A new formula is given according to which it is possible to calculate the volume V of a block of a glass containing one gram atom of O=ions. Over a very wide range V is independent of the number and the nature of the metallic ions present. The two constants occurring in the formula have a well defined physical meaning, which can be understood satisfactorily on the basis of the well‐known theory of the structure of glass by Zachariasen and Warren c.s. and its extensions by Dietzel. The limits of the validity of the formula are also discussed on the basis of Zachariasen's theory. Beside giving much new information concerning the structure of glasses in general, the formula can be transformed very easily into a simple density formula for glasses. This formula has been tested for 44 arbitrarily chosen glasses and it is found that the accuracy of the formula is the same as of the earlier purely empirical formulae of Winkelmann and Schott, English and Turner etc.