Box Fill Calculator

Volume allowances

1. Volume allowance per conductor (12 AWG)
   2.25 cu in
   Ref: NEC 314.16(B)
2. Current-carrying and grounded conductors (equipment grounding excluded)
   6 x 2.25
   13.5 cu in
3. Equipment grounding conductors
   1 x 2.25
   2.25 cu in
4. Switches and receptacles (2 allowances each)
   1 x 4.5
   4.5 cu in
5. Internal cable clamps (1 allowance total)
   0 x 2.25
   0 cu in
6. Total volume required
   20.25 cu in
7. Box volume available
   21 cu in
   Ref: NEC 314.16(A)

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What box fill is about

A box has to have enough volume for everything inside it: the conductors, the splices, the equipment grounding conductors, the devices and the cable clamps. Box fill is the arithmetic that compares what goes into the box with the volume the box provides.

It is a volume check rather than a count of wires. Two boxes holding the same number of conductors can have different requirements, because the allowances depend on conductor size, on how many yokes the box carries, and on items that are not splices at all.

The calculator uses one conductor size for every non-grounding conductor. If the box has mixed sizes, enter the largest; the result is conservative because smaller conductors take less volume. The code itself applies each conductor's own size under NEC 314.16(B)(1).

What counts as an allowance

• Each conductor that originates outside the box and terminates or is spliced inside counts one volume allowance based on its size (NEC 314.16(B)(1)). An unbroken conductor of 12 in or more counts as two.
• A conductor that passes through the box without a splice or termination counts one allowance, unless it is 12 in or longer, in which case it counts as two.
• Equipment grounding conductors count as one allowance for up to four, plus 1/4 allowance for each additional conductor, based on the largest equipment grounding conductor (NEC 314.16(B)(5)).
• Each device or yoke counts as two allowances, based on the largest conductor connected to it.
• Internal cable clamps count as one allowance, based on the largest conductor in the box.

Where the available volume comes from

The volume available is a property of the box. NEC 314.16(A) lists the cubic-inch capacity of common box sizes and shapes, and a box may also carry its volume marked by the manufacturer. A plaster ring or extension ring adds volume only where it is listed and marked for that purpose.

Volume is not the outside dimensions. A deep box of the same footprint has more capacity, and a box with an internal barrier or a raised cover may have less usable space than its shape suggests.

Where this usually goes wrong

• Forgetting the equipment grounding conductor. Up to four count as one allowance, and each additional ground adds 1/4. It is not zero, and it is not one allowance per wire.
• Double-counting a device by adding a yoke allowance and conductor allowances for the same pigtails.
• Borrowing a volume figure from a similar-looking box. Volume is listed per box, not per shape.
• Assuming a plaster ring adds volume. Only rings marked with a volume count toward the total.

Frequently asked

How many wires can be in a box?

There is no fixed count, because the limit is volume rather than number of conductors. Each conductor takes a volume allowance based on its size, devices take two allowances each, and clamps take one, and the total has to fit inside the box capacity. Larger conductors consume more volume each, so the same box holds fewer of them.

Does a ground wire count in box fill?

Yes. Up to four equipment grounding conductors count as one allowance, based on the largest one, and each conductor beyond four adds 1/4 allowance under NEC 314.16(B)(5). Four 12 AWG grounds count the same as one. A fifth adds a quarter. Ignoring them, or counting each one in full, are both wrong.

Does a device count as two conductors?

A yoke or device counts as two volume allowances based on the largest conductor connected to it. That covers the space the device body occupies. A pigtail or jumper that begins and ends inside the box is not counted again under NEC 314.16(B)(1). Counting those internal wires as extra conductors double-counts the device.

What about a box with a plaster ring?

A plaster ring contributes volume only when it is listed and marked with a cubic-inch capacity. Rings that are not marked add nothing to the total, and a ring that is marked adds exactly the volume printed on it rather than the volume of the opening it covers.

Related guides and references

• How to calculate box fill
• Quick reference (printable PDF)