How much volume does each conductor count as?

Each insulated conductor entering or passing through the box counts as one volume allowance from Table 314.16(B): 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, and so on by gauge. A conductor that runs through unbroken still counts once.

Why do devices count as two allowances?

NEC 314.16(B)(4) requires each yoke or strap, such as a receptacle or switch, to count as two volume allowances based on the largest conductor connected to it. This reserves room for the device body itself inside the box.

How are grounding conductors counted?

All equipment grounding conductors in the box together count as a single volume allowance based on the largest grounding conductor, per 314.16(B)(5), no matter how many grounds are present. A separate isolated ground system would add one more.

How do clamps and fixture studs count?

One or more internal cable clamps count as a single allowance based on the largest conductor, while each support fitting such as a fixture stud or hickey counts as one allowance each, both sized to the largest conductor in the box.

What if the box is overfilled?

An overfilled box violates 314.16 and risks damaged insulation and overheating. Move to a deeper or larger box with a higher listed volume, use a box extension ring, or reduce the number of splices in that location.

Box Fill Calculator (NEC 314.16)

Overfilling an electrical box damages insulation, makes terminations unreliable, and fails inspection. NEC 314.16(B) assigns a cubic-inch allowance to every item in the box; this tool sums them under the correct counting rules and checks the total against the box’s listed volume.

How it works

Each category gets a volume allowance, with clamps, studs, devices, and the grounding allowance all based on the largest conductor present:

conductors = count × allowance(conductor gauge)
devices    = device count × 2 × allowance(largest conductor)
clamps     = (any clamps ? 1 : 0) × allowance(largest conductor)
studs      = fitting count × allowance(largest conductor)
grounds    = (any EGC ? 1 : 0) × allowance(largest EGC)
required   = sum of the above

If the required fill is at or below the box’s listed volume the box is compliant; otherwise it is overfilled.

Example and notes

A box with six 12 AWG conductors (6 × 2.25 = 13.5), one device (2 × 2.25 = 4.5), internal clamps (2.25), and a 12 AWG grounding allowance (2.25) needs 22.5 cubic inches, so an 18-cubic-inch box would be overfilled and a deeper box is required. Note that the listed volume of a box is stamped on it or published by the manufacturer; never estimate it from outside dimensions, since internal volume is reduced by the box walls and any plaster ring adds capacity.