Where is box fill in the NEC?

Box fill lives in NEC 314.16. Table 314.16(A) lists standard metal box sizes with their volumes and conductor capacities; 314.16(B) gives the volume allowance per conductor size, from 1.5 cubic inches for 18 AWG to 5 for 6 AWG.

The companion rules sit nearby in Article 314: 314.16(B)(1) through (B)(5) spell out the counting rules for conductors, clamps, fittings, devices, and grounds.

Bigger conductors move you out of box fill entirely: splices at 4 AWG and larger follow the pull-box sizing rules of 314.28, which work by raceway size rather than volume.

Nonmetallic boxes skip the table; their volume is stamped inside the box itself.

The junction box size calculator applies 314.16 or 314.28 depending on what you are splicing.