The Mott-Hubbard gap U and the charge-transfer gap \ensuremath{\Delta} of solid NiO are estimated from ab initio calculations on the ${\mathrm{NiO}}_{6}^{10\mathrm{\ensuremath{-}}}$ cluster. Covalency in the essentially localized ${d}^{n}$ states and localization for the spatially extended O(2p) hole states are introduced by means of a limited configuration-interaction calculation. The localized states induce a large polarization effect in the bulk, accounted for in a semiempirical way. The values obtained for U and \ensuremath{\Delta} are quite similar and in the range of 4.4--5.2 eV, in good agreement with the observed gap.