We present an easy way to calibrate the simple plasma conductivity (PCo) technique for measuring electron plasma densities in gases. We show that calibration can be achieved using a single absolute plasma density measurement through an independent analytical technique, in our case the in-line holographic microscopy (i-HOM). We show the validity and power of the method by comparing the calibrated PCo with results from i-HOM over an extended range of experimental parameters. (C) 2011 Optical Society of America