Quick Answer: For most telecom OSP fiber builds, design to a 40% fill ratio — cable cross-sectional area should occupy no more than 40% of the conduit's inner bore area. The Input Parameters table contains cable and conduit parameters that may be selected with the exception of Cable Area. The selected values are used to populate the two lower tables that have standard values. Once the fill ratio calculator is computed, the program tells you if it falls within Corning's. This calculator will allow you to find the fill ratio using one, two, or three cables within the conduit. Then, under Conduit Size, select the size of your conduit and hit.