This paper presents heat transfer correlation for natural-convection film boiling with a wavy interface. Natural-convection film boiling of this type includes film boiling on horizontal flat plates facing upward, long inclined flat plates, long vertical surfaces, and horizontal cylinders of large diameter. First, it is shown that saturated film boiling along vertical surfaces can be correlated to the capillary length, and the physical meaning of this feature is made clear by focussing attention on the vapor-film-unit length in the author's vapor-film-unit model. Next, using the unit length, a heat transfer correlation for saturated film boiling along long vertical surfaces is developed, and it is shown that this correlating equation is valid for other film boiling systems with a wavy interface if the unit length is given correctly. Finally, a heat transfer correlation for subcooled film boiling with a wavy interface is developed using the heat transfer correlation for subcooled film boiling with a smooth interface and the unit length.