Kramers-Kronig transforms (KKT) constitute a powerful tool to validate experimental impedance data. A new algorithm, containing a Fast Fourier Transform (FFT) sub-routine, for the numerical calculus of the KKT has been developed. This algorithm is based in the convolution theorem and it uses only two FFT calculi. It was finally tested in the validation of an experimental transfer function for hydrogen permeation into an iron membrane. Unlike common algorithms for numerical integration, this algorithm allows the calculation of all frequencies at the same time at a high speed using a regular frequency interval for the whole calculation with the same or even higher precision. The FFT sub-routine is accessible in all languages so that the use of this code is very simple.