What fill percentage does the NEC allow?

NEC 2023 Chapter 9 Table 1 permits: 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. This calculator automatically applies the correct limit based on total conductor count.

Which conduit types are included?

EMT (Electrical Metallic Tubing), IMC (Intermediate Metal Conduit), and RMC (Rigid Metal Conduit). Each has different internal diameters, so the fill area differs even at the same trade size.

What conductor sizes are supported?

AWG 14 through 4/0, plus 250, 300, 350, 400, 500, 600, 700, 750, 800, 900 and 1000 kcmil, with areas for THHN/THWN-2 from NEC Chapter 9 Table 5.

Are these THHN/THWN values?

Yes. Conductor cross-section areas use NEC Table 5 values for THHN/THWN-2 insulation, the most common wire type. For other insulation types (XHHW, RHH, etc.) the area differs; consult NEC Table 5 for exact values.

Can I mix different AWG sizes in one conduit?

Yes. Add a row for each conductor size and enter the quantity. The calculator sums all conductor areas before computing the fill percentage.

Do I count the ground wire in conduit fill?

Yes. Chapter 9 Note 3 says equipment grounding and bonding conductors count toward fill, so add the ground as its own row with its size.

Can I round up the number of wires?

Sometimes. Chapter 9 Note 7 lets you use the next whole number when all wires are the same size and the count works out to a decimal of 0.8 or more. That is why Annex C lists 26 #12 THHN in 1 in EMT (25.98 by the 40% rule). This calculator applies Note 7 when every conductor is the same size, so 26 #12 THHN in 1 in EMT passes at 40.0% fill. Mixed sizes use the strict percentage.

Conduit Fill Calculator

Select conduit type and trade size, add conductors by AWG, and instantly see the fill percentage against NEC 2023 limits.

NEC Chapter 9 Table 1 limits conduit fill to 40% of the inside area for 3 or more conductors, 31% for 2 and 53% for 1.

Conduit Configuration

Conduit Type

Trade Size

Conductors

Fill Result

Configure conduit and add conductors, then click Calculate Fill.

NEC 2023 Chapter 9, Table 1 — Fill Limits

53%

1 conductor

31%

2 conductors

40%

3+ conductors

Quick answer

NEC Chapter 9 Table 1 limits conduit fill to 40% of the inside area for 3 or more conductors, 31% for 2 and 53% for 1. Add up the wire areas (Table 5), divide by the conduit area (Table 4) and compare.

Examples

Conduit	Wires (THHN)	Fill	Result
1/2 in EMT	9 × #12	39.4%	Pass
1/2 in EMT	10 × #12	43.8%	Fail
3/4 in EMT	16 × #12	39.9%	Pass
3/4 in EMT	10 × #10	39.6%	Pass
1 in EMT	16 × #10	39.1%	Pass

Add your wires and conduit.

Summary

This conduit fill calculator applies NEC 2023 Chapter 9 Table 1 limits to EMT, IMC, and RMC conduit types. Enter the trade size and add conductors by AWG size and quantity; the tool computes total conductor area, allowable fill area, and the resulting percentage as a planning estimate. Verify the adopted code edition and installation with the authority having jurisdiction or a qualified electrician.

How it works

Select the conduit type: EMT, IMC, or RMC.
Pick the trade size (½" through 4") from the dropdown.
Add each conductor group: choose AWG size and enter the quantity.
The calculator sums the total conductor area and divides by the conduit internal area.
Compare the result against NEC 2023 Chapter 9 Table 1 limits (40% for 3+ conductors, 31% for 2, 53% for 1). When all conductors are the same size, Note 7 rounds a calculated count with a decimal of 0.8 or more up to the next whole number.
Adjust conductor count or trade size until the fill is within the allowed limit.

Use cases

Verify a panel feeder conduit is not over-filled before pulling wire.
Size a new conduit run for a given set of branch circuit conductors.
Check compliance when adding circuits to an existing conduit.
Quickly compare EMT vs. RMC fill for the same conductor bundle.
Confirm a homerun conduit meets NEC 40% fill rule before rough-in.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu