IllCAT is a python script to help with transforming between coordinates in NAD83-based Illinois legacy projections and coordinates in NAD83-based Illinois systems without NGS sanctioning or other extended support. Support for Illinois East and West, Pearson’s full-state projection, Hutson’s IDOT District 8 low distortion projections (LDPs), and all ICS83 LDPs are included by default. The Python code has been crafted to be extensible, meaning that adding further Transverse Mercator and Lambert Conformal Conic (1-parallel) projections is a relatively trivial procedure. A graphical user interface (IllcatGUI), has been developed to make interacting with the IllCAT script more intuitive. The IllCAT script should work with any operating system if Python version 3 or later is installed on that system. The IllcatGUI tool is only compatible with Microsoft Windows.