How to Calculate Box Fill, Step by Step (and Do Grounds Count?)

NEC 314.16 turns everything in a box into cubic inches and compares the total with the box’s volume. The arithmetic is easy. What goes wrong is the counting: which conductors count, what a clamp or a device is worth, and at which wire size. That’s what this guide spends its time on.

Step 1: Find the box volume

For the metal boxes listed in Table 314.16(A), use the table volume: a 4” square, 1½” deep box is 21.0 in³, the 2⅛” deep version 30.3 in³. Every nonmetallic box, and any other box of 100 in³ or less that isn’t in the table, has to be marked with its volume by the manufacturer (314.16(A)(2)), so read it off the box. Don’t look up a plastic single-gang in the metal-box table.

A plaster ring, extension ring or domed cover adds its volume only if it’s marked with one or is made from a box in the table (314.16(A)). No stamp, no credit.

Step 2: Count the allowances

An allowance is one conductor’s worth of volume. 314.16(B) counts them like this:

What’s in the box	Allowances	At which size
Each conductor that comes in from outside and is spliced or terminated	1 each	its own size
A conductor passing through unbroken	1, or 2 if it’s looped and at least twice the 300.14 free length (12 in)	its own size
Pigtails and jumpers that never leave the box	0
Internal cable clamps, any number	1 total	largest conductor in the box
Fixture studs, hickeys	1 for each type (all the studs together count once)	largest conductor in the box
Each device yoke (switch, receptacle)	2, or 2 per gang for a device wider than a single gang	largest conductor connected to that device
All equipment grounding conductors	1 total, plus ¼ for each one beyond four	largest EGC

Wire connectors don’t count. Neither does a cable connector whose clamping part sits outside the box. 314.16(B)(1) also lets you leave out an EGC and up to four fixture wires smaller than 14 AWG that come into the box from a domed luminaire canopy.

Step 3: Multiply by the volume per conductor

Table 314.16(B) gives the volume per allowance: 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG, up to 5.00 in³ at 6 AWG. Multiply each count by its volume and add it up. If the total is at or below the box volume, the box is big enough.

Worked example 1: a 4” square junction box

Three 12/2 NM cables meet in a 4 × 4 × 1½" box (21.0 in³) with internal clamps. That’s six insulated 12 AWG conductors and three bare grounds.

• Conductors: 6 × 2.25 = 13.50 in³
• Clamps: 1 × 2.25 = 2.25 in³
• Grounds, all three together: 1 × 2.25 = 2.25 in³
• Total: 18.00 in³ against 21.0 in³, so it fits (86% full).

Do grounds count in box fill?

Yes, once for up to four. Up to four equipment grounding conductors in a box count as one allowance at the size of the largest EGC (314.16(B)(5)). In the example, the three grounds cost 2.25 in³. Leave them out and you’d get 15.75 in³, which understates the fill.

The catch: since the 2020 edition, the single allowance covers only four. Every EGC or bonding jumper beyond the fourth adds a quarter allowance at the largest EGC size. Six 12/2 cables meeting in a 4 × 4 × 2⅛" box bring six grounds, so they cost 1.5 allowances, 3.38 in³, and with the twelve insulated conductors and the clamps the box needs 32.63 in³ against 30.3 in³: it doesn't fit. Enter the number of grounds in the calculator and it adds the quarters.

Worked example 2: smallest device box for a switch

A single-pole switch with power in on one 14/2 and on to the light on a second 14/2: four insulated 14 AWG, two grounds, clamps, one yoke. The standard metal device boxes from Table 314.16(A), smallest first:

Only the 3 × 2 × 3½" (18.0 in³) passes. Every smaller metal device box fails, for one ordinary switch. Add a second switch in a two-gang and you’re counting again from scratch.

Worked example 3: mixed sizes on one device

A 4 × 4 × 2⅛" box with two 14 AWG and four 12 AWG conductors, a receptacle on the 12s, and 12 AWG grounds. The receptacle and the grounds count at 12 AWG, so the total is 19.75 in³ against 30.3 in³: it fits.

Move the receptacle onto the 14s and 314.16(B)(4) counts its yoke at 14 AWG, because the allowance follows the largest conductor connected to that device, not the largest in the box: 4.00 in³ instead of 4.50 in³. That’s why the calculator asks for the wire on each device. The grounds still count at 12 AWG, since (B)(5) goes by the largest EGC in the box.

How many 12 gauge wires in a 4x4 box?

With nothing else in it, a 4 × 4 × 1½" box has room for 9 conductors of 12 AWG (or 10 of 14 AWG), and the 4 × 4 × 2⅛" box for 13 of 12 AWG. Clamps, a device and the grounds all come out of that count, so a real box holds fewer.

Run it

Preset to worked example 1. Pick another box, add a device, or turn the grounds off to see what they’re worth.

What gets boxes written up

• Counting a device as one allowance instead of two.
• Counting each fixture stud separately, or forgetting the stud altogether.
• Adding a mud ring’s volume when the ring isn’t marked.
• Using 314.16 for 4 AWG and larger. Those boxes are sized by raceway trade size under 314.28: use the Pull Box calculator.

Raceways use the same idea by area instead of volume: how many 12 gauge wires in 3/4 conduit.

Every number in this guide is computed by the same engine as the Voltkit app. It cites NEC 2023 sections but doesn't reproduce NFPA text or tables; check the wording in your copy of the code and with your authority having jurisdiction.

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