What is the maximum conduit fill percentage?

One conductor may fill 53% of the raceway's internal area, two conductors 31%, and three or more conductors 40%. Short nipples of 24 inches or less are permitted 60% fill.

How is conduit fill calculated?

Compute the internal area of the conduit as A = pi x (id/2)^2, compute the area of each conductor from its insulated outside diameter the same way, multiply by the conductor count, then divide the total conductor area by the conduit area.

Can I calculate fill for mixed conductor sizes?

Yes — sum the area of each group separately. Run the calculator once per size, add the conductor areas together, and compare the total against the raceway's 40% usable area. Fixed chart counts only cover same-size pulls, which is the main reason to use a calculator instead of a table.

Do I count the ground wire in conduit fill?

Yes. All conductors occupy space, so the equipment grounding conductor counts toward the fill area even though it is not counted as a current-carrying conductor for ampacity derating.

Does conduit fill change with conductor type?

Yes, because fill is based on the insulated outside diameter. THHN/THWN-2 is compact; XHHW and older THW insulation are larger, so fewer conductors fit in the same raceway.

Conduit Fill Calculator

Fill percentage against the 53 / 31 / 40 rule, the maximum conductor count for the raceway you picked, and the smallest trade size that legally holds your pull.

Quick answer

Allowable conduit fill is 53% for one conductor, 31% for two, and 40% for three or more. Fill percent is total conductor area divided by the raceway's internal area, where both areas come from A = π × (diameter / 2)². Pick the raceway family below and the calculator applies that pipe's published internal diameter.

Pre-computed charts by conduit type

Want a table instead of one answer? Each raceway family has its own chart.

Raceway and conductors

Conduit typeTrade sizeConductor size (THHN)Number of conductors

A = π × (d / 2)² · fill% = (n × Awire) / Aconduit

12 AWG THHN nominal OD 0.130" = 0.0133 sq in each.

Result

Fill

17.5%

Allowed (3+ wires)

40%

Conduit area

0.3039sq in

Max conductors this size

4 conductors are within the 40% limit for 1/2" EMT.

Smallest EMT that holds 4 × 12 AWG THHN: 1/2"

Conductor area used: 0.0531 sq in of 0.1216 sq in permitted.

Published standard

Where the numbers come from

The calculator solves one pull at a time. Pre-computed counts for every wire size against every trade size live on the chart pages, so they are not repeated here.

Each conductor's area is π × (OD / 2)² using the manufacturer's nominal insulated outside diameter. The raceway's usable area is its internal area multiplied by the fill percentage that matches the conductor count — 53% at one, 31% at two, 40% at three or more. The maximum count is the largest n where n × Awire stays inside that usable area, which is why the limit shifts when the count crosses from two to three.

Looking for a printable table rather than a single answer? The conduit fill chart lists maximum conductors per trade size for EMT, IMC, RMC and PVC, and each wire size there links to its own per-size conduit fill page.

Internal area by trade size

Trade size	Internal dia. (in)	Total area (sq in)	53% (1 wire)	31% (2 wires)	40% (3+ wires)
1"	1.049	0.8643	0.4581	0.2679	0.3457
2"	2.067	3.3556	1.7785	1.0402	1.3422
3"	3.356	8.8457	4.6882	2.7422	3.5383
4"	4.334	14.7526	7.8189	4.5733	5.9010
1/2"	0.622	0.3039	0.1611	0.0942	0.1216
3/4"	0.824	0.5333	0.2826	0.1653	0.2133
1-1/4"	1.380	1.4957	0.7927	0.4637	0.5983
1-1/2"	1.610	2.0358	1.0790	0.6311	0.8143
2-1/2"	2.731	5.8578	3.1046	1.8159	2.3431
3-1/2"	3.834	11.5450	6.1189	3.5789	4.6180

Frequently asked

What is the maximum conduit fill percentage?: One conductor may fill 53% of the raceway's internal area, two conductors 31%, and three or more conductors 40%. Short nipples of 24 inches or less are permitted 60% fill.
How is conduit fill calculated?: Compute the internal area of the conduit as A = pi x (id/2)^2, compute the area of each conductor from its insulated outside diameter the same way, multiply by the conductor count, then divide the total conductor area by the conduit area.
Can I calculate fill for mixed conductor sizes?: Yes — sum the area of each group separately. Run the calculator once per size, add the conductor areas together, and compare the total against the raceway's 40% usable area. Fixed chart counts only cover same-size pulls, which is the main reason to use a calculator instead of a table.
Do I count the ground wire in conduit fill?: Yes. All conductors occupy space, so the equipment grounding conductor counts toward the fill area even though it is not counted as a current-carrying conductor for ampacity derating.
Does conduit fill change with conductor type?: Yes, because fill is based on the insulated outside diameter. THHN/THWN-2 is compact; XHHW and older THW insulation are larger, so fewer conductors fit in the same raceway.