Conduit Fill Calculator
The fill percentage of a conduit from its internal diameter and the cables in it — against the 40% limit — and the most cables of that size the conduit can carry.
Cables must be able to be pulled through without damage and must shed heat, so codes cap how much of a conduit’s cross-section they may fill: 53% for one cable, 31% for two, 40% for three or more (the NEC figures, which most wiring practice follows).
How the conduit fill calculator works
Cables must be able to be pulled through without damage and must shed heat, so codes cap how much of a conduit’s cross-section they may fill: 53% for one cable, 31% for two, 40% for three or more (the NEC figures, which most wiring practice follows). Fill is the cables’ total cross-sectional area divided by the conduit’s internal area. The calculator uses outer diameters, so include the sheath.
Formula: fill % = n × (π d² ÷ 4) ÷ (π D² ÷ 4) × 100; limit 53% / 31% / 40% for 1 / 2 / 3+ cables
Worked examples
| Inputs | Conduit fill | Note |
|---|---|---|
| Three 6.5 mm cables in a 20 mm conduit | 31.7 % | 31.7% — within 40% |
| Four 0.25 in cables in ¾-inch EMT (0.824 in) | 36.8 % | 36.8% |
| One 12 mm cable in a 16 mm conduit | 56.3 % | 56% — over the 53% single-cable limit |
FAQFrequently asked questions
Why is one cable allowed 53% but three only 40%?
A single cable can be pulled straight; several cables jam against each other in bends, and the heat from bundled conductors builds up. The limits are empirical pulling-and-heating rules that codes have kept for decades.
Which diameter do I use?
The conduit’s internal diameter (trade sizes are nominal — a 20 mm conduit is about 17 mm inside, ¾-inch EMT 0.824 in) and the cable’s overall outer diameter including sheath, from the manufacturer’s data.
Does a long run change the answer?
Not the fill limit, but long runs and multiple bends make pulling harder; many electricians keep well under 40% on runs over 30 m or with more than two 90° bends.
Is this the same everywhere?
The 53/31/40 rule is from the NEC and is widely used as good practice; some jurisdictions publish their own tables. Bundling also reduces the current each cable may carry — a separate derating check.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.