Five paragraphs, added up
Box fill has a reputation for being fiddly, and it is really just five rules that get added together. Conductors count once each at their own size. Internal clamps count once in total, no matter how many there are. Each support fitting — a fixture stud or hickey — counts once. Each device yoke counts twice. All the grounds together count once. Then you compare that total against what the box actually holds.
The detail that trips people is which conductor size each rule uses. Only the first rule uses each conductor's own size. Clamps, fittings, yokes and grounds are all taken at the largest conductor in the box. So running a single 12 AWG into a box otherwise full of 14 AWG does not add 2.25 cubic inches — it also raises the clamp allowance and every yoke allowance to the 12 AWG figure. One wire can push a box over on its own.
The device yoke is the expensive one
A switch or receptacle takes a double allowance under 314.16(B)(4). At 12 AWG that is 4.5 cubic inches for the yoke alone, before a single conductor is counted — more than any other single item in a typical box. Anyone estimating fill in their head almost always counts the wires and forgets that the device itself is the largest consumer.
This is why the classic failure is so ordinary. Two 14/2 cables into a 3 × 2 × 2½ device box with one switch: eight cubic inches of conductor, two for the clamps, four for the yoke, two for the grounds. Sixteen cubic inches into a box that holds twelve and a half. That combination is in millions of houses, and it is the default loaded above precisely because it fails — a calculator that opens on a passing example teaches you nothing.
Grounds, and a 2020 change
Grounds have always been generous: however many equipment grounding conductors arrive, they shared a single volume allowance based on the largest one. The 2020 NEC narrowed that. The single allowance now covers up to four grounds, and each conductor beyond four adds a quarter allowance.
For nearly every residential box this changes nothing, because four grounds is already a busy box. It matters in a multi-gang box fed by several circuits, and it matters that the two editions genuinely disagree — so this calculator applies the 2020 rule and warns you on screen once you go past four, rather than silently picking an edition. Confirm which cycle your jurisdiction has adopted before leaning on a marginal result.
Use the marked volume when there is one
Table 314.16(A) lists the standard boxes, and this calculator carries all twenty of them. But the table is a fallback. A box marked with its own volume uses that figure, and where a plaster ring or extension ring is fitted, the assembly is the sum of the marked volumes. Those additions are frequently the difference between failing and passing, and they are stamped on the parts — worth reading before deciding a box has to come out.
Related checks that are separate calculations: how many conductors fit in the raceway feeding the box is a conduit fill question under NEC Chapter 9, and what size those conductors need to be is a wire gauge question under 310.16 and voltage drop. Box fill only asks whether what you have chosen will physically and legally fit in the enclosure.