Issues relating to comfort, like arm fatigue, are well-known in prolonged mid-air interaction with 3D user interfaces (UIs), common in Virtual and Mixed Reality. Solving this problem is hard, due to the lack of tools and guidelines available for design. To address this, we propose a method to estimate the ergonomic cost of interaction at each reachable position in the user’s environment, using existing metrics. We present XRgonomics, a toolkit for visualizing and utilizing the ergonomic cost in real-time, allowing creators to identify UI positions that optimize users’ comfort. We present results from a walkthrough demonstration, which highlight the potential of XRgonomics for making such metrics accessible during the design and development of 3D UIs. We demonstrate scenarios where our toolkit supports the placement of static UI elements and dynamic adaptation of UIs based on spatial constraints. Finally, we discuss its potential to address design goals beyond ergonomics.