Levelling operations are important during the machine tool installation. For more efficient leveling operations, a model which enables the quantitative evaluation of levelling errors is proposed and verified in this paper. This model considers supports as simple linear springs. The relationships between free length change and preload change is expressed as a matrix. Then, a verification experiment is conducted on a three axis machining center. The result suggests that the free length errors of supports can be obtained from the preload changes using the matrix.