Conduit Fill Calculator

Conduit fill calculator for THHN in EMT or PVC, with mixed wire sizes and the 60% nipple rule. Get fill percent, how many wires fit, and the smallest conduit.

By CalcBuilt Editorial TeamUpdated September 13, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Conduit fill is the total conductor area divided by the inside area of the conduit, limited to 40% for three or more conductors, 31% for two and 53% for one. Four 12 AWG THHN conductors in 3/4 in EMT use 0.0532 sq in of a 0.533 sq in raceway, a 9.98% fill, and that conduit holds up to 16 of them.

Your measurements

A column or tube: volume is the circular area (from the diameter) times the height.

Conduit typeEMT indoors and in dry exposed runs; Schedule 40 PVC underground and outdoors. Where the riser must be Schedule 80 (NEC 352.10(F)) the inside area is smaller than the Schedule 40 numbers used here.Conduit trade sizeWire size (THHN / THWN)Stock building wire is dual-rated THHN/THWN-2 and uses these areas. XHHW-2 and THW are thicker (Table 5) and hold fewer per conduit.Number of conductorsCount every conductor in the raceway, including the neutral and the equipment grounding conductor. Fill is one limit; more than 3 current-carrying conductors also derates ampacity, and 9 is the practical cap for 12 AWG on 20 A breakers.Second wire size (optional)Real runs mix sizes. A 100 A feeder is three 3 AWG conductors with an 8 AWG ground, and Chapter 9 Table 1 is applied to the total area of all of them.How many of the second sizeLeave at 0 for a single-size pull. Set it to size a feeder with its own equipment grounding conductor.Third wire size (optional)A third size covers a run that carries a separate control or neutral conductor alongside the feeder.How many of the third sizeLeave at 0 unless the raceway carries three different conductor sizes.Is this a nippleNEC Chapter 9 Note 4 allows 60% fill in a raceway 24 in or shorter between boxes or cabinets, and the ampacity derating of 310.15(C)(1) does not apply to it.

Results update as you type. The link in your address bar saves these entries.

Conduit fill9.98%All results

Results

Conduit fill: 9.98%; OK: within the 40% limit for three or more conductors (4 x 12 AWG THHN in 3/4 in EMT)
Conductor area used: 0.0532 sq in; Allowed 0.213 sq in, which is 40% of the 0.533 sq in inside area
Maximum 12 AWG THHN in 3/4 in EMT: 16 conductors; NEC Chapter 9 Note 7: with all conductors the same size, a fraction of 0.8 or more rounds up
Smallest EMT for 4 x 12 AWG: 1/2 in EMT; Holds up to 9 of this size
Planning estimate only: NEC Chapter 9 Tables 1, 4 and 5; Assumes THHN/THWN conductors and covers EMT and Schedule 40 PVC only; RMC, IMC and Schedule 80 have their own Table 4 areas. If all 4 are current-carrying, ampacity derates to 80% of the 90°C column (Table 310.15(C)(1)): 24 A for 12 AWG THHN. A licensed electrician and local code govern

Conduit Fill formula

Conductor area = Sum over every wire size of Count x Area per conductor (Table 5)   (12 AWG THHN = 0.0133 sq in)
Fill % = Conductor area / Conduit inside area (Table 4) x 100
Limit = 53% for 1 conductor, 31% for 2, 40% for 3 or more (Table 1)
Nipple 24 in or shorter: the limit is a flat 60% and ampacity is not derated (Note 4)
Maximum conductors = Allowed area / Area per conductor, rounded down unless the fraction is 0.8 or more (Note 7, one size only)

Worked example

Eight 8 AWG THHN conductors in 3/4 in EMT: 8 x 0.0366 = 0.2928 sq in. The 3/4 in EMT inside area is 0.533 sq in, so the fill is 0.2928 / 0.533 = 54.9%, well over the 40% limit for three or more conductors. Step up to 1 in EMT: the 40% area is 0.346 sq in, and 0.346 / 0.0366 = 9.45, so nine 8 AWG conductors fit and eight of them fill 0.2928 / 0.864 = 33.9%.

Two 4/0 THHN conductors in 1-1/2 in EMT: 2 x 0.3237 = 0.6474 sq in, or 0.6474 / 2.036 = 31.8% of the inside area. The limit for exactly two conductors is 31%, so they do not fit; 2 in EMT at 0.6474 / 3.356 = 19.3% does. Add a third 4/0 and the limit rises to 40%, so three of them (0.9711 sq in, 47.7%) also need 2 in.

Quick reference

Maximum THHN/THWN conductors of one size in EMT (NEC Chapter 9, 40% fill)
EMT size	14 AWG	12 AWG	10 AWG	8 AWG
1/2 in	12	9	5	3
3/4 in	22	16	10	6
1 in	35	26	16	9
1-1/4 in	61	45	28	16
1-1/2 in	84	61	38	22
2 in	138	101	63	36

How conduit fill is calculated

The NEC sizes raceways by area. Chapter 9 Table 4 lists the inside area of every conduit type and trade size, along with the area at 40% fill; Table 5 lists the area of each conductor size and insulation type, insulation included. Add up the conductor areas, divide by the inside area of the conduit, and compare with the Table 1 limit: 53% for a single conductor, 31% for two and 40% for three or more. Every wire counts, including the neutral and the equipment grounding conductor.

