Tool use extends people's representations of the immediately actionable space around them. Physical tools thereby become integrated in people's body schemas. We introduce a measure for tool extension in HCI by using a visual-tactile interference paradigm. In this paradigm, an index of tool extension is given by response time differences between crossmodally congruent and incongruent stimuli; tactile on the hand and visual on the tool. We use this measure to examine if and how findings on tool extension apply to interaction with computer-based tools. Our first experiment shows that touchpad and mouse both provide tool extension over a baseline condition without a tool. A second experiment shows a higher degree of tool extension for a realistic avatar hand compared to an abstract pointer for interaction in virtual reality. In sum, our measure can detect tool extension with computer-based tools and differentiate interfaces by their degree of extension. CCS CONCEPTS • Human-centered computing → HCI theory, concepts and models; HCI design and evaluation methods; Laboratory experiments; User studies; Empirical studies in HCI ;