Stokes' Law Calculator

About this calculator

This calculator applies Stokes' law, F = 6πηrv, to find the viscous drag force on a small sphere moving slowly through a fluid, where η is the fluid's dynamic viscosity, r is the sphere's radius, and v is its velocity. The law holds in the laminar (low Reynolds number, Re < 1) regime, where drag is dominated by viscous forces rather than fluid inertia — it's also the basis for terminal velocity calculations of a falling sphere, where drag balances gravity minus buoyancy.

Physics and engineering students use it in fluid mechanics coursework, while chemists and lab scientists apply the same principle to sedimentation and centrifugation analysis, particle-size determination (falling-ball viscometers), and estimating settling velocities of particles in suspensions. Enter the fluid viscosity, sphere radius and velocity to get the drag force.