What is box fill in electrical?

Box fill is the NEC's accounting for how much stuff can live in an electrical box. Every item claims a volume allowance, and the box's cubic inches must cover the total. The rules live in 314.16.

The counting: each insulated conductor entering the box counts once, all equipment grounds together count once, all internal clamps together count once, and each device on a yoke counts twice. Each allowance equals the volume for the largest wire in that category.

A 12 AWG allowance is 2.25 cubic inches, so a single-gang box crowds faster than it looks.

Overfilled boxes are why splices fail and devices break loose. When the count does not fit, the fix is a deeper box, not a harder push.

The box fill calculator does the 314.16 count and tells you the minimum box size.