An automatic procedure for direct-dimension definition of stepped-impedance low-pass filters is presented. The procedure does not require any kind of iterative optimization, thus resulting in a very fast design. Two examples of rectangular waveguide low-pass filters are presented. The results are validated through mode matching and finite-element method analyses, showing excellent agreement. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 190–194, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20583