What is the 40% rule?

NEC Chapter 9 limits fill to 40% for three or more conductors.

Does the ground count?

Yes, equipment grounding conductors count toward fill.

What about EMT or PVC?

Inside diameters differ; this tool uses corrugated conduit sizes.

How many 12 AWG wires in ½ in EMT?

Up to nine THHN conductors per NEC Annex C.

EMT indoors; PVC underground or in wet locations.

Does the neutral count toward fill?

Yes — every conductor counts.

Conduit fill calculator

Number of conductors and size: you get the minimum conduit that stays under 40% fill and the actual fill percentage.

Minimum conduit

16 mm Ø

Minimum conduit (outside diameter)16 mm Ø
Fill of that conduit33.96 %
Area taken by conductors30.54 mm²

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How it's calculated (visible formula)

fill = Σ π/4 × d_wire² ÷ (π/4 × d_conduit,inside²) ≤ 40%

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Count conductors in the conduit, including grounds.
Choose conductor size.
Add up the cross-section of all wires.
Find the conduit whose inside area keeps fill at 40% or less.
Check bends and pull-box spacing.

Worked example: 3 conductors of 2.5 mm² (≈ 12 AWG)

3 × π/4 × 3.6² = 30.5 mm²
16 mm conduit (10.7 mm ID): 34% fill

→ 16 mm (½ in trade size) conduit.

Common mistakes

Filling to the limit — no room to pull later.
Not counting the ground wire.
Forgetting bends make pulling harder.

Pro tips

Leave room for future circuits — go one size up.
Add pull points after 360° of bends.
Use pulling lubricant on long pulls.
Don't mix voltage systems in one raceway.

From calculation to quote

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FAQs

What is the 40% rule?: NEC Chapter 9 limits fill to 40% for three or more conductors.
Does the ground count?: Yes, equipment grounding conductors count toward fill.
What about EMT or PVC?: Inside diameters differ; this tool uses corrugated conduit sizes.
How many 12 AWG wires in ½ in EMT?: Up to nine THHN conductors per NEC Annex C.
EMT or PVC?: EMT indoors; PVC underground or in wet locations.
Does the neutral count toward fill?: Yes — every conductor counts.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.