The equivalent radius \(\:{R}_{eq}\) has been used for calculation of strain in Resonant Column (RC) testing. Present calculation methods for \(\:{R}_{eq}\) are not adequate since they are not precise over a wide range of strain. In this study, a new equation of \(\:{R}_{eq}\) that can be used over a wide range of strain is proposed by developing a new method of determining \(\:{R}_{eq}\) based on a numerical integration technique using a theoretical modified hyperbolic model. The new method for obtaining optimum \(\:{R}_{eq}\) can be used for evaluating shear modulus more precisely in an RC test at any strain level. The prediction equation of \(\:{R}_{eq}\) was successfully applied to typical unbound granular materials such as the simulated lunar granular soils to get the shear modulus reduction curves. The Req values computed using the developed equation slope down sharply when the strain value varies from 0.1 to 0.001. The conventional equivalent radius approach could be inadequate at small strains less than 0.01. This means that for obtaining the best results from the RC testing, different values of Req must be used considering the range of shear strain.
