Electrical Box Fill Calculator
Estimate required box-fill volume from conductor groups and explicit code-related allowances, then compare it with the marked box volume.
Box-fill inputs
Box-fill result
Box-fill calculation method
The calculator multiplies the entered conductor count by the entered individual conductor volume, then adds the explicit clamp, device and grounding allowances.
Required volume = conductor volume + clamp + device + grounding allowances
Why an explicit conductor volume is useful
Conductor volume varies with conductor size. Entering the applicable volume directly also avoids falsely implying that one universal number applies to every wire.
Worked example
An 18 in³ box with eight conductors at 2 in³ each has 16 in³ of conductor volume. Adding 2 + 2 + 2 in³ allowances produces 22 in³ required volume, so that example would not fit the entered box.
Code context
NEC 314.16 establishes specific box-fill rules and allowances. The correct calculation can depend on conductor entry, splices, clamps, devices, grounding conductors and internal fittings. Verify the applicable edition and local adoption.
Frequently Asked Questions
Can the box volume be entered from its marking?
Yes. Enter the manufacturer's marked cubic-inch volume where applicable.
Can allowance values be zero?
Yes, when the corresponding condition is genuinely absent.