Conduit fill calculator

Fill percentage = cable count × cable diameter² ÷ conduit internal diameter² × 100.

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Conduit internal diameter (mm)

Cable outside diameter (mm)

More inputs and assumptions

Cable count

Allowed fill (%) from your applicable specification

Answer

…

Fill percentage = cable count × cable diameter² ÷ conduit internal diameter² × 100.

More results

Precision and calculation record

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Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

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Formula and assumptions

Identical round cables inside a round conduit. Uses actual internal and outside diameters; trade sizes are not internal diameters. The allowed percentage is supplied by you, and the example is not a code determination. Mixed cable sizes, ampacity derating, bend radius and pulling constraints need separate assessment.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-29

Using Conduit fill calculator

Fill percentage = cable count × cable diameter² ÷ conduit internal diameter² × 100.

Next: Resistance calculator

A worked example

For conduit internal diameter (mm) = 25, cable outside diameter (mm) = 5, cable count = 8, allowed fill (%) from your applicable specification = 40, geometric fill (%) = 32.

Before you use the result

Example results for conduit internal diameter (mm)

These examples use Cable outside diameter (mm): 5; Cable count: 8; Allowed fill (%) from your applicable specification: 40. They are reference calculations, not recommended settings.

Conduit internal diameter (mm)	Geometric fill (%)
12.5	128
25	32
50	8

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