// Article 314.16 · Standard & custom boxes
Box Fill Calculator
Count what goes in the box and this tells you whether it fits. Conductors, devices, clamps, grounds and studs all get their volume allowance and the total gets checked against the box.
The rules are not complicated, but three of them get counted wrong constantly: devices count double, all the grounds together count once, and internal clamps count once regardless of how many there are.
If you want the reasoning behind the numbers worked all the way through, read the box fill walkthrough.
How this calculation works
Box fill is a volume comparison. Every item in the box is converted to cubic inches using the largest conductor it is associated with, and the total has to be at or under the volume of the box.
What counts, and how much
- Each conductor that enters and terminates or splices counts once, at its own size.
- A conductor passing straight through without a splice counts once, if it is long enough to be a real conductor rather than a loop.
- A conductor that originates and ends inside the box — a pigtail — does not count at all.
- Each yoke or strap holding a device counts as two conductors, at the size of the largest conductor connected to that device. A single receptacle on 12 AWG eats 4.50 cubic inches before you count a single wire.
- All equipment grounding conductors together count as one, at the largest ground present. Six grounds in a box is one allowance, not six.
- All internal cable clamps together count as one, at the largest conductor in the box. Clamps outside the box do not count.
- Each fixture stud or hickey counts as one.
The two mistakes that fail inspections
The first is counting grounds individually. A four-cable box with a ground in each looks like it should be four allowances. It is one.
The second is forgetting that the device allowance uses the largest conductor on that device, not the largest in the box. Put a 12 AWG receptacle in a box that also carries a 10 AWG pass-through and the device still counts at 2.25 each, not 2.50.
A note on ganged and non-standard boxes: gang them and the volumes add. A two-gang assembly of 18 cubic inch boxes gives you 36. Anything not in Table 314.16(A) has its volume stamped inside by the manufacturer — use the custom option above and enter that number.
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Frequently asked questions
Do pigtails count toward box fill?
No. A conductor that both originates and terminates inside the box — a pigtail or jumper that never leaves — gets no volume allowance. The moment it leaves the box it counts.
How many grounds do I count?
One, no matter how many there are. All equipment grounding conductors in the box together count as a single volume allowance, taken at the largest ground present. This is the single most miscounted item in box fill.
Does a receptacle count as one item or two?
Two. Each yoke or strap containing one or more devices counts as double the volume of the largest conductor connected to it. A duplex receptacle is one yoke, so it is two allowances — not four.
What if the box is not in the table?
Non-standard boxes are required to be marked with their volume by the manufacturer, usually stamped inside the back. Use that number in the custom field. Boxes assembled from sections have their volumes added together.
Do plaster rings add volume?
Yes, if they are marked with their volume in cubic inches. Extension rings and plaster rings that carry a marking get added to the box volume. Unmarked ones do not count.
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