by the initial length, or "L." In Poisson's ratio, the compressed area of the object is known as the transverse strain and is a negative number. The area that becomes stretched is known as the longitudinal strain and is a positive number.
Divide the transverse, or negative, strain by the axial, or positive, strain to produce a ratio. This value is Poisson's ratio. The original expression "... = - εt / εl" can be read: "Poisson's ratio is equivalent to the negative transverse strain over the positive longitudinal strain."