We present an experimental method to obtain the effective mass of an unloaded oscillating spring. We measure the period T(n) of the partial springs that result when hanging n of the total N coils of a given spring. Data are correlated with the expectation of a simple model for T(n) that takes into account the effective mass of the hanging spring. We analyze results for metal and plastic springs of different masses and strengths, and conclude that the method provides the effective mass with a good degree of reliability.