Conduit Fill Calculator

This conduit fill calculator checks how full a raceway is under NEC Chapter 9. Choose the conduit type and trade size, list your conductors, and see the fill percentage, the legal limit and the smallest size that works.

Shaded circle = total conductor area shown as one equivalent circle. Dashed circle = NEC maximum fill. Update by pressing Calculate.

Conduit type

Trade size

Conductors (include neutrals and grounds)

Estimates only. By using this calculator you agree to our Terms.

Uses NEC Chapter 9, Tables 1, 4 and 5. Fill is rounded up and remaining space is rounded down. ENT and flexible conduit are not included; use Table 4 directly for those.

How to Use This Conduit Fill Calculator

Pick the conduit type and trade size you plan to use. Then enter each group of conductors: insulation type, AWG or kcmil size, and quantity. Use the add button for each different size, and include neutrals and equipment grounds. Press Calculate Fill, or press Enter in the quantity box.

You get the fill percentage, the NEC limit for your conductor count, the remaining area, and a table showing the fill at every trade size for that conduit type. The smallest size that stays within the limit is marked with a star.

Why This Matters

Conduit fill limits protect two things: the insulation and your ability to pull the wire. Packed conductors trap heat, and the longer the pull, the more insulation damage you risk from friction and pulling tension. Undersized conduit is a frequent cause of failed rough-in inspections, and the fix after drywall is up is expensive.

The numbers surprise people. A single 4/0 AWG THHN conductor has an area of 0.3237 in². In ¾" EMT, which has 0.533 in² of interior area, that is about 60.7% fill. The limit for one conductor is 53%, so it fails. Moving to 1" EMT (0.864 in²) drops it to 37.5%, which passes. Two 4/0 THHN conductors are worse: the limit drops to 31%, and 1½" EMT comes in at 31.8%, just over. They need 2" EMT (19.3% fill).

Electricians use this check for feeders, motor circuits and home runs. Designers use it to size underground PVC runs. Homeowners running a subpanel feed to a detached garage use it to avoid buying the wrong conduit. In every case the question is the same: do the conductors fit within the percentage of conduit area the code allows?

How It's Calculated

Fill is the total cross-sectional area of all conductors, including insulation, divided by the interior cross-sectional area of the conduit:

Fill % = (Σ conductor area × quantity) ÷ conduit interior area × 100

Conductor areas come from NEC Chapter 9, Table 5. Conduit interior areas come from Table 4, which is based on the actual inside diameter of each conduit type. The same trade size has different interior areas in EMT, IMC, rigid, PVC Schedule 40 and Schedule 80, so type matters. The allowed percentage comes from Chapter 9, Table 1:

1 conductor: 53% of interior area
2 conductors: 31%
3 or more conductors: 40%

Max fill area = conduit area × limit % Remaining area = max fill area − conductor area

THW values for 14–8 AWG use the larger Table 5 areas, which is the conservative choice for TW as well. Always check Table 5 for your exact insulation type. This tool does not apply the 60% allowance for nipples of 24 in. or less, and it does not handle multiconductor cable, compact conductors or bare conductors.

Worked Example

Say you are pulling three 4 AWG THHN phase and neutral conductors plus one 8 AWG THHN ground in 1" EMT.

Three 4 AWG THHN: 3 × 0.0824 = 0.2472 in²
One 8 AWG THHN: 1 × 0.0366 = 0.0366 in²
Total conductor area: 0.2838 in² for 4 conductors, so the 40% limit applies
1" EMT interior area: 0.864 in². Max fill area: 0.864 × 0.40 = 0.3456 in²
Fill: 0.2838 ÷ 0.864 = 32.85%, shown rounded up as 32.9%
Remaining space: 0.3456 − 0.2838 = 0.0618 in²

The result is within the limit. Try ¾" EMT (0.533 in²) and the same conductors fill 53.3% (rounded up), well over 40%, so 1" is the smallest suitable EMT. This is exactly what the calculator shows as its defaults.

Tips & Common Mistakes

Count every conductor. Neutrals and equipment grounds count toward fill. Forgetting the ground is the classic way to land a few percent over.
Fill is not ampacity. Passing fill does not mean the wires are sized correctly. More than three current-carrying conductors in one raceway also triggers ampacity adjustment factors under NEC 310.15.
Match the insulation. THHN, XHHW and THW have different diameters at the same AWG, so using the wrong row can swing the answer noticeably.
Leave pulling room. A run at 38% with several bends can be much harder to pull than the same run at 28% and straight. Consider the next size up for long runs, and stay within bend limits (typically 360° between pull points).
When it is close, go up a size. A result a hair under the limit leaves no margin for a table or measurement error.

Frequently Asked Questions

What is the maximum conduit fill percentage allowed by the NEC?

NEC Chapter 9, Table 1 allows 53% for one conductor, 31% for two conductors, and 40% for three or more. Percentages apply to the conduit's interior cross-sectional area. Your local jurisdiction may adopt a different NEC edition or amend these rules.

Does the ground wire count toward conduit fill?

Yes. Equipment grounding conductors count toward fill, and so do neutrals. Add them as a row in the calculator along with your phase conductors. Bare grounds are measured differently in the NEC, but entering an insulated equivalent is a safe, slightly conservative approach.

Why does a conduit with two wires have a lower limit than one with three?

Two round conductors tend to lie side by side and can jam against the conduit wall during a pull, which is why the NEC allows only 31%. With three or more conductors the limit is 40%. This is why a 2-wire run sometimes needs a bigger conduit than a 3-wire run with the same conductors.

Can I mix different wire sizes and insulation types?

Yes. Add a row for each size and insulation type with its quantity. The calculator sums all areas and compares against one limit based on the total conductor count. The NEC does not restrict mixing sizes for fill purposes.

What is the difference between EMT and rigid conduit fill?

From ½" to 2", IMC and rigid (RMC) have slightly larger interior areas than EMT, but from 2½" up EMT is larger. PVC Schedule 40 and 80 are smaller still, with Schedule 80 having thicker walls and less interior space. Select the actual type you are installing.

Is a passing result the same as code approval?

No. It is a fill check only. Inspectors also look at ampacity, bend count, support, bushings, conduit type for the location and local amendments. Treat the result as a planning aid and confirm with your authority having jurisdiction.