Box Fill Calculator
In the workshopNEC 314.16, pass or over
Store name: “Box Fill Calculator”
Coming soon
Not out yet — but it’s close.
Box Fill Calculator answers the question every rough-in comes down to: is this box big enough?
Free web tool
How do you calculate electrical box fill?
NEC 314.16 box fill is a sum of cubic-inch volume allowances, not a wire count. Count each insulated conductor as one volume of its size from Table 314.16(B) (#14 = 2.00 in³, #12 = 2.25 in³, and so on), add one allowance of the largest conductor for cable clamps, one per support fitting, two per device yoke, and one for all grounds together. The total is the minimum box volume the Code requires — compare it to your box's stamped or Table 314.16(A) volume to see if it fits.
Conductors — 314.16(B)(1)
Count every insulated conductor that enters the box — through, spliced, or pigtailed. Each is one volume of its size.
#18
1.50 in³ · fixture / low-voltage wire
#16
1.75 in³ · fixture wire, light fixtures
#14
2.00 in³ · 15 A lighting & receptacle circuits
#12
2.25 in³ · 20 A receptacle & small-appliance
#10
2.50 in³ · 30 A, water heaters, A/C
#8
3.00 in³ · 40 A, ranges, feeders
#6
5.00 in³ · 55 A, large feeders
Devices & fittings
Device yokes / straps
B(4) · each = 2 × largest conductor
Support fittings (studs / hickeys)
B(3) · each = 1 × largest conductor
Grounding — 314.16(B)(5)
Largest ground size
Minimum required volume
18.00in³
4 conductors counted · largest present #12
Test against a box
Smallest standard box that fits: 4 × 1¼ square (18 in³)
Fill breakdown
Per NEC 314.16(B), Tables 314.16(A) & (B) — the same engine as the app. A planning aid; it computes the Code minimum and does not replace the AHJ or a plan review.
Take it with you
The full Box Fill Calculator app works fully offline on the job site, saves and names your boxes, includes the complete Table 314.16(A) box picker and a max-conductors reference, and gives one-tap pass/fail right from your phone.
Coming soon to the App Store
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