Calibrating process gas chromatographs (GCs) can be difficult because a wide range of concentration is usually required. Using gas mixtures as standards is cumbersome, costly, and unreliable because the gases may deteriorate during storage. In this paper, we describe a method using pure gases for the calibration of automatic GCs by varying and measuring precisely the pressure of the sample loop, thus sending known quantities of gas into the column. The concentration of the gas is calculated using the ideal gas law, and we are able to calibrate GCs in a range from 1000 ppm to 100 vol %.