How to calculate box fill
The volume allowances come from your book. The counting is the part that gets it wrong, and it gets it wrong in both directions at once.
Box fill is a small calculation with a surprising number of ways to go astray. Almost none of them involve the volume figures themselves — those are a short column you read off once. The errors are in what gets counted, how many times, and at which size.
The rules, in full
- Each conductor entering and leaving: one allowance, at its own size.
- A conductor beginning and ending in the box: nothing. Pigtails are free.
- A conductor passing through unbroken: one, if short. Two if it is long enough to be spliced.
- All equipment grounds together: one allowance, at the largest ground present. Not one each.
- All internal clamps together: one allowance, at the largest conductor in the box. Clamps outside the box count for nothing.
- Each strap-mounted device: two allowances, at the largest conductor connected to it.
The two that get misremembered
They pull in opposite directions, which is why a box that "obviously fits" sometimes does not.
A device counts twice. A single receptacle on a strap consumes two conductor allowances. People count it once because it looks like one thing. On a small box with a couple of cables, this single error is often the whole difference between fitting and not.
All the grounds count once. Three, four, five grounding conductors together consume one allowance at the largest of them. Counting them individually overstates the fill and leads to buying a deeper box than the job needs — the safer error, but on a retrofit it can mean cutting drywall you did not need to cut.
Why pigtails are free
The allowance reserves room for a conductor's bulk and for the bending it does to get where it is going. A pigtail that starts and finishes inside the box adds bulk but no entry, no exit and no bend through the wall of the box. This is worth knowing because pigtailing is often what you do to avoid a fill problem in the first place.
Volume is not the only limit
A box that passes on volume can still be the wrong box. Conductor bending space at terminals, the depth needed behind a device that sits proud, and the box's own listing for cable entries are separate requirements a fill calculation says nothing about.
What this tool will not do
It contains no code tables. It does not know which edition your jurisdiction has adopted, what local amendments apply, or anything about your job, and it makes no determination of compliance. A licensed professional and the authority having jurisdiction decide that, and where they or the published code differ from the page, they are right.