Box Fill Calculator
Calculate the minimum electrical box fill (volume in cubic inches) required by the NEC for wire conductors, devices, and clamps. Enter your conductor counts and devices to find the box volume you need.
Formula
Volume = Σ(conductor counts × volume per conductor) + device allowances + clamp allowances
- NEC 314.16: #14=2.0 in³, #12=2.25 in³, #10=2.5 in³ per conductor.
- Devices (yoke-mounted): 2× largest conductor volume.
- Ground wires: all grounding conductors together count as one.
4 #14, 2 #12, 1 outlet, 1 clamp
Inputs
- #14 AWG conductors: 4
- #12 AWG conductors: 2
- #10 AWG conductors: 0
- Devices (switches/outlets): 1
- Internal Clamps: 1
4×2.0 + 2×2.25 + 1×2.0 + 1×2×2.25 = 8+4.5+2+4.5 = 19 cu in.
Frequently asked questions
What size box do I need for a standard outlet?
For one outlet with 3 #14 conductors, you need at least 14 cu in — a single-gang 18 cu in box is typically sufficient.
How do I count grounding wires?
All equipment grounding conductors in the box together count as a single conductor of the largest gauge present.
Why do devices count as two conductors?
NEC 314.16(B)(4) requires each yoke-mounted device, like a switch or receptacle, to be counted as two conductors based on the largest wire connected to it.
Do internal cable clamps add to the fill?
Yes. All internal clamps together count as one conductor of the largest gauge in the box.