It is pointed out that Macey's formula, used in the CIBSE Guide to calculate the heat loss from a solid uninsulated ground floor slab, is unsymmetrical in its handling of the floor length and breadth L and B, and further that it implies the existence of a volume of perfectly insulating material embedded in the base of the wall (thickness W). Both flaws are avoided in a simple expression for heat loss which involves the floor dimensions grouped as LB/W( L + B). The expression provides an excellent approximation to an exact expression for heat loss due to Delsante, Stokes and Walsh.