Estimating the sunny crown volume or surface area requires the sunny crown length, as well as the crown profile defined as a change in crown radius from the tree apex toward the ground, because sunny crown volume or surface area can be obtained by rotating the crown profile from the apex to the base of the sunny crown. In this study, we presented a model to estimate sunny crown length in even-aged closed hinoki cypress (Chamaecyparis obtusa ENDL.) stands. The model requires only three attributes: stand density, total height, and height increment of individual trees. To evaluate the model, it was applied to a 36-year-old hinoki cypress stand in which three plots of various stand density were established. The model provided accurate and precise sunny crown length estimates. Combining the model with a total height growth model allows us to estimate sunny crown length because stand density is generally a known variable.