In fluid mechanics, the Rayleigh–Plesset equation or Besant–Rayleigh–Plesset equation is a nonlinear ordinary differential equation which governs the dynamics of a spherical bubble in an infinite body of incompressible fluid.  Its general form is usually written aswhere
: is the density of the surrounding liquid, assumed to be constant
: is the radius of the bubble
: is the kinematic viscosity of the surrounding liquid, assumed to be constant
: is the surface tension of the bubble-liquid interface
:, in which,  is the pressure within the bubble, assumed to be uniform and  is the external pressure infinitely far from the bubble
Provided that  is known and  is given, the Rayleigh–Plesset equation can be used to solve for the time-varying bubble radius .
The Rayleigh–Plesset equation is derived from the Navier–Stokes equations under the assumption of spherical symmetry.