Shear stress is the force per unit area parallel to a surface in a fluid. It measures the force applied tangentially to the surface. The formula for shear stress ($\tau$) in fluid mechanics, as described by Newton’s law of viscosity, is given by:

Where:

• $\tau$ is the shear stress,
• $\mu$ is the dynamic viscosity of the fluid,
• du/dy​ is the velocity gradient or shear rate.