Box Fill Calculator
Engineering Specification Sheet
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Generated: August 10, 2026
Box Fill Calculator
Calculate electrical box fill volume to meet NEC 314.16 requirements. Prevent overcrowding and ensure safe wiring installations.
Box & Contents
Following the smallest box that fits. Turn off to pick your own.
Each gets its own gauge's allowance — mix them freely. Count conductors that originate outside and terminate, splice, or pass through. Pigtails made up entirely inside the box count as zero. Grounds go in their own field below.
Table 314.16(B) only tabulates 18 through 6 AWG. There is no volume allowance for 4 AWG and larger — boxes holding those are sized under 314.28 instead, so this tool stops at #6 rather than quietly computing zero.
Two allowances per yoke, at the largest conductor connected to that yoke — not the largest in the box. A device wider than a single 2 in. device box takes two allowances per gang it needs for mounting.
The first four count as one allowance between them, sized on the largest — they do not have to be the same size. Each one after the fourth adds a quarter allowance. Grounding pigtails that never leave the box count as zero.
One allowance per assembly, at the largest conductor terminated to it. 314.16(B)(6) did not exist before the 2023 edition.
314.16(A) lets you add a ring or cover's volume only where it is marked with it or dimensioned in Table 314.16(A). An unmarked mud ring contributes nothing — leave this at 0.
Box Fill Results
3 x 2 x 2-1/2 Device holds 12.5 in³ — headroom 0.50 in³
Segment heights are true relative volumes. A front view of the box, not a wiring diagram — conductors are not shown where they physically sit.
Allowance Breakdown (Table 314.16(B))
Wire nuts, locknuts, bushings, connectors that clamp outside the box, and pigtails that never leave the box are not counted — 314.16(B) excludes them.
These conductors have to get here
3 conductors enter this box. If they run in a raceway rather than cable, the same conductors have to satisfy Chapter 9 fill on the way in — a box that passes 314.16 says nothing about the conduit feeding it.
Check the conduit feeding this box →Calculations based on NEC 2023 guidelines. Results are for estimation purposes — verify against local code before installation.
Verified against NEC 2023 · Table 314.16(A), 314.16(B)
How this works
A junction or device box has to have enough free volume for every conductor, device, and clamp packed inside it, and NEC 314.16 sets the rules for counting that volume so a box that looks big enough on the shelf doesn't turn into a squeeze once everything's landed.
Not everything inside counts. Wire nuts, locknuts, and bushings are free. Every current-carrying conductor counts by its own gauge, and each device on a yoke, a switch or a receptacle, counts as two allowances based on the largest conductor connected to it.
Grounds get their own rule. Under the 2020 and 2023 NEC, up to four grounding conductors or bonding jumpers landing together count as one allowance sized on the largest of them, and each one past four adds a quarter allowance. Older code cycles, 2017 and before, count any number of grounds as a single allowance with nothing added past four. Pigtails that stay inside the box the whole time don't count at all.
Worked example
A device box with a 14/3 cable feeding a 3-way switch, a 12/2 cable feeding a receptacle, and internal cable clamps.
23.50 in³ needs at least a 4-11/16 x 1-1/4 square box (25.5 in³), smaller than the 4 x 2-1/8 square box (30.3 in³) some published versions of this example default to.
Frequently Asked Questions
How are wire nuts counted for box fill?↓
Wire nuts and other small fittings such as locknuts and bushings are not counted toward box fill volume calculations per the NEC.
How do you count a receptacle or switch for box fill?↓
Each yoke or strap containing one or more devices counts as two volume allowances based on the largest conductor connected to that device.
Do all ground wires count separately in a box?↓
No. Under NEC 314.16(B)(5), up to four equipment grounding conductors or bonding jumpers entering the box together count as one single volume allowance, sized on the largest of them — they do not have to be the same size. Each additional grounding conductor beyond the first four adds one quarter of an allowance. That quarter-allowance rule exists in the 2020 and 2023 editions; under NEC 2017 and earlier there was no additional allowance at all, and any number of grounds counted as one. Grounding pigtails that never leave the box are not counted.
Learn more
Box Fill Calculations: A Field Guide to NEC 314.16
How to calculate electrical box fill under NEC 314.16: conductor, device, and ground allowances, a full box volume table, and a worked example.
NEC Conduit Fill Calculation: How to Read Chapter 9 Table 1
Learn to calculate NEC conduit fill using Chapter 9 Table 1 and Table 4: the 40% rule, a worked example, and how EMT, PVC, and RMC compare.
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