Conduit Fill Calculator

Check NEC conduit fill compliance: calculate fill percentage, compare to code limits, and find maximum conductor count.

Conduit Trade Size

NEC Chapter 9: 1 conductor = 53% fill; 2 conductors = 31%; 3+ conductors = 40% max fill

Conductor Size (AWG)

AWG

Number of Conductors

conductors

NEC Table 1: 1 conductor = 53% fill; 2 = 31%; 3+ = 40% max of conduit inside area

Conductor Type

(1=THHN, 2=XHHW)

THHN #12: 3.96 mm OD; XHHW #12: 4.50 mm OD per NEC Chapter 9 Table 5

Tab to move between fields

Fill Percentage

9.1%

NEC Compliant?

1 (1=yes, 0=no)

Conduit Internal Area387 mm²

Area per Conductor11.68 mm²

Total Fill Area35.04 mm²

Max Conductors Allowed13 conductors

How to Use This Calculator

Select the Conduit Trade Size in mm: 16 mm = 1/2", 21 mm = 3/4", 27 mm = 1", 35 mm = 1-1/4", 53 mm = 2".
Enter the Conductor Size in AWG and the Conductor Type (THHN or XHHW) — conductor OD varies by insulation type.
Enter the Number of Conductors to be pulled through the conduit.
Review the Fill Percentage — NEC Chapter 9 allows 40% fill for 3+ conductors, 31% for 2, and 53% for 1 conductor.
Check the NEC Compliant? output — a value of 1 means the fill is within limits; 0 means you must upsize the conduit.
Note the Max Conductors Allowed to quickly assess how many more wires could be added to an existing conduit.

How the result changes with Number of Conductors

Conduit Trade Size: Conduit trade size per NEC Chapter 9 Tables. Common: 16 (1/2"), 21 (3/4"), 27 (1"), 35 (1-1/4"), 41 (1-1/2"), 53 (2"). Fill limits per NEC Table C1–C12 based on conduit type.
Conductor Size (AWG): AWG wire size: 14, 12, 10, 8, 6, 4, 3, 2, 1, 0. Smaller number = larger wire.
Number of Conductors: Total number of conductors to be installed in the conduit. NEC Chapter 9 Table 1 limits fill to 40% for 3+ conductors to ensure proper heat dissipation and installation ease.
Conductor Type: Conductor insulation type per NEC Article 310. THHN and XHHW have different outer diameters per NEC Chapter 9 Table 5, which directly affects conduit fill calculations.

Identify Input Parameters
4 parameters
Conduit Trade Size = 27, Conductor Size (AWG) = 12, Number of Conductors = 3, Conductor Type = 1 = 4 input(s) provided
Calculate Fill Percentage
Fill Percentage
9.1 = 9.1
Calculate NEC Compliant?
NEC Compliant?
1 = 1
Calculate Conduit Internal Area
Conduit Internal Area
387 = 387
Calculate Area per Conductor
11.68 = 11.68