Young's modulus, E, quantifies the relationship between tensile or compressive stress σ (force per unit area) and axial strain ε (proportional deformation) in the linear elastic region of a material:^[2]E=σε

Young's modulus is commonly measured in the International System of Units (SI) in multiples of the pascal (Pa) and common values are in the range of gigapascals (GPa).

Examples:

• Rubber (increasing pressure: large length increase, meaning low E)
• Aluminium (increasing pressure: small length increase, meaning high E)

A solid material undergoes elastic deformation when a small load is applied to it in compression or extension. Elastic deformation is reversible, meaning that the material returns to its original shape after the load is removed.

At near-zero stress and strain, the stress–strain curve is linear, and the relationship between stress and strain is described by Hooke's law that states stress is proportional to strain. The coefficient of proportionality is Young's modulus. The higher the modulus, the more stress is needed to create the same amount of strain; an idealized rigid body would have an infinite Young's modulus. Conversely, a very soft material (such as a fluid) would deform without force, and would have zero Young's modulus.