When all the conductors are the same size, Annex C of the NEC tabulates the answer directly, and this calculator reproduces those numbers for THHN in EMT and Schedule 40 PVC, including the Note 7 rule that lets a result of 0.8 or more round up. Annex C is built on the 40% column even for two conductors; this calculator applies the stricter 31% from Table 1 in that case, which occasionally moves two large conductors up one conduit size.

Beyond the fill percentage

A conduit that is legal at 40% can still be miserable to pull, so electricians plan for 30 to 35% on long runs. The NEC caps bends at 360 degrees between pull points (358.26 for EMT, 352.26 for PVC): four 90s, and offsets and kicks count. Past that, set a pull box sized by 314.28 or an LB, which is a fight with 6 AWG and up. Mixed conductor sizes are handled the same way: add the individual Table 5 areas and compare with the 40% area. Other insulation types such as THW or XHHW are thicker than THHN and reduce the count.

Fill is only one limit. More than three current-carrying conductors in a raceway derates ampacity from the 90°C column: 80% for 4 to 6, 70% for 7 to 9, 50% for 10 to 20. Nine 12 AWG THHN still make 20 A circuits (30 × 0.7 = 21 A) and the tenth drops all of them to 15 A. That is why 3/4 in EMT holds 16 by fill but 9 by ampacity. The neutral of a 120/240 V multiwire circuit and the ground do not count. Underground and outdoor raceways are wet locations (300.5(B), 300.9), so the wire has to read THWN or THWN-2. A conduit on a hot roof also needs the Table 310.15(B)(1) temperature correction.

What this calculator leaves out

The result is a fill percentage. Not included: fittings and straps (EMT within 3 ft of every box and every 10 ft, 358.30; 1/2 to 1 in PVC every 3 ft, Table 352.30). Also insulated bushings on 4 AWG and larger (300.4(G)), pull boxes, expansion fittings, and burial depth (18 in for PVC, Table 300.5). And the wire itself (500 ft spools, not cut lengths), pulling lube, box fill at each end (314.16), and the permit.

Code limits that apply

NEC Chapter 9 Note 4
A nipple 24 in or shorter between boxes may be filled to 60%, and its conductors are not derated
If you miss it: Sizing a short nipple at 40% buys a larger fitting than the code asks for and can force a bigger box
NEC Chapter 9 Table 1checked
Fill capped at 53% of inside area for one conductor, 31% for two, 40% for three or more
If you miss it: Eight 8 AWG THHN stuffed into 3/4 in EMT at 55% fill jam in the first 90 and skin the insulation; the run is repiped in 1 in
NEC Table 310.15(C)(1)checked
4 to 6 current-carrying conductors at 80%, 7 to 9 at 70%, 10 to 20 at 50% of the 90°C column
If you miss it: Ten 12 AWG THHN in one EMT derate to 15 A under their 20 A breakers; the inspector has the circuits split into a second raceway
NEC Chapter 9 Note 7
With all conductors one size, a fraction of 0.8 or more in the count rounds up
If you miss it: Rounding 9.45 up to ten 8 AWG in 1 in EMT overfills the raceway; the inspector counts nine and the tenth conductor moves to a second pipe
NEC 300.5(B)
Underground raceway interiors are wet locations; conductors are THWN, THWN-2 or XHHW-2
If you miss it: Wire marked THHN only in a buried PVC run fails the underground inspection; it is repulled with THWN-2 before the trench is backfilled
NEC 358.26
No more than 360 degrees of bends between pull points in EMT, offsets and kicks included
If you miss it: A run with four 90s plus an offset will not pull; the wire is cut out and a pull box or LB is set before the inspector sees it
NEC 314.28
Pull box for 4 AWG and up: 8 x trade size for a straight pull, 6 x for an angle pull
If you miss it: A 2 in feeder pulled through an 8 in box with an angle pull is written up; the box is swapped for a 12 in one and the feeder repulled
NEC 358.30
EMT supported within 3 ft of every box and at least every 10 ft along the run
If you miss it: A run strapped only at the boxes is flagged at rough-in; straps are added every 10 ft before the wire is pulled

