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Box Fill Calculator

Calculate required electrical box volume from conductors, devices, and grounds (NEC 314.16).

Introduction

Box Fill Calculator — Calculate required electrical box volume from conductors, devices, and grounds (NEC 314.16). Enter # of 14 AWG conductors, # of 12 AWG conductors, # of 10 AWG conductors, Device yokes (switches/receptacles), to get an instant, accurate result.

Formula

NEC 314.16(B): required box volume = Σ(conductor count × per-conductor volume allowance by gauge) + (2 × largest-conductor allowance × device yokes) + (largest-conductor allowance, counted once, if any equipment grounding conductors are present).

Step-by-step

  1. Enter the # of 14 AWG conductors.
  2. Enter the # of 12 AWG conductors.
  3. Enter the # of 10 AWG conductors.
  4. Enter the Device yokes (switches/receptacles).
  5. Toggle "Ground wires present" on or off as needed.
  6. Click Calculate to see your result instantly.

Real-world example

Example: With # of 14 AWG conductors = 0, # of 12 AWG conductors = 3, # of 10 AWG conductors = 0, Device yokes (switches/receptacles) = 1, Ground wires present = yes, the Box Fill Calculator gives Required Volume Cu In: 13.5, Basis: NEC Table 314.16(B).

Frequently Asked Questions

Why do device yokes (switches/receptacles) count double?
NEC 314.16(B)(4) requires each strap containing one or more devices (like a switch or receptacle) to be counted as two conductors, using the largest conductor size connected to that device.
Why are all grounding conductors counted as just one allowance?
Per NEC 314.16(B)(5), one or more equipment grounding conductors (and insulated equipment bonding jumpers) together count as a single volume allowance, based on the largest grounding conductor present — not one allowance per ground wire.
Does this cover every box-fill exception?
This covers the standard conductor, device-yoke, and grounding-conductor allowances in 314.16(B)(1)-(5); it does not add allowances for internal cable clamps or fixture studs/hickeys, which NEC 314.16(B)(2)-(3) also require counting when present.
What inputs does the Box Fill Calculator need?
You'll need to provide: # of 14 AWG conductors, # of 12 AWG conductors, # of 10 AWG conductors, Device yokes (switches/receptacles), . All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Box Fill Calculator?
The Box Fill Calculator applies the formula exactly as calculated — NEC 314.16(B): required box volume = Σ(conductor count × per-conductor volume allowance by gauge) + (2 × largest-conductor allowance × device yokes) + (largest-conductor allowance, counted once, if any equipment grounding conductors are present). — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Box Fill Calculator free to use?
Yes, the Box Fill Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Box Fill Calculator

The Box Fill Calculator uses a real, verifiable formula — NEC 314.16(B): required box volume = Σ(conductor count × per-conductor volume allowance by gauge) + (2 × largest-conductor allowance × device yokes) + (largest-conductor allowance, counted once, if any equipment grounding conductors are present). — so results are accurate every time, not an approximation.