Rubber modulus is

 

Young's modulus represents the factor of proportionality in Hooke's law, which relates the stress and the strain. However, Hooke's law is only valid under the assumption of an elastic and linear response. Rubber behavior is not like a linear material when applied the stress on it. Rubber modulus or elastic modulus calculate at different specific strain such as 100%, 200% or 300% due to non-linear behavior of the rubber material when applied stress.