Box Fill Calculator
Does it all fit? Counts conductors, devices, clamps and grounds against the box volume.
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Box fill is the total volume conductors, devices, clamps, and grounds require inside an outlet box (NEC 314.16). Each conductor counts as one volume allowance by its size; each device strap counts as two of the largest conductor; all grounds together count as one; all internal clamps together count as one. The box's marked cubic-inch volume must meet or exceed the total.
The box fill formula
Required volume = Σ(conductor allowances) + 2 × (largest-conductor allowance) per device/yoke + 1 × (largest) for all grounds + 1 × (largest) for internal clamps. Table 314.16(B) allowances: #14 = 2.00 in³ · #12 = 2.25 · #10 = 2.50 · #8 = 3.00 · #6 = 5.00.
Worked example
A single-gang box with three #12 conductors, one duplex receptacle, internal cable clamps, and one #12 equipment ground. Conductors: 3 × 2.25 = 6.75. Device: 2 × 2.25 = 4.50. Grounds (all = 1): 2.25. Clamps (all = 1): 2.25. Total = 15.75 in³ — an 18 in³ box passes; a 14 in³ box does not.
NEC Table 314.16(B) — volume allowance per conductor
| Conductor size (AWG) | Volume allowance (in³) |
|---|---|
| #18 | 1.50 |
| #16 | 1.75 |
| #14 | 2.00 |
| #12 | 2.25 |
| #10 | 2.50 |
| #8 | 3.00 |
| #6 | 5.00 |
NEC references
- NEC 314.16(A) — box volume is the sum of its assembled sections' volumes
- NEC 314.16(B) — what counts: conductor, device (×2), clamp, support, and equipment-ground fill
- NEC Table 314.16(B) — volume allowance per conductor size, in cubic inches
How to calculate box fill
- Count the current-carrying conductors by size (#14–#8); each counts once.
- Add two allowances of the largest conductor for each device strap or yoke.
- Add one allowance of the largest for all equipment grounds, and one more if internal clamps are present.
- Compare the total to the box's marked cubic-inch volume — it must be equal or greater.
Frequently asked questions
How do you calculate box fill?
Add a volume allowance for every conductor by its size (Table 314.16(B): #14 = 2.00 in³, #12 = 2.25, #10 = 2.50), two of the largest conductor for each device strap, one of the largest for all grounds combined, and one for internal clamps. The sum must not exceed the box's marked volume.
What counts as box fill in the NEC?
Conductors that enter and terminate or pass through (one each), device straps (two each), internal cable clamps (one total), and equipment grounds (one total). Conductors that begin and end inside the box (pigtails) and fittings entering the box do not add fill.
How many #12 wires fit in an 18 cubic inch box?
18 ÷ 2.25 = eight #12 allowances if nothing else is in the box. With one receptacle (4.50), a ground (2.25), and internal clamps (2.25) you've used 9.0 in³ before conductors, leaving room for about four #12 conductors.
Do ground wires count in box fill?
Yes, but all equipment grounding conductors together count as a single volume allowance of the largest ground present. A second allowance is added only for a separate isolated/insulated equipment grounding conductor.
Built for the field
- Live results as you type — every value recomputes instantly.
- Works fully offline once the app has loaded.
- Cites the governing NEC table for every result.
- Save calculations to a job and build a material list (Pro).
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For guidance only. Always verify against the current National Electrical Code and your local amendments. Fieldwatt does not replace an engineer of record.