Box Fill Calculator

How box fill works

Every electrical box has a volume, measured in cubic inches. The wires, devices and fittings inside take up room, and the code (NEC 314.16) limits how much. Too many wires in a box means cramped splices, hot conductors and wire nuts that get pushed back against insulation. The calculator adds up the room the contents need and compares it to the box.

The room each item needs is called its allowance. A wire’s allowance grows with its size: a 14 AWG wire takes 2 cu in, a 12 AWG wire 2.25 cu in and a 10 AWG wire 2.5 cu in. Type the volume of your box, the wire counts and the extras, and the page shows each line of the sum so you can check it by hand.

What gets counted

• Wires. Count each wire that comes in from outside and ends or splices in the box as one. A wire that passes through uncut counts as one too.
• Loops. An unbroken loop of spare wire, 12 in or longer, counts as two.
• Switches and receptacles. Each device counts as two wires of the largest size connected to it. A wide device takes two for every gang it spans.
• Clamps. If the box has internal cable clamps, add one wire of the largest size in the box. One clamp or four, the count is the same.
• Luminaire stud and hickey. Each one adds one wire of the largest size, once per type.
• Grounds. All the grounds together count as one wire of the largest ground size. Past four grounds, each extra adds a quarter of that allowance (2020 and later editions).
• Not counted. Pigtails that start and end inside the box, locknuts, bushings and other small fittings.

Find the volume of your box

Plastic boxes are stamped with their volume, usually inside the back wall. Metal boxes list it on the label or in the maker’s catalog. If you add a plaster ring, extension ring or raised cover, type its marked volume in the ring box and the calculator adds it to the box.

The page covers boxes up to 100 cu in and wires from 14 to 6 AWG. Bigger boxes and wires larger than 6 AWG follow the pull-box rules in 314.28, which this page does not cover. Barrier boxes are counted one space at a time, and isolated grounds are left out because editions differ on them.

Reading the result

The first tag shows the room the wiring needs, rounded up to the next hundredth of a cubic inch, with a badge for whether it fits. It also shows the same figure in cubic centimetres for boxes sold in metric countries. The second tag shows how much room is left, rounded down, and how many more wires of the largest size would fit. If the box is too full, it shows how far over you are.

The fit check uses the exact total, not the rounded figures. A box whose volume matches the need to the last decimal passes. One that is short by even a hundredth does not.

When the box is too full

You have three ordinary choices. Use a deeper box with more volume, add an extension or plaster ring and type its volume, or reduce the count by moving a splice to another box. Look at the lines: a 2-gang device, a pile of 10 AWG wires or a loop of spare cable often takes more than expected. Changing the largest wire in the box also changes the clamp and fitting counts.

Common mistakes

• Counting each cable as one wire. A two-conductor cable has two insulated wires, plus a ground that goes into the grounds count.
• Forgetting the device. A switch or receptacle takes room like two wires.
• Counting clamps one by one. All internal clamps together count once.
• Using the wrong volume. The marking on the box is the number to use, plus any ring.
• Leaving out a ring. A raised cover or extension ring adds its own marked volume.

This is a planning tool. Your code edition and your local inspector (AHJ) decide what is required, and a licensed electrician should do the work.

Worked example

1. Inputs: 18 cu in box, 4 × 12 AWG, 1 device (12 AWG), internal cable clamps, 2 grounds (largest 12 AWG).
2. 4 × 12 AWG × 2.25 = 9.00
3. 1 device × 2 × 12 AWG × 2.25 = 4.50
4. Clamps: 1 × 12 AWG × 2.25 = 2.25
5. Grounds (2, largest 12 AWG): 1 × 2.25 = 2.25
6. Total required = 18.00 cu in

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