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Free tool · NEC 314.16
Box Fill Calculator
Enter what's in the box. The calculator applies NEC 314.16(B) volume allowances and tells you whether the box is big enough — with the full breakdown so you can see the math.
1 · Conductors
How many of each size terminate or pass through the box. (Pigtails that originate inside the box don't count.)
2 · What's in the box
3 · The box
How the Math Works
Every volume allowance comes from Table 314.16(B): 18 AWG = 1.50, 16 = 1.75, 14 = 2.00, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 AWG = 5.00 in³. The counting rules:
- Each conductor that terminates or is spliced in the box = 1 allowance [314.16(B)(1)]
- A conductor passing straight through unspliced = 1; looped for future use = 2
- Internal cable clamps, all together = 1 allowance at the largest conductor size [314.16(B)(2)]
- Each device yoke = 2 allowances at the largest conductor connected to it [314.16(B)(4)]
- All equipment grounding conductors together = 1 allowance at the largest ground size [314.16(B)(5)]
- Pigtails and conductors originating inside the box = 0
Box volumes from Table 314.16(A). When in doubt, read the volume stamped inside your actual box — the marked volume always wins. Want the full walkthrough? Box fill, step by step.
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