Is your box big enough?
Enter the box volume and what goes inside it. This adds the required conductor, device, clamp and ground allowances under NEC 314.16 and tells you if the box passes before you make up the splices.
Your box
What this means
Required volume by conductor count
| Conductors | Required volume | Your box | Result |
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How box fill works
How NEC Box Fill Is Calculated
NEC 314.16 assigns a volume allowance to everything inside an outlet or device box, then requires the box to be at least that large. Each conductor gets a volume based on its size, each device yoke counts as two conductors, all internal clamps together count as one, and all equipment grounding conductors together count as one.
This calculator adds those allowances and compares the total to your box volume so you know whether the box is legal before you make up the splices.
Volume Allowances by Conductor Size
From NEC Table 314.16(B): 14 AWG is 2.00 cubic inches, 12 AWG is 2.25, 10 AWG is 2.50, 8 AWG is 3.00, and 6 AWG is 5.00. When conductors are different sizes, each is counted at its own value, and clamp, device, and ground allowances use the largest conductor present.
Common Counting Mistakes
People forget that a device yoke counts as two, that all grounds together still add one full conductor volume, and that pigtails entirely inside the box are not counted. Get the count right and an overcrowded box becomes obvious before it fails inspection.
Common questions
Does the ground wire count in box fill?
Yes. All equipment grounding conductors together count as one conductor volume, based on the largest grounding conductor in the box.
How much volume does a receptacle or switch use?
Each device yoke or strap counts as two conductor volumes based on the largest conductor connected to it. A single receptacle on 12 AWG adds 4.5 cubic inches.
Do pigtails count toward box fill?
No. Conductors that begin and end inside the box, such as pigtails, are not counted. Only conductors that enter the box are counted.
What is the volume allowance for 12 AWG?
Each 12 AWG conductor is 2.25 cubic inches under NEC Table 314.16(B).
My box fails. What are my options?
Use a deeper or larger box, add a box extender to gain volume, or reduce the number of conductors and devices in that box.
Estimates only. Always verify against the current NEC and your local amendments. Conductors of different sizes are each counted at their own value.