Selecting a UI element is a fundamental operation on webpages, and the ease of tapping a target object has a significant impact on usability. It is thus important to analyze existing UIs in order to design better ones. However, tools proposed in previous studies cannot identify whether an element is tappable on modern webpages. In this study, we developed Tappy that can identify tappable UI elements on webpages and estimate the tap-success rate based on the element size. Our interviews of professional designers and engineers showed that Tappy helped discussions of UI design on the basis of its quantitative metric. Furthermore, we have launched this tool to be freely available to external users, so readers can access Tappy by visiting the website (https://tappy.yahoo.co.jp).