Abstract Values of ionic radii are determined using the additivity rule and experimental information on X‐ray diffraction and electron density measurements for NaCl crystals. The calculated values of ionic radii are found to produce the nearest‐neighbour interionic separations close to the experimental values in I–VII, II–VI, and III–V compounds. In order to demonstrate the suitability of the calculated radii the interionic distances in tri‐atomic compounds are also predicted. The second‐neighbour overlap repulsive interactions and electronegativity differences are discussed in terms of ionic radii. The calculated radii are found to correlate very well with the electronegativity differences.