For electricians & homeowners
Conduit Fill
Calculator
Every conduit has a fill limit — with three or more wires, conductors can take up at most NEC 40% of the pipe. Overfill it and the wires jam when you pull them and can overheat. This tool checks your fill against NEC Chapter 9 in seconds.
A conduit fill calculator built for electricians — clear enough for anyone.
Pack a conduit past 40% and the wires jam and overheat; keep it at or under 40% and they pull clean. This tool sizes the pipe for any bundle in seconds.
NEC-based
Chapter 9 fill tables
Transparent
Every tool shows its formula
Free & private
No signup, runs in your browser
Conduit Fill Calculator
Check conductors fit a conduit within the NEC 40% fill limit.
Add conductors to check the fill.
Fill vs. the allowable area
Compare trade sizes — tap to inspect (fill %)
Method — how the fill is checked
Fill factor (NEC Ch.9 Table 1): 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%; a short nipple (≤24 in) = 60%. Conduit internal areas from Table 4; insulated conductor areas from Table 5. Grounds count. Mixed sizes use exact-area fill (no round-up).
How to use it
How to use the conduit fill calculator
It takes three quick inputs — your conduit, the conductors going in, and whether it's a short nipple. The tool sums the conductor areas, compares them to the conduit's allowable fill, and shows the fill percentage — and whether it stays under the NEC 40% limit.
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01
Pick your conduit
Choose the type (EMT, PVC, RMC) and trade size — or leave it on Auto and the tool finds the smallest size that fits.
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02
Add your conductors
Set the insulation (THHN, XHHW…), size and quantity. Add a row for each group — hots, neutral and grounds all count.
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03
Flag a nipple
If the raceway is a short nipple (24 in or less), tick the box — the fill limit rises from 40% to 60%.
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04
Read your fill
Get the fill % instantly, whether it's under the 40% limit, the smallest conduit that fits, and how many more wires you have room for.
The core idea
What determines conduit size
One ratio decides it — how much of the pipe your conductors fill.
Conduit size comes down to a single ratio: the total area of all the conductors versus the conduit's inside area. That fill must stay at or below 40% for three or more wires. More or bigger conductors raise it; a roomier conduit lowers it.
One rule — keep the fill under 40%
must stay ≤ 40%
for 3+ conductors
Other limits: 53% for a single wire, 31% for two, 60% for a short nipple (≤24 in).
Fill % = Σ conductor area ÷ conduit area — keep it ≤ 40%
The method
How conduit fill is calculated
The conduit fill formula is one ratio: add up the cross-sectional area of every conductor (NEC Table 5), then divide by the conduit's inside area (Table 4). Keep the result under the fill limit — 40% for three or more wires.
The fill limit changes with the number of wires
1 wire
53%
2 wires
31%
3+ wires
40%
(≤24 in)
Why do two wires get less room than three? Two conductors sit side-by-side and trap heat between them, so the limit drops to 31%. With three or more they pack rounder and shed heat better — the limit rises to 40%.
Worked example
How many 12 AWG THHN fit ¾″ EMT?
Sixteen 12 AWG THHN conductors fill a ¾″ EMT to 39.9% — just under the 40% limit, so they fit. Add a seventeenth and the fill jumps to 42.4%, over the line — step up to 1″ EMT.
16 conductors
16 × 0.0133 in² = 0.2128 in²
allowable = 0.533 × 40% = 0.213 in²
0.2128 ≤ 0.213 → fits
17 conductors
17 × 0.0133 in² = 0.2261 in²
allowable = 0.213 in² (unchanged)
0.2261 > 0.213 → over the limit
16 × 12 AWG THHN is the most a ¾″ EMT holds. Need 17 or more? Step up to 1″ EMT — 0.346 in² allowable, about 60% more room.
The code behind it
The NEC rules behind conduit fill
Conduit fill runs on NEC Chapter 9: Table 1 sets the fill percentages, Table 4 gives each conduit's inside area, and Table 5 gives each conductor's area. Grounds and neutrals count too — and filling to 40% doesn't exempt you from ampacity derating.
| Trade size | EMT | PVC 40 | RMC |
|---|---|---|---|
| ½″ | 0.304 | 0.285 | 0.314 |
| ¾″ | 0.533 | 0.508 | 0.549 |
| 1″ | 0.864 | 0.832 | 0.887 |
| 1¼″ | 1.496 | 1.453 | 1.526 |
| 1½″ | 2.036 | 1.986 | 2.071 |
| 2″ | 3.356 | 3.291 | 3.408 |
Inside area in in². Multiply by the fill factor (× 40% for 3+ wires) to get the usable area. EMT runs a touch roomier than PVC, a touch tighter than rigid.
Fill limits
53% for one wire, 31% for two, 40% for three or more, 60% for a short nipple (≤24 in).
Conduit areas
Inside cross-sectional area for every conduit type and trade size — EMT, PVC, RMC, IMC and more.
Conductor areas
Area of each insulated conductor — THHN, XHHW, THW differ, so the same AWG can take more or less room.
Ready-made counts
Pre-computed "how many fit" tables for same-size conductors in each conduit — a quick field lookup.
Grounds count
Every conductor counts toward fill — hots, neutrals, and equipment grounds, insulated or bare.
What affects it
What raises fill — and how to fit more
Fill climbs with the number of conductors, their size, and how thick their insulation is — and two-wire runs fill faster because their limit is only 31%. To fit more, the biggest lever by far is a larger conduit; after that, pick compact THHN, trim conductors, or split the run.
