Box Fill Calculator
Required cubic inches vs your box — conductors, devices, grounds and clamps counted right.
Calculator
How this works
Box fill isn't about physically cramming wires in — it's a volume accounting system. Every item in the box gets a volume allowance based on conductor size, the allowances are summed, and the total must not exceed the box's cubic-inch rating:
The counting rules (widely published practice) are where people slip. Each circuit conductor entering or passing through the box counts once. All equipment grounds together count as one allowance of the largest ground. Each device on a yoke — switch, receptacle — counts as two allowances of the largest conductor connected to it. Internal cable clamps add one allowance of the largest conductor in the box, regardless of how many clamps. Pigtails that never leave the box count zero.
The calculator applies all of that and shows the line-by-line breakdown under the result, so you can see exactly where the cubic inches went — and how much headroom is left before you add "just one more cable" to the box.
Max circuit conductors by box size (grounds + one device present)
| Box | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|
| 3 × 2 × 2-1/4 in device box (10.5 in³) | 2 | 1 | 1 |
| 3 × 2 × 2-3/4 in device box (12.5 in³) | 3 | 2 | 2 |
| 3 × 2 × 3-1/2 in device box (18.0 in³) | 6 | 5 | 4 |
| 4 × 1-1/4 in square (18.0 in³) | 6 | 5 | 4 |
| 4 × 1-1/2 in square (21.0 in³) | 7 | 6 | 5 |
| 4 × 2-1/8 in square (30.3 in³) | 12 | 10 | 9 |
| 4-11/16 × 2-1/8 in square (42.0 in³) | 18 | 15 | 13 |
Assumes all equipment grounds together (one allowance) and one device (two allowances) of the same size, no internal clamps. Different mix? Use the calculator above.
Worked example: adding a receptacle mid-run
You're cutting in a receptacle on an existing 12-2 run — one cable in, one cable out, receptacle on the yoke. The plan is a standard 18 in³ old-work box. Does it fit the accounting?
Count it out. Two 12-2 cables mean four circuit conductors: 4 × 2.25 = 9.00 in³. The two bare grounds together count once: 2.25 in³. The receptacle takes two allowances of 12 AWG: 4.50 in³. No internal clamps on this box. Total: 15.75 in³ against an 18 in³ box — PASS, with 2.25 in³ of margin.
Now watch how fast that margin evaporates. Say you later want to tap a third 12-2 from this box to feed an outdoor light. Two more conductors is 4.50 in³, blowing the total to 20.25 in³ — over the box by 2.25. Same box, one extra cable, failed inspection. The fix is a deeper cut-in box (21 in³ or more) or a different junction point.
This is why box fill is the calculation worth doing before the drywall saw comes out: the difference between the right box and a callback is usually one size, known thirty seconds in advance.
Practical tips and common mistakes
- Read the stamp. Steel boxes usually have their cubic inches stamped inside; nonmetallic boxes have it molded in. Don't guess from outside dimensions — mud rings and raised covers add volume that plaster ears don't.
- Grounds count once, not per wire. Counting every ground separately is the most common way to oversize; forgetting them entirely is the most common way to fail. All grounds together = one allowance of the largest.
- Two devices means four allowances. A duplex receptacle is one yoke (two allowances), but a two-gang box with a switch and a receptacle carries two yokes — four allowances. Count yokes, not gangs.
- Unbroken pass-through wires count once each. A conductor that runs through the box without a splice still occupies volume. Only true pigtails (start and end in the box) are free.
- When in doubt, go deeper. The cost difference between a 18 in³ and a 22.5 in³ box is under a dollar. The cost of swapping a box behind finished drywall is a morning.
Frequently asked questions
How many 12 AWG wires can go in an 18 cubic inch box?
Eight, if nothing else is in the box: each 12 AWG counts 2.25 in³, so 8 × 2.25 = 18.0 exactly. But a real box has grounds (one shared 2.25 allowance) and usually a device (two allowances), which brings a switch box down to five circuit conductors. Run your actual box above.
Why does a switch or receptacle count as two conductors?
A device on a yoke takes up disproportionate room — the body, the ears, the folded conductors behind it. The widely used accounting assigns it two volume allowances of the largest conductor connected to it, which tracks the space it genuinely consumes in the box.
Do ground wires count in box fill?
Yes, but all equipment grounding conductors in the box together count as a single allowance, based on the largest ground present. Four grounds twisted under one wirenut still count once — that's why the calculator asks whether grounds are present rather than how many.
Do pigtails count toward box fill?
No. A conductor that originates and ends inside the same box — a pigtail from a wirenut to a device — doesn't count as a volume allowance. Only conductors entering or leaving the box count, plus wires that pass through unbroken, which count once each.
What size box do I need for three 12-2 cables and a receptacle?
Six circuit conductors (6 × 2.25 = 13.5), grounds together (2.25), and a device (4.5) totals 20.25 in³ — more than a common 18 in³ box. You'd need a 21 in³ or deeper box. This exact miss is one of the most common inspection failures; the calculator catches it in seconds.
Do internal cable clamps count?
Yes — one allowance of the largest conductor in the box, no matter how many clamps. External connectors like locknut-style NM connectors outside the box don't count. Check the box: molded-in clamps are common on nonmetallic new-work boxes.