An electrical box fill calculator determines whether a junction box has enough internal volume for the conductors, devices, clamps, and grounds you plan to install — as required by NEC Article 314.16. Overcrowding a box traps heat, damages wire insulation, and creates fire hazards. This calculator does the math so your installation is code-compliant and inspection-ready.
Wire Gauge
Select the largest wire size in the box. This determines device, ground, and clamp allowances.
Conductors
Count each insulated wire (hot, neutral) entering or originating in the box. Do not count ground wires here.
Devices, Grounds & Clamps
Each device yoke = 2× largest conductor volume (NEC 314.16(B)(4)).
All grounds count as 1× largest conductor volume total (NEC 314.16(B)(5)).
All internal clamps count as 1× largest conductor volume (NEC 314.16(B)(2)).
Each bonding jumper = 1× largest conductor volume (NEC 314.16(B)(5)).
Box to Check
Fill in the inputs and click Calculate Box Fill to see results.
Volume Breakdown (NEC 314.16)
| Item | Qty | Each (cu in) | Total (cu in) |
|---|
Recommended Minimum Box
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How to Use the Electrical Box Fill Calculator
Electrical box fill calculations determine whether a junction box, device box, or outlet box has enough internal volume to safely contain all the conductors and hardware installed inside it. The National Electrical Code (NEC) Article 314.16 sets specific cubic-inch allowances for each wire size and item type. This electrical box fill calculator automates the entire process so you can verify compliance before you start wiring — or troubleshoot an existing installation that needs to pass inspection.
Step 1: Select the Largest Wire Gauge in the Box
The largest AWG conductor in the box sets the per-unit volume for device, ground, clamp, and bonding allowances. Under NEC 314.16, wire volumes are: 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in, 8 AWG = 3.0 cu in, and 6 AWG = 5.0 cu in. If you have mixed gauges, select the largest one present — it drives the multipliers for everything else.
Step 2: Enter Your Conductor Counts
Count every insulated conductor (hot and neutral wires) that enters or originates inside the box. Each one gets its own cubic-inch allowance based on its AWG size. Use the "Add another gauge" button if your box contains wires of more than one size — for example, a 12 AWG feed with 14 AWG switch loops. Do not count bare or green equipment grounds here; those go in the ground field below.
Step 3: Add Devices, Grounds, and Clamps
Each switch, receptacle, or other device mounted on a yoke or strap counts as two conductors at the size of the largest wire connected to it — per NEC 314.16(B)(4). All equipment grounding conductors in the box combined count as just one conductor allowance — the size of the largest ground present — per NEC 314.16(B)(5). If the box has internal cable clamps (Romex connectors inside the shell, not external locknuts), all clamps together count as one conductor allowance equal to the largest wire in the box.
Step 4: Select or Enter Your Box Volume
Choose a standard box from the dropdown: 4-inch square shallow (21.0 cu in), 4-inch square deep (30.3 cu in), 4-11/16 inch square boxes (29.5 or 42.0 cu in), single-gang device box (18.0 cu in), or double-gang device box (34.0 cu in). If you have an unusual box, select "Custom" and enter the volume stamped or listed on the box label. NEC requires every box to have its volume marked.
Step 5: Review the Results
The calculator shows your total required volume, your selected box's capacity, the remaining headroom, and a pass/fail verdict. If the box fails, the recommended box section tells you the smallest standard box that would accommodate your load. A color-coded fill bar gives you an instant visual of how full the box is. Always verify with your local inspector — some jurisdictions adopt amended versions of the NEC.
Common Box Fill Mistakes to Avoid
The most frequent errors in box fill calculations are forgetting to count pigtail conductors that originate inside the box, missing the device allowance for GFCI receptacles (which are still one device, not two), and overlooking clamps when using Romex cable with built-in connectors. Also remember that wires passing straight through a box without a splice or connection do not count toward box fill under NEC 314.16(B)(1).
FAQ
Is this electrical box fill calculator free?
Yes, this NEC 314.16 box fill calculator is completely free with no signup, no account, and no limits. All calculations run locally in your browser — your wiring data is never sent to any server.
Is my data private when I use this tool?
Absolutely. Every calculation runs client-side in your browser using JavaScript. No wire counts, device numbers, or any other information are ever transmitted or stored. Your data stays entirely on your device.
What is NEC 314.16 and why does box fill matter?
NEC Article 314.16 is the National Electrical Code section governing how many conductors, devices, and fittings are allowed inside an electrical box. Overfilling a box causes heat buildup, insulation damage, and arcing — which are leading causes of electrical fires. Proper box fill ensures safe wiring and is required to pass inspection.
How are conductor volumes calculated under NEC 314.16?
Each conductor is assigned a volume allowance based on its AWG size: 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in, 8 AWG = 3.0 cu in, and 6 AWG = 5.0 cu in. All conductors originating inside the box, plus device and equipment ground allowances, are added together to find the total required volume.
How are switches and receptacles counted for box fill?
Under NEC 314.16(B)(4), each yoke or strap containing one or more devices counts as two conductors based on the largest wire connected to that device. So one switch or receptacle on a 12 AWG circuit counts as 2 × 2.25 = 4.5 cu in. Two devices on a single yoke still count as two conductors, not four.
How are ground wires counted under NEC 314.16?
All equipment grounding conductors in a box count together as a single conductor allowance equal to the volume of the largest ground wire present. For example, if your largest ground is 12 AWG, all grounds combined add 2.25 cu in — regardless of whether there are two or ten ground wires in the box.
Do clamps inside the box add to the fill calculation?
Yes. Internal cable clamps, whether single or multiple, count as one conductor allowance based on the largest conductor in the box — NEC 314.16(B)(2). External clamps, connectors, and locknuts outside the box do not count. If you use push-in connectors inside the box, count them as internal clamps.
What are common junction box volumes I should know?
Standard box volumes: 4-inch square × 1-1/2 inch = 21.0 cu in; 4-inch square × 2-1/8 inch = 30.3 cu in; 4-11/16 inch square × 1-1/2 inch = 29.5 cu in; 4-11/16 inch square × 2-1/8 inch = 42.0 cu in; single-gang device box = approximately 18 cu in; double-gang device box = approximately 34 cu in. Always verify the exact volume stamped inside your specific box.