Ab initio calculations give, with an accuracy depending on the sophistication of the method, a bond length as an equilibrium value, r(e). The experimental bond lengths are always vibrationally averaged and may be expressed in different ways (r(g), r(z), r(a), etc.). Since high-quality ab initio calculations now are capable of giving bond lengths that are approximately of experimental accuracy, it is important to be able to interconvert these values. We find that the bond lengths optimized at the TZ2P+f CCSD level may be considered as the converged r(e) values and that the MM3 and MM4 force fields successfully convert r(g) to r(e) values. We also evaluated the performance of quantum mechanics at the 6-31G* MP2 and the 6-31G* B3LYP levels and found that the bond lengths (r(e)) at the 6-31G* B3LYP level are better than these at the 6-31G* MP2 level for molecules with only first-row atoms. However, the bond lengths for the bonds involving second-row atoms are too long at the 6-31G* B3LYP level, and for these, the 6-31G* MP2 level is recommended. An empirical formula is given for the conversion of the theoretical r(e) values calculated at these levels to the r(g) values.