Double isotope dilution is a powerful methodology to measure accurately the ratio between two isotopes of two different elements (e.g.²³⁸Pu/²³⁸U, ¹⁴⁸Nd/²³⁸U or ²³⁰Th/²³⁸U ratios). To obtain the lowest possible uncertainty, some parameters must be optimized: the elaboration of the spike and the proportion of the spike in the sample–spike mixture. A piece of code is also provided to easily calculate the optimal parameters. As an example, the application of the code to ²³⁸Pu/²³⁸U and ¹⁴⁸Nd/²³⁸U ratio determination in an irradiated sample will be discussed.