A method for determining a stiffness of a tube bundle heat exchanger. The heat exchanger has a core tube and a plurality of coil tubes coiled around the core tube to form a tube bundle having a plurality of coil layers at a respective layer coiling angle. The method determines a geometric strength parameter for a coil layer, the strength parameter being an area ratio of a coil-tube cross-sectional area to a cell cross-sectional area resulting from the axial spacing of the coil tubes and an outer diameter of the coil tubes. The area ratio is corrected by a correction factor taking the orientation of the coil tubes of the coil layer in relation to the force of gravity acting on the coil tubes into consideration. The stiffness of the respective coil layer is determined from the corrected area ratio and a modulus of elasticity of the coil-tube material.