In physics, the Young–Laplace equation () is a nonlinear partial differential equation that describes the capillary pressure difference across the interface between two static fluids, such as water and air. This difference is due to the phenomenon of surface tension or wall tension. Wall tension can only be used for very thin walls. The Young–Laplace equation relates the pressure difference to the shape of the surface or wall. It is very important in the study of static capillary surfaces.