Conduit Fill Calculator
Instantly calculate NEC-compliant conduit fill, check if your selected conduit size passes, and find the minimum conduit size for any wire combination.
How to Use This Calculator
- Select the wire gauge — choose the AWG size of your conductors. Wire areas are based on THHN/THWN insulation (NEC Chapter 9, Table 5).
- Enter the number of conductors — include all wires: hot, neutral, and ground. Each counts toward fill.
- Choose your conduit type — EMT is the most common for indoor work. PVC and RMC have different interior areas.
- Select a conduit size to check — the calculator shows the fill percentage and whether it passes NEC limits, then recommends the minimum adequate size.
NEC Conduit Fill Calculation
- Step 1 — Total Wire Area:
Total Wire Area = Wire Area (sq in) × Number of Conductors - Step 2 — NEC Fill Limit:
1 wire = 53%, 2 wires = 31%, 3+ wires = 40% of conduit interior area - Step 3 — Available Fill:
Available Fill = Conduit Interior Area × Fill Percent - Step 4 — Pass / Fail:
Pass if Total Wire Area ≤ Available Fill - Step 5 — Minimum Size:
Find smallest conduit where Total Wire Area ≤ Interior Area × Fill %
Wire cross-section areas are from NEC Chapter 9, Table 5 (THHN/THWN-2). Conduit interior areas are from NEC Chapter 9, Table 4. Fill percentages are from NEC Chapter 9, Table 1.
Pro Tips
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Conduit & Pulling Tools
Southwire 1/2" EMT Conduit (10-ft)
Standard EMT for indoor wiring runs
View on Amazon →Southwire 3/4" EMT Conduit (10-ft)
Most versatile size for mixed circuits
View on Amazon →Klein Tools Fish Tape (50 ft)
Pull wire through conduit runs
View on Amazon →Ideal Industries Wire Pulling Lubricant
Reduce friction when pulling multiple conductors
View on Amazon →Frequently Asked Questions
What is the NEC conduit fill rule?
NEC Chapter 9, Table 1 limits how much of a conduit's interior area can be occupied by conductors. For one conductor the limit is 53% of the interior area, for two conductors it is 31%, and for three or more conductors it is 40%. These limits ensure wires can be pulled without damage and allow heat from current flow to dissipate safely.
Does conduit type affect how many wires fit?
Yes. EMT (Electrical Metallic Tubing), PVC Schedule 40, PVC Schedule 80, and RMC (Rigid Metal Conduit) all have different interior diameters for the same nominal trade size. EMT has the largest interior area for a given trade size, meaning it fits the most wire. PVC Schedule 80 has the smallest interior area. This calculator uses EMT dimensions as the reference.
Can I run 4 wires in 1/2" EMT?
It depends on the wire gauge. Four 14 AWG THHN conductors have a total area of 4 × 0.0097 = 0.0388 sq in. The 40% fill limit for 1/2" EMT (0.304 sq in interior area) is 0.1216 sq in. Since 0.0388 < 0.1216, four 14 AWG wires fit in 1/2" EMT. However, four 12 AWG conductors (0.0532 sq in) also fit, but four 10 AWG (0.0844 sq in) require 3/4" EMT.
Do ground wires count toward conduit fill?
Yes, equipment grounding conductors (EGC) must be counted toward conduit fill. All conductors inside a conduit — including grounds — occupy space and generate heat. Include your ground wire in the total conductor count for an accurate fill calculation.