Standard Dimension Ratio
The Dimension Ratio, DR, is the ratio of the pipe outside diameter to the pipe minimum wall thickness. As diameters change, the pressure rating remains the same for the same material, dimension ratio and application. Certain DRs that meet an ASTM International specified number series (32.5, 26, 21, 17, 13.5, 11, 9 and 7.3) are referred to as Standard Dimension Ratios, SDR. There is approximately 25% difference in minimum wall thickness between each SDR.