The laboratory method of determining aggregate stability introduced by Childs in 1940 is reconsidered. A convenient and quick method of interpreting the experimental results is presented and provides the structural index for a given soil treatment. The ratio of the structural indices for two soil treatments provides the stability ratio, which is of relevance to erosion and pedogenetic studies of structure. Illustrative results show that pretreatments such as air drying, grinding and sieving affect the structural index and stability ratio.