Conduit Fill Calculator
Add each wire gauge and how many of that gauge are in the run. You get the total conductor area, the NEC fill limit that applies to that conductor count, and the smallest standard EMT trade size that keeps you within it -- with the full reference table shown underneath.
Conductors in the run
Total conductors
3
fill limit: 40% of conduit area
Total conductor area
0.0399 sq in
sum of each wire's THHN area
Smallest EMT that fits
1/2"
13.1% fill
| EMT trade size | Total area (100%) | Max at 40% fill | Your fill |
|---|---|---|---|
| 1/2" | 0.304 sq in | 0.1216 sq in | 13.1% |
| 3/4" | 0.533 sq in | 0.2132 sq in | 7.5% |
| 1" | 0.864 sq in | 0.3456 sq in | 4.6% |
| 1-1/4" | 1.496 sq in | 0.5984 sq in | 2.7% |
| 1-1/2" | 2.036 sq in | 0.8144 sq in | 2.0% |
| 2" | 3.356 sq in | 1.3424 sq in | 1.2% |
| 2-1/2" | 5.858 sq in | 2.3432 sq in | 0.7% |
| 3" | 8.846 sq in | 3.5384 sq in | 0.5% |
| 3-1/2" | 11.545 sq in | 4.6180 sq in | 0.3% |
| 4" | 14.753 sq in | 5.9012 sq in | 0.3% |
Common wire counts, smallest EMT that fits
Typical residential and light-commercial conductor groups, all THHN/THWN-2, and the smallest standard EMT trade size that keeps them within the NEC fill limit.
| Conductors | Total area | Fill limit | Smallest EMT |
|---|---|---|---|
| 3× 12 AWG (standard 120V branch circuit) | 0.0399 sq in | 40% | 1/2" |
| 4× 10 AWG (30A circuit, hot/neutral/ground + spare) | 0.0844 sq in | 40% | 1/2" |
| 3× 8 AWG (50A circuit) | 0.1098 sq in | 40% | 1/2" |
| 3× 4 AWG (100A subpanel feeder) | 0.2472 sq in | 40% | 1" |
| 4× 2 AWG (feeder with isolated ground) | 0.4632 sq in | 40% | 1-1/4" |
These are the fill limit only -- a real pull also depends on bends, offsets and how much the actual jacket diameter differs from the Table 5 value for your specific wire brand.
Where the fill limits actually come from
NEC Chapter 9, Table 1 sets the fill percentage; Table 4 gives each raceway type's internal area by trade size; Table 5 gives each conductor type's cross-sectional area by AWG. This calculator multiplies the fill percentage by the EMT area from Table 4, sums the real conductor areas from Table 5, and finds the smallest trade size where the conductors fit within that limit.
The reasoning behind the limit is mechanical, not electrical: conduit fill isn't primarily about heat (that's handled separately by the ampacity derating in NEC 310.15(C) for more than three current-carrying conductors) -- it's about whether the conductors can actually be pulled through the conduit without jamming or damaging the insulation.
Not applied
Bends and offsets (a run with several 90° bends pulls harder than the fill percentage alone suggests, even at the same nominal fill), rigid/PVC/IMC internal areas (all different from EMT's), non-THHN insulation types, and local amendments. Those are exactly what the electrician and the inspector check beyond this calculator.
Frequently Asked Questions
What counts as a conductor for fill purposes?
Every current-carrying conductor in the run -- hots and neutrals -- plus the equipment grounding conductor if one is pulled in the same conduit. A standard 120V circuit with hot, neutral and ground is 3 conductors for fill math, even though only 2 of them carry current under normal load.
Why is the limit lower for exactly 2 conductors (31%) than for 1 (53%) or 3+ (40%)?
It looks backwards, but it's intentional (NEC Chapter 9, Table 1 and its notes). A single conductor has nothing to jam against and gets the most generous limit. Two conductors can wedge against each other when pulled, which is genuinely harder to fix mid-pull than a bundle of three or more, so NEC drops the allowance below the 3+ case rather than above it.
Does this apply to any conduit type, not just EMT?
The fill percentages (53%/31%/40%) are the same for any raceway type, but the internal area they apply to is not -- rigid metal conduit, IMC and PVC each have different wall thickness and therefore different internal area at the same trade size. This calculator uses EMT, the most common type in residential and light commercial work; rigid and PVC will fit slightly fewer conductors at the same nominal size.
Does wire insulation type change the numbers?
Yes. This calculator uses THHN/THWN-2, the standard building wire for conduit runs. Other insulation types (XHHW, USE-2, etc.) have different outer diameters and therefore different cross-sectional areas per NEC Chapter 9, Table 5 -- swapping insulation type without rechecking fill is a real, common mistake.
Can I mix wire gauges in one conduit?
Yes, and it's common -- a feeder might run larger hots with a smaller ground, for example. Fill is based on total area, not conductor count alone, so this calculator lets you add multiple gauges and sums their real areas rather than assuming every wire is the same size.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.