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.

What is the Box Fill Calculator (NEC 314.16)?

Tabulates the NEC 314.16(B) volume allowances for conductors, devices, equipment grounding conductors, internal clamps, and fixture studs in cubic inches, then compares the total to the box's listed volume. For electricians selecting device and junction boxes. It runs free in your browser on Gera Tools, with nothing uploaded.

Box Fill Calculator (NEC 314.16)

Name: Box Fill Calculator (NEC 314.16)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Volume allowances by conductor gauge (NEC Table 314.16(B))

AWG	Cubic inches per conductor
18	1.50
16	1.75
14	2.00
12	2.25
10	2.50
8	3.00
6	5.00

These are the allowances for insulated conductors entering or passing through the box. Remember that each conductor counts once regardless of whether it enters, leaves, or runs completely through (a pass-through still counts as one allowance for the box it passes through).

Worked example — a typical device box

Imagine a single-gang box with:

Six 12 AWG conductors (two 12/2 cables, one 12/3): 6 × 2.25 = 13.5 in³
One duplex receptacle (device): 2 × 2.25 = 4.5 in³
Internal cable clamps (present): 1 × 2.25 = 2.25 in³
One equipment grounding conductor (12 AWG): 1 × 2.25 = 2.25 in³

Required fill: 22.5 cubic inches

A standard 14-cubic-inch single-gang box would be heavily overfilled. Even an 18 in³ box falls short. A 25 in³ or 26 in³ box, or a two-gang 36 in³ box used as a device box, would comfortably accommodate this fill.

Common boxes and their listed volumes

Box type	Approximate listed volume
Single-gang 2×4 switch box, 2-1/4” deep	12 in³
Single-gang 2×4 switch box, 2-3/4” deep	14 in³
Standard 2×4 box, 3-1/2” deep	21 in³
Old-work 2×3 box (20.5 series)	18 in³
Two-gang box, 3-1/2” deep	36 in³
4” square box, 1-1/2” deep	21 in³
4” square box, 2-1/8” deep	30.3 in³
4-11/16” square box, 2-1/8” deep	42 in³
Octagon box, 1-1/2” deep	15.5 in³

Listed volumes are marked on the box or available in manufacturer data. Never estimate volume from outside dimensions; wall thickness reduces internal space.

Tips for inspectors and electricians

When a box is overfilled, the solutions are: a deeper box of the same gang, a box extension ring (which adds its listed volume to the total), moving splices to a separate junction box, or reducing conductors in that location.
Pigtails that begin and end in the same box are not counted separately — they are already captured by the conductor that enters the box.
The grounding allowance covers all equipment grounding conductors collectively — not one per conductor. However, an isolated ground (IG) system adds one additional allowance beyond the standard EGC.