Box fill is how the NEC decides whether an outlet, device, or junction box is big enough for what lives inside it. Each insulated conductor entering or leaving the box counts one volume allowance from Table 314.16(B) — 2.00 in³ for 14 AWG rising to 5.00 in³ for 6 AWG — and further allowances stack on top: internal cable clamps together add one allowance of the largest conductor present, each stud or hickey adds another, every device yoke counts two allowances of the largest conductor connected to it (a device wider than a single gang counts as two yokes), and the equipment grounding conductors collectively add one, plus a quarter allowance apiece beyond four under the 2023 code.
The bookkeeping has real teeth. Two 12/2-with-ground NM cables feeding one duplex receptacle in a clamped single-gang box demand 9.00 + 2.25 + 2.25 + 4.50 = 18.00 in³ — an 18.0 in³ box sits exactly at the limit. Pigtails that begin and end inside the box count nothing; a conductor looped through with twice the 300.14 free length counts twice. The available volume is the manufacturer marking on nonmetallic boxes or Table 314.16(A) for standard metallic ones (a 4 × 1-1/2 in octagon is 15.5 in³, a 4-11/16 × 2-1/8 square 42.0 in³), plus any marked rings and covers. An overfilled box is a genuine hazard, not a paperwork problem: crowded conductors get damaged during device installation, splices are stressed, and trapped heat degrades insulation toward arcing faults.