Abstract Archaeologists are increasingly in need of quantitative measures of stone artefact retouch. Existing techniques fail to provide a generic measure of retouching for all chipped stone artefacts. A fast and reliable index is proposed which measures retouch in terms of the invasiveness of flake scars on the surfaces of complete stone artefacts. Unlike other measures, the index of invasiveness is well suited to the analysis of bifacially worked artefacts such as bifacial points and bifaces. Experimental tests demonstrate a strong correlation between the index and measures of reduction based on diminishing flake weight and numbers of retouch blows. Inter-observer reliability is also demonstrated through the use of a blind test. Limitations of the method are discussed, although potential techniques for surmounting problems are identified. An archaeological application of the index demonstrates the utility of the method in the context of regional assemblage variability in northern Australia.