Box fill calculator
Count what goes in the box. The calculator adds up the allowances from 314.16(B) and tells you the smallest box volume you can use.
Built on the 2026 NEC · Free, no sign-up · Updated October 2026
Result
29.25 in³
How it was calculated
- allowances = 8 conductors + 0 clamp + 1 grounding + 4 device
- required volume = 13 x 2.25 cu in
Assumptions
- 12 AWG conductors
- 2 device yoke(s) counted as 4 conductor allowance(s)
- 2 equipment grounding conductor(s) counted as 1 allowance(s)
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
How to count the box
Each item in the box counts as a number of allowances. Each allowance is a volume that depends on conductor size, from Table 314.16(B)(4): 2.00 in³ for 14 AWG, 2.25 for 12, 2.50 for 10, 3.00 for 8, and 5.00 for 6.
- Conductors: each conductor that comes into the box from outside and is spliced or terminated counts once. A conductor that passes straight through without a splice counts once. A pigtail that never leaves the box doesn't count.
- Device yokes: each yoke or strap holding a receptacle, switch, or similar device counts as two allowances, sized by the largest conductor connected to that device.
- Internal cable clamps: one allowance total if the box has any, sized by the largest conductor in the box. A connector clamped outside the box counts nothing.
- Equipment grounding conductors: up to four count as one allowance together. Each one past four adds a quarter allowance.
- Luminaire studs and hickeys add one allowance for each type present, and terminal blocks add one per block. This calculator doesn't model those two, so add them by hand if you have them.
Locknuts, wire connectors, and bushings don't count.
Finding the box volume
Metal boxes in Table 314.16(A) have their volume listed in the table. Nonmetallic boxes and boxes of 100 in³ or less that aren't in the table are marked with their volume by the manufacturer. Plaster rings, extension rings, and domed covers marked with a volume add to the box volume.
If the box volume is equal to or larger than the result, the fill is OK.
Common mistakes
- Counting each ground as a full allowance. All grounds up to four count as one.
- Counting the device once. A yoke is a double allowance.
- Using the smallest conductor size when sizes are mixed. Clamps, grounds, and devices are sized by the largest conductor involved. If conductor sizes differ, count each size at its own volume. This calculator uses one size for everything, so enter the largest to stay on the safe side.
2023 to 2026: our list has 5 changes in Article 314. See the changes
Worked example
A switch box with 8 insulated 12 AWG conductors, 2 grounds, 2 devices, and no internal clamps:
- Largest conductor size (AWG): 12
- Insulated conductors: 8
- Equipment grounding conductors: 2
- Device yokes or straps: 2
- Box has internal cable clamps: no
Result
29.25 in³
How it was calculated
- allowances = 8 conductors + 0 clamp + 1 grounding + 4 device
- required volume = 13 x 2.25 cu in
Assumptions
- 12 AWG conductors
- 2 device yoke(s) counted as 4 conductor allowance(s)
- 2 equipment grounding conductor(s) counted as 1 allowance(s)
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
Questions electricians ask
What does a box fill calculator count, and does this one follow the NEC?
It adds up the conductors, device yokes, internal clamps, and equipment grounds in a box and returns the minimum box volume in cubic inches. This calculator follows the NEC 2026 method in 314.16(B) and Table 314.16(B)(4).
Does it work for the Canadian Electrical Code?
No. It is built on the NEC, not the Canadian code. If you work under the Canadian code, size the box with that code's own tables.
Does it handle large conductors such as 500 kcmil?
No. The allowance table stops at 6 AWG, and so does this calculator. Boxes that hold larger conductors are sized under a different rule, so don't enter them here.
Do grounding conductors count in box fill?
Yes. Up to four equipment grounding conductors count as one allowance, sized by the largest one. Each additional ground adds a quarter allowance, per 314.16(B)(5).
Do wire nuts count in box fill?
No. Splicing connectors, locknuts, and bushings need no allowance under 314.16(B).
How much does a receptacle or switch count?
Each yoke or strap counts as two conductors of the largest size attached to it. A device wider than a single 2 in. gang counts double for each gang it needs.
Does a pigtail count?
No. A conductor that never leaves the box isn't counted.
Practice it hands-on
Click what counts inside the box, pick the smallest box that holds it, and close the cover.
Teach it in one class
Free slides, quiz, and instructor note: Box fill (NEC 2026 314.16).
How this calculator was built
The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.
How we test the calculators. See something wrong? Tell us and we will check it against the code.
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Have a code question the calculator doesn’t cover?
Ask Shorty in plain English. You get a straight answer with the NEC 2026 section it came from.