The GUM Tree Calculator (GTC) is a data processing tool with full support for uncertainty calculations involving real and complex quantities. GTC is a stand-alone executable for Windows. It can handle instructions typedat the command prompt or evaluate text files containing instructions. It is closely integrated with the Windows Explorer for ease of use and does not depend on any other software. GTC can be used on small problems, or as a sophisticated data processing tool for more demanding tasks. Instructions use the Python programming language. An integrated development environment, with syntax highlighting and on-line help, is distributed with the calculator. For more information, visit http://www.uncertainnumbers.com/.