Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
Working the problem backwards
The fill calculator answers "does this fit?". This one answers the question you usually actually have: "what size do I need?" It walks the sizes in NEC Table 4 from smallest up and returns the first that passes the correct Table 1 limit for your conductor count.
| Conductors | Total area | Smallest EMT | Fill at that size |
|---|---|---|---|
| 3 × 12 AWG | 0.0399 in² | 1/2 in | 13.1% |
| 6 × 12 AWG | 0.0798 in² | 1/2 in | 26.3% |
| 9 × 12 AWG | 0.1197 in² | 1/2 in | 39.4% |
| 4 × 10 AWG | 0.0844 in² | 1/2 in | 27.8% |
| 4 × 8 AWG | 0.1464 in² | 3/4 in | 27.5% |
| 4 × 6 AWG | 0.2028 in² | 3/4 in | 38.0% |
| 4 × 4/0 | 1.2948 in² | 2 in | 38.6% |
Reasons to go up a size anyway
The minimum legal size is often not the size to install:
- Pulling tension. A pull at 39% fill through several bends is genuinely hard work and risks damaging insulation. One size up can turn an afternoon into ten minutes.
- Future circuits. Conduit is cheap; opening a wall again is not. Upsizing a run you might extend later is usually the best money on the job.
- Bends. 360° total between pull points is the maximum, and every bend multiplies tension. More bends, more room.
- Derating. If bundling adjustment pushes you to larger conductors, the conduit has to grow with them. Work out the conductor size first.
Trade sizes are nominal
Conduit sizes are trade designations, not measurements. "Half-inch" EMT has an internal diameter of 0.622 in and an internal area of 0.304 in²; nothing about it measures half an inch. The same nominal size in a different type has a different area, because wall thicknesses differ — which is why Table 4 lists every type separately and why you cannot substitute PVC for EMT in a fill calculation without redoing it.
Common questions
What size conduit do I need for 9 12 AWG wires?
Half-inch EMT, at 39.4% fill against a 40% limit. That is legal but a tight pull — three-quarter inch is the practical choice.
What size conduit for four 4/0 THHN conductors?
Two-inch. Four 4/0 THHN is 1.295 in², and 2 in EMT allows 1.342 in² at 40%. One-and-a-half inch only allows 0.814 in².
Should I install the minimum size conduit?
Usually go one size up. The minimum passes the code but makes pulling harder, risks insulation damage through bends, and leaves no room for future circuits. Conduit is the cheapest part of the job.
Is half-inch conduit half an inch across?
No. Conduit sizes are trade designations. Half-inch EMT has an internal diameter of 0.622 inches and 0.304 square inches of internal area.