# Electrical Conduit Fill Calculator

> Check conduit fill from the conduit’s internal diameter and the conductor diameters, against a fill limit you take from your wiring code.

Category: Construction · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/electrical-conduit-fill-calculator

Conduit fill is the total cross-section of the conductors divided by the conduit’s internal cross-section. Enter the internal diameter and the fill limit from the tables you work to.

Also called: Conduit sizing calculator, Wire fill calculator.

## Inputs
- Conduit internal diameter (unit mm; default 25)
- Number of conductors (default 6)
- Conductor outside diameter (with insulation) (unit mm; default 4)
- Second conductor group: number (default ; optional)
- Second conductor group: diameter (unit mm; default ; optional)
- Allowable fill (unit %; default 40)

## Results
- Conduit fill
- Against your limit
- Conductor cross-section
- Conduit internal cross-section
- Allowed conductor area
- Maximum conductors of the first size

## Formula

```
Fill = Σ n × π d² ÷ 4 ÷ (π D² ÷ 4) × 100 = Σ n d² ÷ D² × 100
```

Where:
- n, d: number and outside diameter of each conductor group
- D: conduit internal diameter

## Worked example
1. Fill = 6 × 4² ÷ 25² = 96 ÷ 625 = 15.36%.
2. Conduit area = π × 25² ÷ 4 = 490.87 mm²; 40% of it is 196.35 mm².
3. Each conductor is 12.57 mm², so up to 15 fit within 40%.

**Result:** Six 4 mm conductors fill a 25 mm-bore conduit to 15.36%, well within a 40% limit.

## What the result means

Fill limits keep conductors from being damaged when pulled and allow heat to escape. Grouped circuits may also need current derating, which this calculator does not check.

## Assumptions and limits
- Conductors are round; their overall outside diameter is used.
- Fill limits depend on the number of conductors, conduit type and code; enter the one that applies.
- Jamming, bends and pull tension are not checked.

## Questions

### Why does the limit change with the number of conductors?

Many codes allow a higher fill for a single conductor than for several, because several round conductors pack less efficiently and are harder to pull.

### Can I use the nominal conduit size?

No. Use the internal diameter for your conduit type; it differs between materials with the same trade size.

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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/electrical-conduit-fill-calculator