Box Fill Calculator
Estimate whether an electrical box has enough room for the entered conductors, grounds, devices, and fittings.
Use this as a practical planning tool to compare required box volume against the box you plan to use.
Different box styles offer different usable volume, and box fill depends on conductor count, wire size, grounds, devices, and fittings.
Choose the box and conductor details to estimate box fill.
Estimated result
Required box volume
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Selected box volume
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Remaining room / overfill
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Status
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Run the calculator to compare the entered conductors and devices against the box volume.
How to think about box fill
Box fill is about making sure the box has enough room for conductors and devices
Ground wires count differently than insulated conductors, but they still take up space
Devices and internal clamps also take up room inside the box
Crowded boxes make work harder and can create avoidable safety concerns
Reference approach used
Conductor allowances
Each insulated conductor counts by wire size allowance. Grounds are grouped together as one conductor allowance, while each device yoke counts as two allowances of the connected conductor size.
Practical planning focus
This is meant to help answer three practical questions: does this box work, is it too small, and what is a reasonable common box size to start with.
This calculator is a practical planning reference only. Actual box fill depends on conductor count, conductor size, devices, fittings, and code requirements. For precise requirements, refer to NEC 314.16 or the applicable local code.
Related problems and reference
Planning and safety note
This content is for general learning and planning. Always verify work against current NEC requirements, manufacturer instructions, and local code rules. If you are unsure or see heat, sparks, burning smell, repeated trips, or damaged wiring, contact a licensed electrician.