Box Fill Calculator — NEC 314.16 Cubic Inches
Box fill is a volume calculation with five allowance categories, and the trick is that they do not all count the same way. Conductors count individually. All the grounds count as one. All the clamps count as one. Each device yoke counts as two. Get those multipliers right and the arithmetic is easy — get them wrong and you either fail inspection or buy boxes you didn't need.
Count what's in the box
Clamp, device, and fitting allowances are all based on this size.
All of them together count as one.
Receptacles, switches, dimmers — each counts double.
Fixture studs or hickeys — each counts double.
Volume required
3" × 2" × 3-1/2" device box meets the minimum with almost no margin — go one size up for a workable box.
Smallest box that works
3" × 2" × 3-1/2" device box
Spare volume in that box
0.00 in³
See the breakdown
Plaster rings, extension rings, and domed covers add their own marked volume — add that to the box volume before comparing. The Code minimum is not the same as a comfortable install.
The formula, explained in plain English
One unit volume, set by the largest conductor in the box, multiplied by five different counts.
Grounds are one, always
Two grounds or eight, they collectively cost one conductor volume. This is the allowance people most often over-count, which leads to buying boxes a size too big.
Devices are the expensive part
At two volumes each, a single receptacle costs 4.50 in³ on a 12 AWG job — a quarter of an 18 in³ box before any wire goes in. Two devices in one box is where things get tight fast.
Largest conductor sets everything
One 10 AWG conductor in a box otherwise full of 12 AWG raises the clamp, device, and fitting allowances to 2.50 in³ each. Mixed-gauge boxes cost more volume than people expect.
Through-conductors count once
A conductor passing through unbroken counts as one, not two, even though it physically crosses the box twice. Splice it and it is still one — the splice itself adds nothing.
Worked examples
The everyday receptacle, a three-way switch that forces a 4-inch square, and the smallest legal box on the market.
Mid-run receptacle — two 12/2 cables
12 AWG · 2 hots · 2 neutrals · 2 grounds · internal clamps · 1 receptacle. The defaults above.
grounds (2 count as 1): 1 × 2.25 = 2.25 in³
clamps (all count as 1): 1 × 2.25 = 2.25 in³
device yoke: 2 × 2.25 = 4.50 in³
→ 18.00 in³ required
Result: an 18 in³ box meets this exactly — zero spare volume. It is legal, but a 20.3 or 22.5 in³ box costs pennies more and leaves room to actually fold the wires in.
Three-way switch — two 12/2 plus one 12/3
12 AWG · 4 hots (including the traveller pair) · 2 neutrals · 3 grounds · internal clamps · 1 switch.
grounds (3 count as 1): 2.25 in³
clamps: 2.25 in³
device yoke: 4.50 in³
→ 22.50 in³ required
Result: past every standard single-gang device box except a 22.5 in³ plastic box, which it fills exactly. In metal, this is a 4-inch square box with a plaster ring. Two extra conductors added 4.50 in³ and cost a box size.
Switch loop — one 14/2 cable
14 AWG · 1 hot · 1 neutral · 1 ground · internal clamps · 1 switch.
grounds: 2.00 in³ · clamps: 2.00 in³ · device: 4.00 in³
→ 12.00 in³ required
Result: a 12.5 in³ box works. Note that two-thirds of this total comes from the clamps, the ground, and the device — not from the conductors. On small boxes the fixed allowances dominate.
Common box volumes
Standard boxes with the maximum number of conductors each holds on its own — no clamps, devices, or grounds. Add those allowances and the usable conductor count drops fast, which is what the calculator above accounts for.
| Box | Volume | Max 14 AWG | Max 12 AWG | Max 10 AWG |
|---|---|---|---|---|
| 3" × 2" × 2" device box | 10.0 in³ | 5 | 4 | 4 |
| 3" × 2" × 2-1/4" device box | 10.5 in³ | 5 | 4 | 4 |
| 3" × 2" × 2-1/2" device box | 12.5 in³ | 6 | 5 | 5 |
| 3" × 2" × 2-3/4" device box | 14.0 in³ | 7 | 6 | 5 |
| 3" × 2" × 3-1/2" device box | 18.0 in³ | 9 | 8 | 7 |
| Single-gang plastic box, 20.3 in³ | 20.3 in³ | 10 | 9 | 8 |
| 4" square × 1-1/2" box | 21.0 in³ | 10 | 9 | 8 |
| Single-gang plastic box, 22.5 in³ | 22.5 in³ | 11 | 10 | 9 |
| 4-11/16" square × 1-1/2" box | 29.5 in³ | 14 | 13 | 11 |
| 4" square × 2-1/8" box | 30.3 in³ | 15 | 13 | 12 |
| 4-11/16" square × 2-1/8" box | 42.0 in³ | 21 | 18 | 16 |
Sources & standards: NEC (NFPA 70) 2023 — 314.16(A) and Table 314.16(A) box volumes, 314.16(B)(1) through (B)(5) fill allowances, 314.16(B)(2) clamp allowance, 314.16(B)(4) device yoke allowance, 314.16(B)(5) equipment grounding conductor allowance, 314.16(C) for conduit bodies. Non-metallic boxes must be marked with their volume. Local amendments override the model code.
Frequently asked questions
Common questions about NEC 314.16 box fill and what counts.
How do I count grounds in box fill?
All equipment grounding conductors in the box together count as one single conductor volume, based on the largest EGC present — per NEC 314.16(B)(5). Six grounds pigtailed together still count as one. If there is a separate isolated equipment grounding conductor group, that adds one more volume. This is the single most misunderstood part of 314.16.
Do wire nuts, pigtails, and cable staples count?
No. NEC 314.16(B) has exactly five allowance categories: conductors, cable clamps, support fittings, device yokes, and equipment grounding conductors. Wire connectors, pigtails made up entirely inside the box, and external staples or straps are not on the list and get no allowance. A conductor that both originates and terminates inside the box — a pigtail — is not counted at all.
How many wires can go in a single-gang box?
It depends entirely on the box's marked volume and what else is inside. A common 18 in³ single-gang box holds 8 × 12 AWG conductors on their own. Add internal clamps and one receptacle and the same box only takes 4 current-carrying conductors plus grounds — which is exactly two 12/2 cables. That is the default shown above.
Does a smart switch or dimmer change the count?
Not for the box-fill arithmetic — NEC 314.16(B)(4) gives every device yoke two conductor volumes regardless of what the device is, unless the device is wider than a single 2-gang yoke, which gets four. Physically, though, deep smart switches and dimmers crowd a box badly. Most manufacturers recommend a 20+ in³ box even when 314.16 permits smaller, and the Code minimum is not the same as a workable install.
Do all the clamps count once, or each?
Once, in total. NEC 314.16(B)(2) allows a single conductor volume for all internal cable clamps in the box combined, based on the largest conductor present. Four clamps still cost you one volume. Note this applies only to clamps inside the box — external cable connectors on a metal box are not counted.
What about a conductor that just passes through?
It counts as one volume. NEC 314.16(B)(1) counts each conductor that runs through the box without splice or termination the same as one that terminates there. A conductor entering and leaving unbroken is one conductor, not two — even though it crosses the box twice.
Where do I find my box's cubic inches?
Non-metallic boxes are required to be durably and legibly marked with their volume, usually moulded into the inside back or on the mounting flange. Standard metal boxes are not always marked, so their volumes come from NEC Table 314.16(A) by trade size. Plaster rings, extension rings, and domed covers add their own marked volume to the total.
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