A free conduit fill calculator built on NEC Chapter 9 Table 1 (53% / 31% / 40% rules) and Table 5 (conductor cross-sectional areas). Handles EMT, PVC Schedule 40, RMC, and IMC across every common trade size.
Where the calculator lives. Conduit fill is the fourth tab of the full Wire & Cable calculator. We kept them together because most people checking conduit fill also need to confirm the conductor sizes that go in. Open the calculator, then click the “Conduit Fill” tab.
Open the conduit fill calculatorEach conduit type has a different internal area at the same trade size. PVC Schedule 40 typically has the largest internal area, followed closely by EMT, then RMC, then IMC. The differences are small (within a few percent) but they matter when a pull is borderline. The calculator automatically pulls the right NEC Chapter 9 Table 4 row.
Common rule of thumb: at 3/4" trade size, you can fit roughly 9 12-AWG THHN conductors at the 40% fill rule across PVC, EMT, RMC, and IMC. Larger conduits widen the differences. For mixed-size pulls, the calculator handles the math directly — you specify each wire size and quantity, and it returns the percent fill.
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped raceway design package from a working engineer — typically within 48 hours of receiving your one-line and load data.
NEC Chapter 9 Table 1 caps fill at 53% for one conductor, 31% for two conductors, and 40% for three or more. The percentages are based on the conduit's internal cross-sectional area minus the conductors' total cross-sectional area (NEC Chapter 9 Table 5 for THHN/THWN areas).
Heat dissipation, jam-resistant pulls, and physical clearance all change with conductor count. With one conductor, more space is available. With two conductors, alignment is harder and jams are more likely, so fill drops to 31%. With three or more, the conductors lay together and the 40% rule applies.
Slightly. PVC, EMT, RMC, and IMC all have different internal cross-sectional areas at the same trade size. NEC Chapter 9 Table 4 lists each. The calculator picks the right table automatically based on conduit type selection.