Equations, in the IEEE Standard, for finding the coefficients of piezoelectric materials are based on one-dimensional models of slabs, rods, and discs. In the Standard, recommendations are also made for the sizes of samples to be used for characterization. The errors in the coefficients of samples determined using the Standard are presented here. The errors are, in effect, determined by using three-dimensional finite element models to determine the coefficients and comparing them with those determined using the Standard equations. It is difficult to find the piezoelectric coefficients using a three-dimensional model and measured quantities such as the input electrical admittance. Therefore, the quantities that are required as input to the Standard equations are first computed using a set of piezoelectric coefficients and a three-dimensional finite element code, ATILA. These quantities are then used to find the coefficients. The difference between the coefficients input to ATILA and those obtained from the Standard equations is the error. The error is computed for slabs, rods, and discs of various sizes. Some sizes satisfy the Standard recommendations but most do not. The error is small in several cases. This is of significance for transducer developers who use samples of non-Standard size.