We studied the diffusion rate of stable isotopes of water ice in a controlled laboratory experiment, where isotopically different layers of fabricated firn were stacked. This experiment was done in order to study the diffusion rate between the isotopically different layers and to compare the measurements with forward-calculated diffusion rates of the fabricated firn stack, using three different analytical and numerical methods. The layers varied in thickness within the stack, and the studied period includes variations in firn temperature. We sampled the firn stack on five different dates along the 144 day long experiment. We obtained a quantitatively good match between the calculated and measured diffusion rates, with a root-mean-square similarity of 67–80% in the depth-averaged diffusion rates. Despite the good average match, we found some qualitative discrepancy between our calculations and our measurements. The calculated diffusion rates predict >5% faster isotopic decay of thinner layers and 13% slower decay of thicker layers than our measurements show. We speculate that grain size and grain fabric may influence the tortuosity of the firn pack, and that the current generalization of tortuosity, being a function of firn density as the only free parameter, is an oversimplification.