About Conduit Fill Calculator

Free NEC conduit fill calculations built for working electricians — no sign-up, no paywall, no guesswork.

Why We Built This

Conduit fill calculations are repetitive, table-lookup-heavy work. Every conduit type, trade size, wire insulation type, and gauge combination requires cross-referencing NEC Chapter 9, Tables 4 and 5 — and then applying the right fill percentage limit based on conductor count. On a busy job site, that's slow. On a bid estimate, it's tedious. Getting it wrong costs money in re-pulls.

Conduit Fill Calculator automates that process. Enter your conduit type, trade size, wire type, gauge, and count — and get the fill percentage plus NEC compliance status in seconds. The calculation applies the correct Table 4 and Table 5 values from NFPA 70 (NEC) 2023 Edition automatically.

The tool is free because this is the kind of resource that should exist for the trades without a subscription attached to it. Electricians, contractors, inspectors, and apprentices all benefit from a fast, accurate fill calculator — so we built one and made it openly available.

NEC-Accurate Calculations

Every fill percentage uses exact cross-sectional area values from NEC 2023 Chapter 9, Tables 4 and 5. No rounding shortcuts, no approximations.

Built for Real Job Sites

Quick presets cover common scenarios — residential branch circuits, HVAC feeds, sub-panel runs. Results include remaining area and max wire count, not just pass/fail.

Transparent Methodology

The calculation formula, NEC references, and data sources are documented on the calculator page. You can verify any result against the published NEC tables.

For Every Skill Level

A licensed master electrician can verify a quick field calculation. An apprentice can use it to learn how Table 4 and Table 5 interact. The tool works for both.

Our Editorial Process

Every calculation value on this site is sourced from primary standards documents:

  • NFPA 70 — National Electrical Code 2023 Edition: All conduit interior areas (Table 4), conductor cross-sectional areas (Table 5), and fill percentage limits (Table 1)
  • NEC 310.15 and associated tables: Ampacity values and derating factors referenced in blog posts
  • NEC 430: Motor feeder sizing requirements referenced in industrial content

Content is reviewed for factual accuracy before publication and updated when the NEC cycle changes values. If you find a calculation error or outdated reference, please report it via the contact email below — we take accuracy seriously.

Get in Touch

Found a calculation error, want to suggest a new feature, or have a question about the NEC values used?

contact@example.com