In this article, simple but efficient expressions for determining the dual‐frequencies of equilateral triangular compact microstrip antennas (ETCMA) with a shorting‐pin are presented. Two expressions related to offset factor functions were derived with the use of a differential evolution optimization algorithm. The dual‐frequencies of an ETCMA with a shorting‐pin were then calculated by multiplying the offset factor functions with the single resonant frequency formulation of a conventional equilateral triangular microstrip antenna. The proposed approach was demonstrated through successful comparison with simulated and measured data. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1227–1230, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27580