This article describes a Microsoft Excel macro designed to calculate PCR master mix amounts based on variations in the DNA template amounts added to each tube, the total number of PCR assay tubes being set up or both. This macro uses a dynamic dialog box to quickly calculate and display the new component volumes after changes in one or both of the variables. The PCR assay mix protocol can then be printed in a format suitable for record keeping. This macro was designed for use with the Windows version of Excel but will also run on Macintosh computers.