An expression for the volume of a general hexahedron has been obtained. The faces of the hexahedron had first to be defined as doubly-ruled surfaces, but once that was done the actual volume, for a given set of eight corner points, is unique. Hexahedral cells so defined have the practical merit that they can be fitted together to fill the whole space without any consideration of matching of diagonals of the face quadrilaterals. Moreover the cell faces are completely smooth. The expression obtained is longer than that of W. Kordulla and M. Vinokur, but it need be evaluated only once for each cell in a finite volume method and the result stored. The penalty for evaluating the longer expression is tolerable when it is set against the advantage of the ease of fitting the cells together.
