An approach for measuring energy of cosmic-ray particles with energies E > 1012 eV using an ultrathin calorimeter is presented. The method is based on the analysis of the correlation dependence of the cascade size on the rate of development of the cascade process. In order to determine the primary energy, measurements are made based on the number of secondary particles in the cascade, N e , at two observation levels Z 1 and Z 2, separated by an absorber layer. Based on the obtained measurements, a correlation analysis of the dependence of logN e (Z 1) on th e difference d N = logN e (Z 1) − logN e (Z 2) is carried out. The correlation curves (logN e from d N) in the negative part of the dN axis are almost parallel to each other and practically do not depend on the depth of the cascade development. It makes it possible to determine the primary energy using an ultrathin calorimeter. The best option for applying the method is a calorimeter, which has a unit with a heavy target, leading to the rapid development of the cascade, and a homogeneous measuring and absorption block.