📦What size box do you need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Box fill calculator

14 AWG conductors in the box

12 AWG conductors

10 AWG conductors

Devices (each switch or receptacle yoke)

Internal cable clamps

Grounding conductors

Box you plan to use

20.25 cu in needed

Smallest listed box that takes it: 4 x 1-1/2 square at 21 cu in

0.75 cu in spare

Largest conductor sets the allowance: 12 AWG at 2.25 cu in
Conductors counted: 6
Volume of the box you picked: 21 cu in

A jumper that starts and ends inside the box counts as nothing (314.16(B)(1)).

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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How do you calculate box fill?

The AmpSizer box fill result for six 12 AWG conductors, one device and a ground is 20.25 cubic inches, which needs a 4 × 1½ inch square box at 21.0 cu in and leaves 0.75 to spare.

The counting rules are all in 314.16(B), and each one has a trap in it. Every conductor entering and terminating gets one allowance. Each device yoke gets twice the largest conductor, not once. All the internal clamps together get one allowance, however many there are. All the grounding conductors together get one — also just one, no matter how many.

A pigtail that begins and ends inside the box counts as nothing at all, and a conductor passing straight through counts once rather than twice.

The allowances are short enough to memorise, and worth memorising because they turn a box question into mental arithmetic on site.

Conductor	Allowance each	Device yoke	Clamps, all together	Grounds, all together
14 AWG	2.00 cu in	4.00 cu in	2.00 cu in	2.00 cu in
12 AWG	2.25 cu in	4.50 cu in	2.25 cu in	2.25 cu in
10 AWG	2.50 cu in	5.00 cu in	2.50 cu in	2.50 cu in
8 AWG	3.00 cu in	6.00 cu in	3.00 cu in	3.00 cu in
6 AWG	5.00 cu in	10.00 cu in	5.00 cu in	5.00 cu in

The right-hand three columns are all keyed to the largest conductor in the box, so one 10 AWG conductor in a box otherwise full of 12s raises the cost of the device, the clamps and the grounds as well as its own allowance.

The boxes and what they hold

A quick way to read the standard sizes: a single-gang 3 × 2 device box is between 10 and 18 cubic inches depending on depth, a 4-inch square is 21 or 30, and a 4-11/16 square is 29.5 or 42. The jump from a shallow single-gang to a deep one is 8 cubic inches, which is three and a half 12 AWG conductors — often the whole difference between a legal box and an illegal one, for the price of a deeper can.

Why inspectors look here first

Box fill is the rare code requirement that can be checked from the doorway, without a meter, without turning anything off, and without trusting anybody’s word. Count the cables, look at the box, done. That makes it the cheapest thing on an inspector’s list — and the cheapest thing on yours to get right before they arrive.

Frequently asked questions

Do I count the pigtail in box fill?

No. 314.16(B)(1) only counts conductors that originate outside the box and terminate or pass through it. A jumper made up entirely inside the box — the classic pigtail from a wire nut to a receptacle screw — adds nothing to the count. People either forget this and end up with a bigger box than they need, or apply it to the incoming conductors too and end up with one that fails.

Why does a receptacle count as two conductors?

Because 314.16(B)(4) counts a double volume allowance for each yoke or strap holding a device, based on the largest conductor connected to it. It is a proxy for the physical space the device body occupies inside the box, which is why it scales with wire size and not with how many terminals you used.

How many 12 gauge wires fit in a standard plastic box?

A common 18 cu in single-gang plastic box takes six 12 AWG conductors with one device and a ground: that is 13.5 plus 4.5 plus 2.25, which is 20.25 — so it does not fit, and you need a 21 cu in box or larger. Four conductors with one device and a ground comes to 15.75 and fits comfortably. This is the calculation that decides whether a two-cable switch box is legal.

Do cable clamps count once each or once in total?

Once in total. All internal cable clamps together get a single allowance based on the largest conductor in the box, so four clamps cost the same as one. External connectors — the kind that clamp on the outside of a metal box — cost nothing at all, which is a practical reason to prefer them on a crowded box.

Where do I find the volume of a plastic box?

Moulded into it, usually on the inside back wall or on a mounting ear. Non-metallic boxes are required to be marked with their volume, and the marked figure is the one to use rather than a table. Metal boxes without a marking use the listed volumes in Table 314.16(A), which are the sizes in the dropdown here.

How we calculate this

Volume allowances are NEC 314.16(B) Table 314.16(B): 2.00 cu in per 14 AWG conductor, 2.25 per 12 AWG, 2.50 per 10 AWG, 3.00 per 8 AWG and 5.00 per 6 AWG. Each conductor originating outside the box and terminating or passing through counts once; a conductor entirely inside the box counts as none (314.16(B)(1)). Each yoke or strap containing a device counts twice the allowance of the largest conductor connected to it (314.16(B)(4)). One or more internal cable clamps together count one allowance for the largest conductor (314.16(B)(2)), and one or more equipment grounding conductors together count one (314.16(B)(5)). Box volumes are the listed figures of Table 314.16(A) for standard metal boxes; non-metallic boxes are marked with their own volume and that marking governs.

Worked example

With 14 AWG conductors in the box 0, 12 AWG conductors 6, 10 AWG conductors 0, Devices (each switch or receptacle yoke) 1, Internal cable clamps None — external connectors, Grounding conductors Yes — all of them count once, Box you plan to use 4 x 1-1/2 square — 21 cu in, this page works out 20.25 cu in needed. Smallest listed box that takes it: 4 x 1-1/2 square at 21 cu in

Largest conductor sets the allowance12 AWG at 2.25 cu in
Conductors counted6
Volume of the box you picked21 cu in

Sources:

The three ways a box gets overfilled

Adding a circuit to an existing box. The box was right for two cables and now has three. This is the most common overfill in a renovation, and it is invisible until the cover will not sit flat.

Smart devices and dimmers. The allowance for a yoke assumes an ordinary device body. A dimmer with a heat sink or a smart switch with a radio inside it physically occupies far more than the arithmetic allows for, even when the arithmetic passes.

Grounding pigtails. All grounds together get one allowance, so the maths rarely fails. The physical bundle of five grounds plus a nut, folded behind a device, very much can.