Raises the fill
what fills the pipe faster
- More conductorsEvery wire adds area — hots, neutrals and grounds all count.
- Bigger conductorsA jump up in AWG or kcmil adds cross-sectional area fast.
- Thicker insulationXHHW and THW take more room than THHN at the same size.
- Two-wire runsThe limit drops to 31%, so two conductors fill sooner than you'd expect.
How to fit more
ranked by leverage
- 01Upsize the conduitThe biggest lever by far — one trade size up adds a lot of room.
- 02Choose THHNThe most compact common insulation — more wires per pipe than XHHW or THW.
- 03Trim or downsize conductorsWhere the design and ampacity allow — fewer or smaller wires.
- 04Split into two racewaysRun parallel conduits for a large bundle instead of one overfull pipe.
The biggest lever
One trade size up ≈ 60–75% more room
Going from ¾″ to 1″ EMT lifts the usable area from 0.213 to 0.346 in² — about 62% more. Upsizing the raceway is almost always the fastest fix for an overfull run.
Already have a conduit run? Use the calculator's Fill % mode to see how full it is now and exactly how many more conductors you can safely add.
Quick chart
How many wires fit — by conduit & size
Pick a conduit and a conductor to see the maximum that fit under the NEC fill limit. For the classic field question — how many 12 AWG THHN in ¾″ EMT — the answer is 16.
Maximum conductors
16 wires
12 AWG THHN in ¾″ EMT
fills to 39.9% · 40% limit
Common circuits → the conduit that fits
20 A branch
3 × 12 AWG THHN
½″ EMT
30 A circuit
3 × 10 AWG THHN
½″ EMT
50 A EV charger
3 × 6 AWG THHN
¾″ EMT
100 A feeder
3 × 3 AWG + 8 AWG grd
1″ EMT
Typical branch/feeder bundles. Always confirm the exact conductor count and insulation for your job.
| Wire size (AWG) | ½″ EMT | ¾″ EMT | 1″ EMT | 1¼″ EMT | 1½″ EMT | 2″ EMT |
|---|---|---|---|---|---|---|
| 14 AWG THHN | 12 | 21 | 35 | 61 | 83 | 138 |
| 12 AWG THHN | 9 | 16 | 26 | 44 | 61 | 100 |
| 10 AWG THHN | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG THHN | 3 | 5 | 9 | 16 | 22 | 36 |
| 6 AWG THHN | 1 | 4 | 6 | 11 | 16 | 26 |
| 4 AWG THHN | 1 | 2 | 4 | 7 | 9 | 16 |
| 3 AWG THHN | 1 | 1 | 3 | 6 | 8 | 13 |
| 2 AWG THHN | 1 | 1 | 2 | 5 | 7 | 11 |
| 1 AWG THHN | 1 | 1 | 1 | 3 | 5 | 8 |
| 1/0 THHN | 0 | 1 | 1 | 3 | 4 | 7 |
| 2/0 THHN | 0 | 1 | 1 | 2 | 3 | 6 |
| 3/0 THHN | 0 | 1 | 1 | 1 | 3 | 5 |
| 4/0 THHN | 0 | 0 | 1 | 1 | 1 | 4 |
How many wires fit in ½″, ¾″, or 1″ EMT?
For 12 AWG THHN: ½″ holds 9, ¾″ holds 16, and 1″ holds 26. For 10 AWG THHN it's 5, 10, and 16. Bigger conductors or thicker insulation lower the count — check the chart above for any size.
What's the 40% fill rule, and why 40%?
With three or more conductors, they can occupy at most 40% of the conduit's inside area (NEC Ch.9 Table 1). The empty space lets you pull the wires without scraping insulation and lets heat escape, so nothing overheats.
Why do two wires get only 31%?
Two conductors sit side-by-side and trap heat between them, so the limit drops to 31%. Three or more pack rounder and shed heat better, so it rises to 40%. A single conductor gets 53%.
Do ground wires count toward fill?
Yes. Every conductor counts — hots, neutrals, and equipment grounds, whether insulated or bare. Add them all when you total the conductor area.
Does wire type (THHN vs XHHW) change how many fit?
Yes. XHHW and THW have thicker insulation than THHN, so the same AWG takes more room and fewer fit. Always use the area for your actual insulation type (NEC Table 5) — the calculator does this automatically.
Can I add wires to an existing conduit?
Yes — as long as the new total stays within the limit. Add the area of the new conductors to what's already in the pipe and confirm it's still ≤ 40% (for 3+ wires). The calculator's Fill % mode shows exactly how many more you can add.
Does conduit fill affect ampacity?
They're separate checks — but once you have more than three current-carrying conductors in the raceway, their ampacity is derated (NEC 310.15(C)). You may need to upsize the wire even when the fill is comfortably under 40%.
Is EMT the same as PVC for fill?
No. At the same trade size, EMT has a slightly larger inside area than PVC (and PVC Sch 80 is tighter than Sch 40), so EMT fits a touch more. And a short nipple (≤24 in) of any type is allowed up to 60% fill.
Keep going
Related electrical calculators
Fitting the raceway is the last step. These cover the ones before it — size the conductor, check the run, and protect the circuit — and run on the same NEC-based engine.
How we keep this accurate
Conduit fill follows the National Electrical Code (NEC 2023) Chapter 9: fill limits from Table 1 (53 / 31 / 40 / 60%), conduit inside areas from Table 4, and conductor areas from Table 5, with Annex C for ready-made counts. Results are for planning and estimating. Code adoption, local amendments, and field conditions vary — verify with your AHJ and a licensed electrician before installation.