This paper describes flexurba, a software library written in R, with the first open reconstruction of the Degree of Urbanisation algorithm to classify cities, towns, and rural areas. The R package offers enhanced flexibility and facilitates constructing alternative versions of the Degree of Urbanisation classification by customising parameters such as the minimum population size required for a city, and more ‘hidden’ implementation details including the contiguity rules and edge smoothing procedures. To illustrate how the package can be employed, we briefly demonstrate the grid classification and spatial units classification for Belgium. In addition, we compare results generated by the flexurba package with the official classification and discuss potential use cases. The package enables a broad range of analyses beyond the Degree of Urbanisation’s original application, including evaluating alternative urban delineations, sensitivity analyses, and comparative research.