Box Fill Calculator

How it’s calculated. Every allowance comes from NEC 314.16(B), using the volumes in Table 314.16(B)(1): 18 AWG = 1.50, 16 = 1.75, 14 = 2.00, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 = 5.00 in³. The table ends at 6 AWG; 4 AWG and larger are sized by 314.28, not by this tool — the pull-box guide covers those.

Cables. A 12/2 w/g is two insulated 12 AWG conductors plus a bare 12 AWG ground; a 14/3 is three 14s plus a 14 ground. Each cable row expands that way, so two 12/2 cables become four counted conductors and two grounds. The ground size follows the cable as built — 14/x, 12/x and 10/x NM-B carry a ground of the same size, while 8/x and 6/x NM-B carry a 10 AWG ground; Type MC follows the same pattern except 6/x MC, which usually carries an 8. Check the print legend and override the ground size if yours differs. Conductors counted directly in the second block simply add to what the cables became.

Counting conductors — 314.16(B)(1). A conductor that comes in from outside and terminates or is spliced inside counts one. A conductor that passes through unspliced counts one — but if it is looped or coiled with at least 12 in. of unbroken conductor (twice the 300.14 free length) it counts two. A conductor no part of which leaves the box (a pigtail) counts zero. The cables’ grounds are never counted as conductors; they get their own allowance below.

Devices, clamps and fittings. Each device yoke counts 2× the largest conductor connected to that yoke (314.16(B)(4)) — not the largest in the box — and a device on a yoke or strap wider than 2 in. takes that double allowance for each gang it occupies, so enter the extra gangs those wide devices take (a 2-gang timer is one device plus one extra gang, 4×). Internal cable clamps add 1× the largest conductor in the box, once, however many clamps there are (314.16(B)(2)); MC and EMT connectors that clamp outside the box add nothing. A luminaire stud and a hickey each add 1× the largest conductor (314.16(B)(3)), one allowance per type.

Grounds — 314.16(B)(5). All equipment grounding conductors and equipment bonding jumpers together count one allowance at the size of the largest, for up to four; every one beyond four adds another ¼ allowance. The count defaults to one per cable and the size to the largest the cables carry; both can be overridden. If you count conductors by hand with no cable rows, Auto has nothing to read — pick the ground size yourself (the tool flags it), or choose None when the raceway is the ground. An isolated ground — the insulated green run to an isolated-ground receptacle under 250.146(D) — is an additional set, and 314.16(B)(5) gives that set its own full allowance at the largest conductor in it. Enter it in the isolated-ground field, not in the count above: folding it into the first group as a ¼ under-counts. Beyond four in that set the tool adds the same ¼ per conductor, the safe reading of the text.

The box — 314.16(A). The list is the complete 2023 Table 314.16(A), grouped the way the table groups it, each with its minimum volume. A box that is not in the table — every nonmetallic box, and any metal box of 100 in³ or less that the table does not list — must carry a marked volume (314.16(A)(2)), and that marking governs: type it in the volume field. “Other box — by inside dimensions” multiplies L × W × D for a box with no marking in front of you; use inside dimensions, because outside dimensions overstate the interior, and treat the result as a check, not a listing. Plaster rings, extension rings and domed covers add their volume when they are marked with it (or are made from a listed box size) — enter that marking in the ring field; an unmarked mud ring adds nothing.

What the result tells you. Required volume against the box (plus ring). When it does not fit, the flag names the shortfall in cubic inches and the two legal outs: more volume (a deeper box in the same family, or a marked ring or extension) or fewer conductors entering the box. “Smallest box that fits” is the smallest Table 314.16(A) row that holds the required volume; when you picked a box, the smallest in that family is shown too. This is an estimate — confirm against the box listing and the full design.

Worked example. The page loads with a duplex receptacle fed by one 12/2 NM-B and feeding another downstream: two cables become four 12 AWG conductors (4 × 2.25 = 9.00), the yoke counts twice the 12 landed on it (4.50), the box’s internal NM clamps count once (2.25), and the two 12 AWG grounds together count once (2.25) — 18.00 in³. A 4×1½ square (21.0 in³) passes with 3.00 to spare; a 3×2×3½ device box (18.0) is exactly full and passes; a 3×2×2¾ (14.0) is over. Add a third cable — a 12/3 NM-B bringing a second circuit through — and the count becomes seven conductors (15.75) plus the yoke (4.50), the clamps (2.25) and three grounds still at one allowance (2.25) = 24.75 in³: the 4×1½ square is now 3.75 short, and the tool names the smallest square that fits (a 4-11/16×1¼ at 25.5) — or a 4×2⅛ square (30.3), the usual answer off the truck. A deeper box is cheap; a failed rough-in inspection is not.

Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.