Young's modulus ${\displaystyle E}$, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied lengthwise. It quantifies the relationship between tensile/compressive stress ${\displaystyle \sigma }$ (force per unit area) and axial strain ${\displaystyle \varepsilon }$ (proportional deformation) in the linear elastic region of a material and is determined using the formula:

Young's moduli are typically so large that they are expressed not in pascals but in gigapascals (GPa).

Example:

• Silly Putty (increasing pressure: length increases quickly, meaning low ${\displaystyle E}$)
• Aluminum (increasing pressure: length increases slowly, meaning high ${\displaystyle E}$)

Higher Young's modulus corresponds to greater (lengthwise) stiffness.

Although Young's modulus is named after the 19th-century British scientist Thomas Young, the concept was developed in 1727 by Leonhard Euler. The first experiments that used the concept of Young's modulus in its current form were performed by the Italian scientist Giordano Riccati in 1782, pre-dating Young's work by 25 years. The term modulus is derived from the Latin root term modus which means measure.