Conduit Fill: NEC Calculator +

Trusted NEC conduit fill with the exact code tables behind every result. Reverse sizing, derating, box fill.

Conduit Fill is the NEC conduit and box fill calculator built for electricians who cannot afford a wrong answer. Enter your conduit and conductors and see the fill percentage, the applicable maximum, and a clear pass or fail in under a second, fully offline on the job.

What sets it apart is proof. Tap Show math on any result to see the exact NEC Chapter 9 table references and figures behind the number, so you can stand behind your work in front of an inspector. It covers the whole rough-in decision: forward fill, reverse auto-sizing to the smallest compliant conduit, ampacity derating, and box fill per 314.16. Nipple mode applies the 60 percent allowance for runs of 24 inches or less, and a jam advisory flags tight three-conductor pulls. Switch between the 2017, 2020, and 2023 code editions. Big, glove-friendly controls and full dark mode keep it fast in the field.

Included in free tier: full forward conduit fill for every supported conduit type and conductor insulation, the fill percentage with pass or fail, the applicable maximum, nipple mode, the jam advisory, the code-year selector, the complete Show math breakdown with NEC table references, and up to three saved calculations.

Included in premium tier: Conduit+ unlocks reverse auto-sizing across every conduit type, the box fill calculator per 314.16, ampacity derating for more than three current-carrying conductors with ambient-temperature correction, unlimited saved calculations grouped by job or area, and text and PDF export for inspection prep. Premium is a subscription with a weekly plan that includes a three-day free trial, or an annual plan.

No account, no login, no ads, and no network connection are ever required, and every calculation runs on your device. Download Conduit Fill and pass inspection before you pull.

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