🧰 Field calculators · ⚡ Electrical
Box Fill Calculator — NEC 314.16
Add up the NEC 314.16(B) volume allowances for a box — conductors, device yokes, clamps, and grounds — and get the minimum cubic-inch volume plus the smallest standard box from Table 314.16(A) that fits.
Conductor size
Insulated circuit conductors
Devices (yokes — each counts double)
Equipment grounding conductors
Internal cable clamps
How box fill counting works
NEC 314.16(B) scores everything in the box as conductor-equivalent allowances. Each insulated circuit conductor entering the box counts once. Each device yoke or strap — a receptacle or a switch — counts double. Internal cable clamps count once total, no matter how many. Equipment grounding conductors count once for up to four combined, then a quarter allowance for each additional EGC — the quarters refinement is 2020-NEC and later. Conductors passing through the box unbroken still count once.
The total allowance count is then multiplied by the volume per conductor from Table 314.16(B) for your wire size: 2.0 in³ for 14 AWG, 2.25 for 12 AWG, 2.5 for 10 AWG, 3.0 for 8 AWG, and 5.0 for 6 AWG. This calculator assumes one conductor size in the box — the rough-in norm. Finally, it compares the required volume against the common standard metal boxes of Table 314.16(A) and suggests the smallest one that fits.
Worked example
A classic duplex-receptacle rough-in on 12 AWG: four circuit conductors (4) + one device yoke (2) + internal clamps (1) + two ground wires (1) = 8 allowances. At 2.25 in³ each, the box needs 18.0 in³ — a 3×2×3-1/2 device box exactly. If the math outgrows the largest common standard box (42.0 in³), use an extension ring or a larger enclosure.
Frequently asked questions
Do ground wires count in box fill?
Yes, but as a group: all equipment grounding conductors together count as one allowance for up to four, then each additional EGC adds a quarter allowance. The quarters rule appears in the 2020 NEC and later editions.
How much does a receptacle or switch count?
Every device yoke or strap counts as two conductor allowances at the volume of the largest conductor connected to it. A single duplex receptacle adds two allowances, not one.
Do wires that pass through the box count?
Yes — a conductor passing through the box unbroken still counts as one allowance. Only fixture wires and certain small equipment-bonding jumpers get excluded under 314.16(B)'s detailed provisions.
What box do I need for a duplex receptacle on 12/2 with ground?
A typical rough-in with four 12 AWG circuit conductors, one device, internal clamps, and two grounds totals 8 allowances × 2.25 in³ = 18.0 in³ — a 3×2×3-1/2 device box is the smallest common standard box that fits.
Learn the code behind this calculator
Master the table, not just the answer
This calculator reads the same tables inspectors quiz you on. CraftIQ turns them into real-jobsite puzzles — every answer cites the actual standard. Free for individual apprentices, in English or Spanish.
Practice electrical puzzles free