Provision	Limit	If you miss it
NEC Chapter 9 Note 4	A nipple 24 in or shorter between boxes may be filled to 60%, and its conductors are not derated	Sizing a short nipple at 40% buys a larger fitting than the code asks for and can force a bigger box
NEC Chapter 9 Table 1checked	Fill capped at 53% of inside area for one conductor, 31% for two, 40% for three or more	Eight 8 AWG THHN stuffed into 3/4 in EMT at 55% fill jam in the first 90 and skin the insulation; the run is repiped in 1 in
NEC Table 310.15(C)(1)checked	4 to 6 current-carrying conductors at 80%, 7 to 9 at 70%, 10 to 20 at 50% of the 90°C column	Ten 12 AWG THHN in one EMT derate to 15 A under their 20 A breakers; the inspector has the circuits split into a second raceway
NEC Chapter 9 Note 7	With all conductors one size, a fraction of 0.8 or more in the count rounds up	Rounding 9.45 up to ten 8 AWG in 1 in EMT overfills the raceway; the inspector counts nine and the tenth conductor moves to a second pipe
NEC 300.5(B)	Underground raceway interiors are wet locations; conductors are THWN, THWN-2 or XHHW-2	Wire marked THHN only in a buried PVC run fails the underground inspection; it is repulled with THWN-2 before the trench is backfilled
NEC 358.26	No more than 360 degrees of bends between pull points in EMT, offsets and kicks included	A run with four 90s plus an offset will not pull; the wire is cut out and a pull box or LB is set before the inspector sees it
NEC 314.28	Pull box for 4 AWG and up: 8 x trade size for a straight pull, 6 x for an angle pull	A 2 in feeder pulled through an 8 in box with an angle pull is written up; the box is swapped for a 12 in one and the feeder repulled
NEC 358.30	EMT supported within 3 ft of every box and at least every 10 ft along the run	A run strapped only at the boxes is flagged at rough-in; straps are added every 10 ft before the wire is pulled

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Four 90s is the 360 degree limit between pull points (NEC 358.26, 352.26), and every offset and kick counts; add a pull box or LB.
Pull boxes for 4 AWG and larger follow NEC 314.28: 8 times the trade size for a straight pull, 6 times for an angle pull.
Nine current-carrying 12 AWG THHN per conduit still carry 20 A (30 × 0.7 = 21 A); a tenth derates them all to 15 A.
Underground and outdoor conduit is a wet location (NEC 300.5(B), 300.9); plain THHN is dry only, so check the jacket says THWN or THWN-2.
EMT comes in 10 ft sticks with set-screw fittings for indoors; outdoors and underground need raintight compression fittings or PVC (NEC 358.42).
PVC grows about 4 in per 100 ft across a 100 F swing (NEC Table 352.44); exposed runs need expansion fittings or couplings pull apart.

Key facts

A 100 A feeder of three 3 AWG THHN conductors and one 8 AWG ground is 0.3285 sq in of conductor, which fits 1 in EMT at 38% fill but not 3/4 in.
NEC Chapter 9 Table 1 limits conduit fill to 53% of the inside area for one conductor, 31% for two and 40% for three or more.
The limit for two conductors is stricter than for three because two conductors lie side by side and can jam in the bends; three or more pack more evenly.
12 AWG THHN has an area of 0.0133 sq in including insulation, so 3/4 in EMT (0.533 sq in inside) holds 16 of them and 1/2 in EMT holds 9.
Ampacity must be derated when more than three current-carrying conductors share a raceway: 80% for 4 to 6, 70% for 7 to 9, and 50% for 10 to 20 (NEC Table 310.15(C)(1)).
Note 4 to Table 1 allows a nipple of 24 in or less between enclosures to be filled to 60%, and the conductors in it are not subject to the bundling derate.

Frequently asked questions

How do I calculate conduit fill with different wire sizes?

Add the area of every conductor from NEC Chapter 9 Table 5, then compare the total against the 40% column for the conduit. The Note 7 round up does not apply once the sizes differ, so the summed area has to sit inside the limit outright.

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

16. The 40% fill area of 3/4 in EMT is 0.213 sq in and each 12 AWG THHN conductor is 0.0133 sq in, so 0.213 / 0.0133 = 16.0. In 3/4 in Schedule 40 PVC the answer is 15.

How many wires can go in 1/2 in conduit?

In 1/2 in EMT, 12 of 14 AWG THHN, 9 of 12 AWG, 5 of 10 AWG or 3 of 8 AWG. Schedule 40 PVC is slightly smaller inside and holds 11, 8, 5 and 3.

Does the ground wire count toward conduit fill?

Yes. Chapter 9 Note 3 says equipment grounding and bonding conductors are included in the fill calculation, using their Table 5 area. They are not counted as current-carrying conductors for ampacity derating.

How many 90 degree bends can a conduit run have?

No more than 360 degrees between pull points, which is four 90s, and offsets and kicks count toward it (NEC 358.26 for EMT, 352.26 for PVC). Beyond that you break the run with a pull box, sized per 314.28 for 4 AWG and larger, or a conduit body. Fewer bends is the real target: a legal 40% fill with four 90s and no lube is where wire insulation gets skinned.

Sources and references

NFPA 70, National Electrical Code (NFPA)
Re3 Conduit Fill Calculator (Southwire)
Code Hunter: Calculations, IAEI Magazine (IAEI)

Next steps for this project

The usual order for a electrical & plumbing project. See all electrical & plumbing calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.