How do you calculate conduit fill?

Add up the area of every wire, including the equipment ground, using NEC Chapter 9 Table 5 for insulated wire and Table 8 for bare wire. Divide by the conduit's internal area from Table 4 and compare the percent with the Table 1 limit: 53% for one wire, 31% for two, 40% for three or more. Example: three 6 AWG THHN (0.0507 in² each) and one 10 AWG THHN ground (0.0211 in²) are 0.1732 in². In 3/4 in EMT (0.533 in²) that is 32.5%, under 40%. In 1/2 in EMT (0.304 in²) it is 57%, too full.

What is the 40% rule for conduit fill?

NEC Chapter 9, Table 1 limits the wires in a conduit to 40% of its internal area when there are more than two. One wire may fill 53% and two wires 31% (two round wires jam more easily). A nipple 24 inches or shorter between boxes may be filled to 60% (Note 4). When every wire is the same size and type, Note 7 lets you round up to the next whole wire if the count works out to a decimal of 0.8 or more.

How many 12 AWG THHN wires fit in 3/4 inch EMT?

Sixteen. 3/4 in EMT has 0.533 in² inside, so 40% is 0.213 in², and one 12 AWG THHN is 0.0133 in²: 0.2132 ÷ 0.0133 = 16. In 1/2 in EMT the answer is 9, and in 1 in EMT 26. These match NEC Annex C, Table C.1, which lists the same counts for wires that are all one size.

How do you calculate box fill?

Count conductor volumes under NEC 314.16(B) and multiply by the cubic inches per conductor in Table 314.16(B): 2.00 for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. Each wire entering the box counts one; internal cable clamps count one; each device yoke counts two; up to four grounds count one, plus a quarter for each ground over four. Six 12 AWG wires, clamps, one receptacle and three grounds are 10 volumes, 22.5 in³, which fits a 4 × 2-1/8 in square box (30.3 in³).

Conduit Fill Calculator

Check how many wires fit in EMT, PVC, rigid or flexible conduit by trade size, with the NEC 40% fill rule, and the box fill count for a device box. Add each group of wires with its insulation and size to get the percent fill, pass or fail at the NEC Chapter 9 limits, the smallest trade size that passes and the most of one wire that fits. Based on the 2023 NEC. Local amendments and the authority having jurisdiction (AHJ) govern.

Your numbers

Conduit type

Trade size

Nipple, 24 in or shorter

Allows 60% fill (Chapter 9, Note 4).

Wires in the conduit

Row 1

Number of wires

Size

Insulation

Up to 6 rows, one per wire size and insulation. Count the ground wire too.

Conduit fill

–

NEC Chapter 9, Table 1: –
Wire area: –
Conductors: –
Conduit area (100%): 0.533 in², 3/4 in EMT
Smallest EMT that passes: –
Most of one wire alone: –

Based on the 2023 NEC. Local amendments and the authority having jurisdiction (AHJ) govern.

Include the equipment grounding conductor in the wire rows (Chapter 9, Note 3). For a single wire size, NEC Annex C lists the same counts.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How the conduit fill is calculated

Each wire's area comes from NEC Chapter 9: Table 5 for insulated conductors (THHN/THWN-2, XHHW-2, THW) and Table 8 for bare stranded conductors. The calculator multiplies each area by the count, adds the rows, and divides by the internal area of the conduit from Table 4, which is listed separately for EMT, rigid metal conduit, IMC, PVC Schedule 40 and 80, flexible metal conduit and liquidtight flexible metal conduit. Equipment grounding conductors count toward the fill (Chapter 9, Note 3).

Worked example: three 6 AWG THHN and a 10 AWG THHN ground. 3 × 0.0507 + 0.0211 = 0.1732 in². That is four conductors, so the limit is 40%. 3/4 in EMT has 0.533 in², so the fill is 32.5% and it passes. 1/2 in EMT (0.304 in²) would be 57%. In PVC Schedule 80, whose thicker wall leaves less room, 3/4 in (0.409 in²) is 42.3% and fails, so the smallest Schedule 80 that passes is 1 in.

The maximum-count line works the way NEC Annex C does: 40% of the conduit area divided by one wire's area, rounded down, except that a decimal of 0.8 or more rounds up (Note 7). For a single wire size the Annex C tables give the same answers without any arithmetic; the calculator is for mixed sizes and for checking a table you have on site.

Why 53%, 31% and 40%

The limits in Chapter 9, Table 1 leave room to pull the wires without damaging the insulation and to let heat escape. One conductor may take 53% of the area, two conductors 31% (two round wires in a round pipe can wedge across the diameter), and three or more 40%. A nipple 24 inches long or less between enclosures may be filled to 60% (Note 4), and the ampacity adjustment for more than three current-carrying conductors does not apply to it (310.15(C)(1)).

Fill is a maximum, not a target. Long pulls, several bends and large conductors pull more easily well below 40%. And the more current-carrying conductors share a conduit, the more their ampacity must be reduced under 310.15(C)(1): the wire size may change before the conduit does.

Box fill under NEC 314.16

Box fill counts conductor volumes, each worth the cubic inches in Table 314.16(B) for its size: 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00 and 6 AWG 5.00. Every conductor that enters the box and is spliced or terminated counts one, and so does one that passes straight through; an unbroken loop at least twice the free length in 300.14 (12 inches) counts two. Pigtails that start and end inside the box do not count.

Then add: one volume for internal cable clamps (whatever their number), one for each type of support fitting such as a fixture stud or hickey, both at the largest conductor in the box; two for each device yoke, at the largest conductor connected to that device; and one for up to four equipment grounding conductors, plus a quarter volume for each one over four, at the largest ground. Compare the total with the box volume from Table 314.16(A), or the volume marked inside a nonmetallic box, plus the volume marked on a plaster ring, extension ring or raised cover.

What the calculator does not cover

Compact conductors (Chapter 9, Table 5A), multiconductor cables pulled into a conduit (counted as one conductor of their outside diameter, Note 9), 3/8 in flexible conduit (its own tables in 348.22 and 350.22) and other raceways such as ENT, LFNC and HDPE are not included. Use the manufacturer's diameter for any wire whose insulation differs from the Table 5 types.

The result checks fill only. Ampacity adjustment for bundled conductors, the conductor's temperature rating and the total degrees of bends between pull points (360° under 358.26 and the matching sections) are separate rules. The calculator follows the 2023 NEC; local amendments and the authority having jurisdiction decide what passes inspection.

For electricians and estimators on a walk-through

On a walk-through the circuit list grows in conversation: two more 20 A circuits for the shop, a 10 AWG ground, a separate run for the charger, PVC underground and EMT inside. Each change can move a run up a trade size, and the fittings, straps and labor follow.

Run each homerun here while the customer is still talking, so the takeoff and the quote use the conduit sizes the job actually needs.

Based on the 2023 NEC. Local amendments and the authority having jurisdiction (AHJ) govern. Check the insulation printed on the wire and the volume marked on the box before relying on the result.

Sources, read on 2026-10-24:

Choosing the wire first? The wire size calculator checks ampacity and voltage drop together, and the voltage drop calculator checks a long run.

The circuit list changes on the walk-through. Keep it.

Homeruns, grounds and conduit runs get decided out loud: two more 20 A circuits, PVC underground, a 10 AWG ground for the charger. Record the walk-through in ParrotNotes and get a structured note, a summary and the action items, so the takeoff matches what the customer asked for. Free for 100 minutes of recording a month.

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