Milling is one of the most frequently used manufacturing processes. End milling cutters, which are available in a wide variety of designs and sizes, are used very frequently due to their universal applicability and low manufacturing costs. During NC programming and the associated path planning, the tool to be used for machining a specific area is determined. The choice of tools plays a crucial role here. In particular, their diameter has a significant influence on the efficiency of processing. In order to choose the best tool, it is necessary to know which tool can be used for machining a specific part of the differential volume. The geometric accessibility sets an upper limit for the tool diameter. This paper presents an algorithm for calculating the largest possible tool diameter for end mills for any point within concave polygons. The heuristic determines the largest possible tool diameter and the associated position of the tool guide point of the end mill within an adjustable error corridor. The algorithm allows for the efficient calculation of the distribution of the largest possible tool diameters for any arbitrarily shaped 3-axis milling feature. Possible areas of application are optimization of tool usage, automated tool selection, feature recognition, automated NC programming and automated process planning.