Box Fill Calculator · NEC 314.16 helper

Calculator

Box Volume (cubic inches)

Tip: If using a box plus ring, add their volumes.

Equipment Grounds (count)

Ground Gauge (largest)

Conductors by Gauge

Count all insulated conductors that enter or leave the box (hots, switched legs, travelers, neutrals). Do not count pigtails that do not leave the box.

Gauge (AWG)	Insulated conductors (qty)	Volume each (cu in)	Row volume

Integral Clamps inside box

Count integral clamps only (not external connectors).

Device Yokes (switches, receptacles)

Each yoke counts as two conductors of the largest conductor attached.

Largest Device Conductor Gauge

Result

Required volume: 0.00 cu in
Box volume: 0.00 cu in
Remaining: 0.00 cu in
Fill: 0%

Component	Count	Factor	Volume

Formula used

This tool applies the volume allowance method from NEC 314.16(B). Total required volume is the sum of:

Insulated conductors: 1 allowance per conductor by its gauge (cu in per Table 314.16(B)).
Equipment grounding conductors: All grounds together count as one allowance of the largest grounding conductor present.
Internal clamps: Each integral clamp counts as 1 allowance of the largest conductor present in the box.
Device yokes: Each yoke (switch/receptacle) counts as two allowances of the largest conductor connected to that device.

Required Volume = Σ(conductors × allowance_by_gauge) + (grounds ? 1 × allowance_by_ground_gauge : 0) + (clamps × allowance_by_largest_gauge) + (devices × 2 × allowance_by_device_gauge)

Notes: Pigtails that do not leave the box are not counted. External cable connectors do not count as clamps inside the box. Always confirm with the edition of the code enforced locally.

How to use this calculator

Choose a preset box volume or enter a custom volume in cubic inches (add ring volumes if used).
Add one or more gauge rows and enter the number of insulated conductors of each gauge entering or leaving the box.
Enter the total equipment grounds and select the largest ground gauge (often same as circuit conductors).
Enter the number of integral clamps located inside the box (skip external connectors).
Enter the number of device yokes and select the largest conductor gauge connected to those devices.
Review the result status. If PASS, remaining volume is positive. If FAIL, choose a bigger box or reduce fill.
Use CSV/PDF export to document your calculation for records.

Example data table

Scenario	Gauge	Cond.	Grounds	Yokes	Clamps	Box Volume (cu in)	Req. Volume	Margin (cu in)	Status

FAQs

1) Do neutrals and travelers count?
Yes. All insulated conductors that enter or leave the box count once by their gauge, including neutrals, travelers, and switched legs.

2) Do pigtails count?
Pigtails that originate and terminate within the same box are not counted. If a pigtail leaves the box, count it like any other conductor.

3) How do devices on a yoke count?
Each yoke (switch or receptacle strap) counts as two conductor allowances of the largest conductor connected to that device.

4) What about equipment grounding conductors?
All grounds together count as a single allowance of the largest grounding conductor present in the box.

5) Do external connectors or locknuts count as clamps?
No. Only clamps that are integral to the box interior count toward volume.

6) Multiple gauges in one box?
Yes. Count each insulated conductor using its own gauge allowance. For devices and clamps, use the largest relevant gauge as specified.

7) Can I use metric units?
This tool calculates in cubic inches per NEC tables. Convert metric volumes to cubic inches before entry (1 cubic inch ≈ 16.387 cm³).

Informational tool only. Always follow the latest applicable electrical code and inspector guidance.

Conductor volume allowances (cu in per Table 314.16(B))

AWG	cu in per conductor

These values are widely used references. Verify the edition enforced in your jurisdiction.

Quick tips

When mixing gauges, devices often require using the largest connected gauge for their allowance.
Add plaster ring volume to box volume where applicable.
Grounds of different sizes still count as one allowance of the largest ground present.
Consider multi-gang boxes or deeper boxes to reduce crowding.