Strain rate (˙ε˙) is a measure of the rate at which deformation occurs in a fluid flow. It quantifies how quickly the fluid is being deformed or sheared as it flows. Strain rate is defined as the rate of change of velocity gradient within a fluid. The SI unit of strain rate is per second (s⁻¹) or reciprocal time. The formula for strain rate is given by:

Where:

• ˙ε˙ is the strain rate, 
• $du$ is the change in velocity along a direction,
• $dy$ is the change in distance along that direction.