Box Fill Calculator

Calculate electrical box fill percentage per NEC. Enter box volume, conductors, and devices. Get total volume, fill percentage, and compliance check. Based on NEC standards. Informational only—consult a qualified electrician.

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Published: October 23, 2025 | Updated: October 23, 2025 | Reviewed by: Science/Engineering Editor

⚠️ Electrical Safety Warning: This calculator is for informational purposes only. All electrical work must be performed by a licensed electrician and comply with NEC and local regulations.

Box Information

Conductors

Type Size (AWG) Number Action

Devices

Device Type Number Action

Preset Boxes

NEC Volume Reference

NEC 314.16(A) - Box fill calculation based on conductor and device volumes per NEC Table 314.16(B).

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How it works

Fill % = (Total Conductor Volume + Total Device Volume) / Box Volume × 100. We use NEC tables for the volume of conductors and devices and compare it to the box volume.

The NEC requires that the total volume of conductors, devices, and other components does not exceed 75% of the box volume when devices are present.

Inputs explained

  • Box Type: Predefined standard boxes or custom dimensions
  • Box Volume: Total internal volume in cubic inches
  • Conductors: Wire type, size (AWG), and quantity
  • Devices: Switches, receptacles, dimmers, GFCIs, and quantity

Example

Scenario: 4x4x1.5 Box (21 in³)

  • 2× 14 AWG Copper Conductors: 2 × 2.25 in³ = 4.5 in³
  • 1× Switch: 2.25 in³
  • Total Fill: 4.5 + 2.25 = 6.75 in³
  • Fill %: (6.75 / 21) × 100 ≈ 32.1%
  • Status: Compliant (< 75%)

Tips & notes

  • The NEC limits the fill to 75% of the box volume for boxes that contain devices
  • Overfilling a box can cause overheating and create a fire hazard
  • Always check the latest NEC edition for the most current rules
  • Count equipment grounding conductors as one conductor of the largest size
  • Internal cable clamps count as one conductor of the largest size
  • Each device (switch, receptacle) counts as two conductors

FAQs

The percentage of a box's volume that is occupied by conductors and devices.

75% for boxes that contain devices, 100% for boxes that only contain conductors.

Using the volume per conductor specified in NEC Table 314.16(B).

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Disclaimer

Informational tool based on NEC standards. Electrical work must be done by a licensed professional and comply with local regulations. Consult a qualified electrician for all electrical installations.

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