Conduit fill calculator

Check how full a conduit gets before you pull. Enter the conduit interior diameter (mm or inches) and each cable size by diameter or AWG — the calculator sums the cable areas and shows the fill percentage against the 40% NEC limit for raceways with more than two cables.

Conduit

Conduit interior diameter

Unit

Interior, not outside, diameter — 1 in = 25.4 mm.

Cables

Enter each cable type by diameter (mm) or AWG size — use “count” for runs of the same size.

Count

Diameter (mm)

…or AWG

e.g. 14, 4/0 or 0000

Count

Diameter (mm)

…or AWG

e.g. 14, 4/0 or 0000

Fill result

Conduit interior area: 201.1mm²
Total cable area: 61.0mm²
Conduit fill: 30.3%

Within the NEC 40 % raceway limit.

Keep the fill legal

Conduit fill keeps cables cool and pullable. Match the conduit to the right conduit fittings and cable glands, and use the correct wire connectors at terminations.

Frequently asked questions

What is the NEC conduit fill limit?

The NEC raceway limit depends on how many conductors it carries: 53% for one conductor, 31% for two, and 40% for more than two. The calculator applies these from NEC Chapter 9 Table 1 and flags the fill in red when the result exceeds the applicable limit.

How do you calculate conduit fill percentage?

Fill is (Σ cable area) ÷ conduit interior area, where each cable is treated as a solid circle with A = π·(d/2)² and the conduit uses its interior diameter. Because real cables have insulation and a non-circular fill factor, actual fill is usually higher than this bare-conductor estimate.

Does AWG or diameter matter more for conduit fill?

Both work — the calculator accepts cables by conductor diameter in mm or by AWG size, converting AWG with d = 0.127 × 92^((36 − AWG) / 39). For the same reason it matters that you enter the conduit interior diameter, not the outside diameter (1 in = 25.4 mm).

How it works

Fill = (Σ cable area) ÷ conduit interior area. Each cable is treated as a solid circle A = π·(d/2)². Real cables have insulation and a non-circular fill factor, so actual fill is usually higher than this bare-conductor estimate — verify against the cable manufacturer's fill tables before a final design.

← Back to all calculators

More calculators

← Back to all calculators