How the calculation works
NEC 314.16(B): each conductor = 1 allowance (volume from Table 314.16(B)); all internal clamps together = 1; equipment grounding conductors count as one allowance for up to four, plus 0.25 allowance for each one beyond four; each device yoke = 2 allowances. This tool assumes all conductors are the same size; mixed sizes use the largest per allowance rule. CEC Rule 12-3034 uses similar methods.
Worked example
6 × 12 AWG + grounds + 1 device in a 4×2⅛" square (30.3 in³): (6+1+2)×2.25 = 20.25 in³ → pass.
Common mistakes to avoid
- Counting a device once; each yoke or device counts as two conductor allowances.
- Forgetting internal cable clamps or the grounding allowance.
- Counting pigtails that start and end inside the box; they are not counted.
Frequently asked questions
Do fittings count?
Yes: fixture studs and hickeys add 1 allowance each (314.16(B)(3)); not modeled here.
Do wire connectors count toward box fill?
No. Wire nuts and similar splicing devices are not counted.
What if the box volume is not marked?
Standard metal boxes use the volumes in NEC Table 314.16(A). Boxes with a stamped volume use that marking.
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