Conduit Fill Calculator
Smallest legal conduit for your wires. 1 wire = 53%, 2 = 31%, 3+ = 40% max fill.
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Conduit fill is the share of a raceway's internal cross-sectional area taken up by the conductors inside it. NEC Chapter 9, Table 1 caps it at 53% for one conductor, 31% for two, and 40% for three or more. Add each conductor's area from Chapter 9, Table 5, then pick a conduit whose allowed area exceeds the total.
The conduit fill formula
Fill % = (total conductor area ÷ conduit internal area) × 100. Total conductor area = Σ(per-conductor area, NEC Ch.9 Table 5). Allowed fill (NEC Ch.9 Table 1): 1 conductor = 53% · 2 conductors = 31% · 3 or more = 40%. Conduit internal areas come from NEC Ch.9 Table 4.
Worked example
Four #8 THHN copper conductors in EMT: each #8 THHN is 0.0366 in² (Ch.9 Table 5), so the bundle is 4 × 0.0366 = 0.1464 in². Three or more conductors use the 40% limit. 1/2" EMT allows only 0.122 in² (40% of 0.304), but 3/4" EMT allows 0.213 in² (40% of 0.533) — so 3/4" EMT is the minimum, filled to about 27.5%.
EMT internal area and 40% fill limit (NEC Ch.9 Table 4)
| EMT trade size | Internal area (in²) | 40% fill area (in²) |
|---|---|---|
| 1/2" | 0.304 | 0.122 |
| 3/4" | 0.533 | 0.213 |
| 1" | 0.864 | 0.346 |
| 1-1/4" | 1.496 | 0.598 |
| 1-1/2" | 2.036 | 0.814 |
| 2" | 3.356 | 1.342 |
NEC references
- NEC Chapter 9, Table 1 — maximum fill percentage: 53% (1 wire), 31% (2 wires), 40% (3 or more)
- NEC Chapter 9, Table 4 — internal cross-sectional area of each conduit type and trade size
- NEC Chapter 9, Table 5 — cross-sectional area of each conductor by size and insulation (THHN, etc.)
- NEC Annex C — ready-made conductor counts when every conductor is the same size and type
How to calculate conduit fill
- Total the cross-sectional area of every conductor (NEC Ch.9 Table 5 by size and insulation).
- Pick the fill limit: 53% for one conductor, 31% for two, 40% for three or more (Table 1).
- Look up each candidate conduit's internal area (Table 4) and multiply by that limit.
- Choose the smallest conduit whose allowed area meets or exceeds your conductor total.
Frequently asked questions
How do you calculate conduit fill?
Add the cross-sectional area of every conductor (NEC Ch.9 Table 5), then compare it to the conduit's internal area (Table 4) times the fill limit from Table 1 — 53% for one conductor, 31% for two, and 40% for three or more. The smallest conduit whose allowed area beats the conductor total is your minimum size.
What is the 40% conduit fill rule?
When three or more conductors run in a raceway, NEC Chapter 9, Table 1 limits them to 40% of the conduit's internal area. The headroom leaves space to pull the wire without damaging insulation and to dissipate heat. One conductor may fill 53% and two are limited to 31%.
How many wires can I put in 3/4 inch conduit?
3/4" EMT has 0.213 in² available at the 40% limit (40% of its 0.533 in² internal area), which holds roughly 16 #12 THHN or 5 #8 THHN conductors. Confirm the exact count against NEC Chapter 9, Table 1 and the Annex C tables for your conductor type.
Does the fill limit change for one or two conductors?
Yes. A single conductor in a conduit may occupy up to 53% of the internal area, two conductors are capped at 31%, and three or more drop to 40% (NEC Chapter 9, Table 1). The two-conductor 31% figure is the most restrictive.
Built for the field
- Live results as you type — every value recomputes instantly.
- Works fully offline once the app has loaded.
- Cites the governing NEC table for every result.
- Save calculations to a job and build a material list (Pro).
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For guidance only. Always verify against the current National Electrical Code and your local amendments. Fieldwatt does not replace an engineer of record.