Conduit Fill Calculator
Check conduit fill percentage against the NEC 40% limit.
Fill limits, derating for more than three current-carrying conductors, and conduit type all interact. Verify against NEC Chapter 9 and have the installation inspected by a licensed electrician.
How this calculator works
fill % = (conductor area × count) ÷ conduit internal area × 100
The NEC allows 53% fill for a single conductor, 31% for exactly two, and 40% for three or more. The two-conductor case is the strictest and is the one most people get wrong.
Worked example
With conduit internal area 0.533 , area per conductor 0.0133 , number of conductors 3 , this calculator returns 7.49 .
Conduit fill by conductor count
Figures below assume conduit internal area 0.533 × area per conductor 0.0133 . Change any measurement in the calculator to get your own numbers.
| Number of conductors | Conduit fill | Area used |
|---|---|---|
| 1 | 2.50 | 0.01 |
| 2 | 4.99 | 0.03 |
| 3 | 7.49 | 0.04 |
| 4 | 9.98 | 0.05 |
| 6 | 14.97 | 0.08 |
| 8 | 19.96 | 0.11 |
| 10 | 24.95 | 0.13 |
| 12 | 29.94 | 0.16 |
THHN 12 AWG in 3/4 inch EMT. The 40% limit applies once there are three or more conductors; two conductors are limited to 31% and a single conductor to 53%.
Conduit fill for common wire pulls
| Job | Size | Conduit fill |
|---|---|---|
| 3 × 12 AWG in 1/2 in EMT | 0.304 × 0.0133 × 3 | 13.13 |
| 4 × 12 AWG in 3/4 in EMT | 0.533 × 0.0133 × 4 | 9.98 |
| 9 × 12 AWG in 3/4 in EMT | 0.533 × 0.0133 × 9 | 22.46 |
| 3 × 10 AWG in 3/4 in EMT | 0.533 × 0.0211 × 3 | 11.88 |
| 4 × 8 AWG in 1 in EMT | 0.864 × 0.0366 × 4 | 16.94 |
| 3 × 4/0 in 2 in EMT | 3.356 × 0.3237 × 3 | 28.94 |
Fill is only half the problem. A long run with several bends can exceed pulling tension well below 40% fill, and code caps total bends at 360 degrees between pull points.
Conduit internal areas
| Type | Area (sq in) | Notes |
|---|---|---|
| 1/2 in EMT | 0.30 | 40% fill: 0.122 sq in |
| 3/4 in EMT | 0.53 | 40% fill: 0.213 sq in |
| 1 in EMT | 0.86 | 40% fill: 0.346 sq in |
| 1-1/4 in EMT | 1.50 | 40% fill: 0.598 sq in |
| 1-1/2 in EMT | 2.04 | 40% fill: 0.814 sq in |
| 2 in EMT | 3.36 | 40% fill: 1.342 sq in |
Areas are from NEC chapter 9, table 4. PVC and rigid conduit have different internal areas for the same trade size — check the right table for your raceway type.
Frequently asked questions
How do I calculate conduit fill?
Add the cross-sectional areas of every conductor, then divide by the internal area of the conduit. Conductor areas come from NEC chapter 9 table 5, conduit areas from table 4.
How many wires can I put in 1/2 inch conduit?
Nine 12 AWG THHN conductors, or twelve 14 AWG. Half-inch EMT has 0.304 sq in of internal area, and the 40% limit leaves 0.122 sq in usable.
Does derating apply to conduit fill?
They are separate rules that often bite together. Fill is a physical limit; ampacity derating under 310.15 kicks in past three current-carrying conductors and can force a larger wire even when fill is fine.
What counts as a current-carrying conductor?
Phase and neutral conductors in most cases. Equipment grounding conductors never count for derating, and a neutral carrying only the unbalanced current of a multi-wire branch circuit does not count either.
What is the maximum conduit fill?
Forty percent for three or more conductors, 31% for exactly two, and 53% for a single conductor. The limits come from NEC Chapter 9, Table 1.
How many 12 AWG wires fit in 3/4 inch EMT?
Sixteen THHN conductors at the 40% limit. In practice pulling that many is difficult, and derating for more than three current-carrying conductors usually becomes the real constraint first.
Why is the two-conductor limit lower than for three?
Two round conductors in a round raceway leave an awkward geometry that jams during a pull. The 31% figure reflects pulling practicality rather than heat, which is why it looks inconsistent.
Does the ground wire count toward fill?
Yes. Every conductor in the raceway counts toward the fill calculation, including equipment grounds and neutrals. Only for ampacity derating do you count just the current-carrying conductors.