The structural data on compounds which crystallize with the (cubic) pyrochlore structure have been analyzed to obtain a model equation that predicts their lattice constants. The model, which is based on the ionic radii and the electronegativities of the constituting ions, is applied to simple ternary and to more complicated mixed cation and anion pyrochlores. The average error between the experimental and calculated lattice constants is similar to 0.45% for the ternary pyrochlores and similar to 0.68% for the mixed compounds. The lattice constant of any new pyrochlore can be estimated from the model equation, which has been derived in this work.