This study introduces the "spline" functions with the purpose of testing their utility in estimating tree volume. The "spline" functions consists in a group of cubic equations that fittings to a series of points interpolate with exactness values between each point. These functions estimate the value of diameters to given heights or commercial volumes and heights to given diameters. This study presents the development of the "spline" functions and their application in estimating volume in Lenga (Nothofagus pumilio).