Conduit & duct fill calculator

Fill is by cross-sectional area, not diameter. Take the area of one cable (pi times half its outside diameter, squared), multiply by the number of cables, and divide by the inside area of the conduit. For same-size cables it simplifies to the number of cables times the square of the cable OD divided by the conduit ID. The calculator above does this and compares it to the code limit.

NEC Chapter 9, Table 1 sets the ceiling by cable count: 53 percent for a single cable, 31 percent for two, and 40 percent for three or more. Communications and fiber work often uses the same 40 percent figure as a rule of thumb. Staying under the limit leaves room to actually pull the cable without damaging it.

Jam ratio is the conduit inside diameter divided by the cable outside diameter, and it only matters when you pull exactly three same-size cables. When that ratio lands between about 2.8 and 3.2, the three cables can wedge into a triangle that locks against the wall and jams the pull. Keep the ratio clear of that band, or pull a different count.

It depends on both diameters. Divide the code fill fraction by the square of the cable OD over the conduit ID to get the count. For example half-inch cable in a two-inch (about 2.05 inch ID) conduit fills about 6 percent each, so six fit inside the 40 percent limit. The calculator shows the most that fit for your sizes.

Always the inner diameter, the actual bore. Trade size is a nominal label, and the real inside diameter changes with the wall: a Schedule 80 pipe has a smaller bore than Schedule 40 of the same trade size, and HDPE by SDR changes again. Using the trade size overstates the space and can push a pull over the real limit.