Reference standards reduce variation in reported promoter activities under different measurement conditions. We measured the activity of 2 promoters, J23101 (white columns) and J23150 (grey columns) under seven different measurement conditions and measurement instruments. We varied the media, temperature, cell strain, and plasmid copy number of the promoter test construct. (A) To estimate the per cell GFP synthesis rate we reported the change in fluorescence over a 1 hour period in exponential phase divided by the average absorbance during this period. The coefficient of variation of the GFP synthesis rates across the seven measurement approaches was 49% for J23101 and 39% for J23150. (B) We used the same data and divided the GFP synthesis rate of J23150 (grey bars) by that of J23101 (white bars) in order to calculate the relative promoter activity of J23150 in RPUs. The coefficient of variation of the relative promoter activity across the seven measurement approaches was only 17% suggesting that the relative promoter activity is less sensitive to conditions then absolute activity measured by per cell GFP synthesis rate.