Conduit Fill Calculator
This free conduit fill calculator turns your conduit and conductor list into a fill percentage, the applicable NEC limit, and a clear pass or over-limit answer — instantly.
- Free · no sign-up
- Runs in your browser
- Estimates only — always verify with your AHJ
- 1Choose conduit
- 2Add conductors
- 3Read fill & verdict
How it works
- Step 1
Pick conduit type & trade size
Choose your raceway type and trade size from the two dropdowns.
- Step 2
List your conductors
Add a row for each group of wires: insulation, wire size and quantity.
- Step 3
Read fill %, limit and verdict
See the fill percentage, the applicable NEC limit, and a clear pass or over-limit answer.
Method & sources
Fill limits come from NEC Chapter 9, Table 1, cited by reference: one conductor may occupy up to 53% of a conduit's internal area, two conductors up to 31%, and three or more up to 40%. A multi-conductor cable counts as one conductor. Short runs, certain fittings and other exceptions can change which rule applies — this tool indicates the common rule only.
Conductor areas come from manufacturer-published dimensions for each insulation type and wire size. Conduit internal areas come from published dimension data for each conduit type and trade size. Where standard tables exist, we cite them by reference rather than reproduce them.
Reference edition: NEC 2026. States and local jurisdictions adopt code editions on different schedules, so the edition enforced where you work may differ; results always note the reference edition used.
WiringCalc is an estimating tool for trade users, not a code compliance certification. Always verify against the currently adopted code and consult a licensed electrician or your Authority Having Jurisdiction (AHJ) before installation.
Frequently asked questions
What is conduit fill percentage?
Conduit fill percentage is the share of a conduit's internal cross-sectional area that the conductors inside it take up. This free conduit fill calculator compares your conductor list against the conduit's published internal area and shows the percentage, the applicable limit, and a pass or over-limit verdict instantly.
Why are the limits 53%, 31% and 40%?
They come from NEC Chapter 9, Table 1 (cited, not reproduced): one conductor may use up to 53% of the conduit's internal area, two conductors up to 31%, and three or more up to 40%. A multi-conductor cable counts as one conductor. Short runs and certain fittings can change which rule applies — this tool flags the common rule only.
Does the insulation type matter?
Yes. Insulation changes a conductor's outside diameter, so the same wire size takes different area: THHN/THWN-2 is the most compact common type, while THW, XHHW-2 and USE/RHH are progressively larger. Pick the insulation printed on the wire you are actually installing.
Which conduit types are supported?
EMT, IMC, RMC, PVC Schedule 40 and PVC Schedule 80, in trade sizes from 1/2 in through 4 in. Conduit internal areas come from published dimension data for each type and trade size.
Is it really free, and are my inputs stored?
Yes — it is free with no sign-up. Calculations run locally in your browser. Your conductor inputs are not uploaded or stored.
Which NEC edition does the calculator use?
The reference edition is NEC 2026. States and local jurisdictions adopt code editions on different schedules, so the edition enforced in your area may differ. Always verify against the currently adopted code in your jurisdiction.
More calculators in the works
- Wire Size CalculatorPlanned
- Electrical Load CalculatorPlanned
- Transformer Sizing CalculatorPlanned
- Conduit Bend CalculatorPlanned