Conduit Fill Calculator
Fill percentage and pass/fail for THHN in EMT, PVC and rigid — mixed sizes welcome.
Calculator
How this works
Conduit fill is an area problem. Every conductor has a published cross-section — insulation included — and every conduit type has a published internal area. The calculation sums your conductor areas and compares the total against the conduit's area multiplied by a fill factor:
The widely published fill factors are 53% for a single conductor, 31% for two, and 40% for three or more. The dip at two isn't a typo — a pair of round conductors lying side by side wastes space in a round pipe, so the allowance tightens, then relaxes again once three or more can nest.
The tool uses manufacturer-published THHN/THWN dimensions (a 12 AWG THHN is 0.0133 in², a 4/0 is 0.3237 in²) and per-type conduit areas — EMT, PVC Schedule 40 and rigid all differ at the same trade size because wall thicknesses differ. For a single-size pull, it also computes the maximum number of that conductor the conduit will take, which is usually the number you actually want when planning a run.
Max THHN/THWN conductors in EMT (40% fill)
| Conductor | 1/2" EMT | 3/4" EMT | 1" EMT | 1-1/4" EMT | 1-1/2" EMT | 2" EMT |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 21 | 35 | 61 | 83 | 138 |
| 12 AWG | 9 | 16 | 25 | 44 | 61 | 100 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 3 | 5 | 9 | 16 | 22 | 36 |
| 6 AWG | 2 | 4 | 6 | 11 | 16 | 26 |
Three or more conductors (40% factor), manufacturer-published THHN dimensions and EMT internal areas. PVC Sch 40 and rigid differ — select them in the calculator.
Worked example: three circuits to a kitchen remodel
You're piping a kitchen remodel and want to run three 20 A small-appliance circuits from the panel in one EMT home run: three hots, three neutrals, and one ground, all 12 AWG THHN — seven conductors. Does 1/2" EMT take it, or do you need 3/4"?
Each 12 AWG THHN occupies 0.0133 in², so seven of them total 0.0931 in². Half-inch EMT has 0.304 in² of internal area, and with seven conductors the 40% factor applies: 0.304 × 0.40 = 0.1216 in² of allowed fill. You're at 0.0931 ÷ 0.304 = 30.6% — PASS, with room to spare. In fact the max-count readout shows 1/2" EMT takes nine 12 AWG THHN, so seven fits with two to grow.
Before you commit, think past the fill math: seven current-carrying conductors in one raceway triggers ampacity derating, and a crowded 1/2" pull around a couple of 90° bends is no fun even when it's legal. Many electricians would still reach for 3/4" here (0.533 in², fill drops to 17.5%) purely for pull comfort and future capacity — the material cost difference is pocket change on one run.
That's the pattern this tool supports: get the hard pass/fail instantly, then apply judgment about the pull.
Practical tips and common mistakes
- Count every conductor, including grounds and travelers. If it's in the pipe, it takes up area. The only things you don't count are the ones that aren't there.
- Fill limits don't cover pull difficulty. A 39% fill through four bends and 80 feet is a brutal pull. Long runs, multiple bends, or large conductors all argue for going up a size even when the math passes.
- Watch the conductor count for derating. More than three current-carrying conductors in a raceway means ampacity derating on top of fill limits — fill can pass while ampacity fails. They're separate checks.
- Same trade size ≠ same capacity. EMT, PVC Sch 40 and rigid have different internal areas at the same nominal size. Calculate with the conduit you're actually installing, not the one you priced first.
- Different insulation types have different areas. This calculator covers THHN/THWN. XHHW, USE and others are fatter for the same AWG — don't reuse THHN numbers for them.
Frequently asked questions
How many 12 AWG THHN wires fit in 1/2 inch EMT?
Nine, using the widely published 40% fill factor for three or more conductors. Half-inch EMT has about 0.304 in² of internal area; 40% of that is 0.1216 in², and each 12 AWG THHN occupies 0.0133 in². The calculator shows this max-count figure for any single-size pull.
Why is the fill limit 53% for one wire but 40% for three or more?
The widely published factors — 53% for one conductor, 31% for two, 40% for three or more — reflect how conductors actually sit in a raceway. A single cable can use more of the space; two conductors tend to lie side by side inefficiently; three or more nest together, so the factor comes back up.
Do I count the ground wire in conduit fill?
Yes. Every conductor in the raceway occupies area, including equipment grounding conductors and neutrals, whether or not they carry current in normal operation. Add a row for the ground at its actual size and insulation type.
Can I mix wire sizes in the same conduit?
Yes, and the calculator supports it — add a row for each size. The math simply sums the individual conductor areas and compares the total against the conduit's internal area times the fill factor for the total conductor count.
Why does the same trade size have different capacity in EMT vs PVC?
Trade size names don't equal internal dimensions. Half-inch EMT has about 0.304 in² inside, while half-inch PVC Schedule 40 has about 0.285 in² because its wall is thicker. Rigid metal conduit is different again. Always calculate with the actual conduit type you're installing.
What happens if I overfill a conduit?
Hard pulls, damaged insulation, and heat. Conductors packed too tight dissipate heat poorly and can be nicked during the pull — failures that may not show up until the circuit is loaded. If you're over the limit, step up a trade size; conduit is cheaper than a re-pull.