The lengths of the hydrogen bond, d(-H) in inorganic and biochemical groups have been evaluated from the observed donor (X) to acceptor (Y) distances, d(XH...Y)(obs) using the covalent and phi-based ionic radii of X and Y, where phi is the Golden ratio. It is found that d(-H) = nd(H+) + md(H), where d(H) and d(H+) = 2d(H)/phi(2) = 0.28 angstrom are, respectively, the covalent and cationic radii, n = 1 or 2 and m = m(X) + m(Y) = 0, 1, 2 or 3. In all cases, d(XH...Y)(obs) = d(XH...Y)(cal) = d(X) + d(H) + d(-H) + d(Y). (c) 2006 Elsevier B.V. All rights reserved.