Junction Box Sizing Calculator — NEC 314.16(B)
Size your junction, device or utility box correctly using the NEC 314.16(B) fill method. Enter the conductor gauges and counts, ground wires, devices and internal clamps — the calculator adds up the required cubic-inch volume and recommends a standard box.
Ground wire gauge
Internal cable clamps
Minimum box: 4 × 1¼ in square (18 in³)
- 1
Conductor fill
4×2.00 = 8NEC 314.16(B): 14 AWG = 2.00 in³, 12 = 2.25, 10 = 2.50, 8 = 3.00 per conductor. - 2
Ground credit
1 × 2 = 2 - 3
Device fill
0 (no devices) = 0 - 4
Cable clamp fill
0 (no clamps) = 0 - 5
Total required fill
8 + 2 + 0 + 0 = 10
How does this calculator work?
NEC 314.16(B) box fill: each 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³, 8 AWG = 3.00 in³. All grounds = one conductor credit; each device = two conductors of the largest size; all clamps = one conductor credit. Pick a listed box whose cubic-inch rating meets or exceeds the total.
Formula
How this is calculated
NEC Article 314.16(B) assigns a standard fill volume to each conductor size: 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³, and 8 AWG = 3.00 in³. Each current-carrying conductor (hot wire and neutral) is counted individually. Grounding conductors — no matter how many — are counted as just one conductor at the volume of the largest grounding wire in the box. Each wiring device (switch, receptacle, or dimmer) mounted on a strap counts as two conductors at the volume of the largest conductor connected to it. All internal cable clamps together count as one conductor at the volume of the largest wire entering the box.
Add up all these fill contributions to get the total required cubic-inch volume. Then select a listed metal box whose marked cubic-inch capacity equals or exceeds that total. Common square 4-inch boxes run from 18 in³ (1¼ in deep) to 30.3 in³ (2⅛ in deep); the larger 4-11/16 in square box at 42 in³ handles most residential multi-circuit situations.
This calculator covers the standard box-fill method only. It does not account for conduit locknuts, fixture stems, luminaire hickeys or conductors entering from conduit (which may require a raceway fill calculation). Always consult the current adopted edition of the NEC and your local inspector for final approval.
Frequently asked questions
NEC 314.16(B)(5) groups all grounding conductors — regardless of count — as a single volume credit equal to the largest grounding conductor size. The rationale is that grounding wires are typically bare or insulated green and easily managed in a bundle, so they take proportionally less space than individual insulated conductors.
NEC 314.16(B)(4) defines a device as any yoke- or strap-mounted component: a standard single-gang switch, receptacle, GFCI, AFCI or dimmer each count as one device (= two conductor volumes). Double-ganged or multi-pole devices may count as two devices — check the device's listing or NEC Table 314.16(B) note.
If the fill calculation exceeds 42 in³ (the largest common 4-11/16 in square box), the NEC requires either splitting conductors into a second box, using a deeper listed enclosure whose marked volume meets the need, or routing some conductors into a separate pull box. Never force more wires into a box than its marked volume allows — overcrowding causes heat build-up and insulation damage.
Also known as
TG we-Calculate Editorial Team. (2026). Junction Box Sizing Calculator — NEC 314.16(B) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/junction-box-sizing-calculator
TG we-Calculate Editorial Team. "Junction Box Sizing Calculator — NEC 314.16(B)." TG we-Calculate. 2026. https://we-calculate.com/calculator/junction-box-sizing-calculator.
TG we-Calculate Editorial Team, "Junction Box Sizing Calculator — NEC 314.16(B)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/junction-box-sizing-calculator
@misc{wecalculate_junction_box_sizing_calculator, title = {Junction Box Sizing Calculator — NEC 314.16(B)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/junction-box-sizing-calculator}}, year = {2026}, note = {TG we-Calculate} }
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