Free Box Fill Calculator

Add up a volume allowance for everything in the box, then compare the total to the box's volume. Each insulated conductor counts once at its own size. All internal cable clamps together count once, at the largest conductor size in the box. Each fixture stud or hickey counts once at the largest size. Each device yoke (a switch or receptacle strap) counts twice, at the largest conductor landing on that device. All equipment grounds together count once, at the largest ground. The total required volume must not exceed the box volume.

The allowance depends on the conductor size and is set in NEC 314.16(B) — for example, a 12 AWG conductor counts 2.25 in³. Bigger wire takes a bigger allowance. This calculator applies the allowance for each size automatically; check the table in your adopted edition for any size you need to confirm by hand.

A conductor running unbroken through the box counts as one volume allowance. A conductor that enters and terminates also counts as one. Pigtails and jumpers that don't leave the box are not counted.

All equipment grounding conductors together count as a single allowance of the largest ground (a separate isolated ground adds one more). Each device yoke (a receptacle or switch strap) counts as two allowances of the largest conductor connected to that yoke.

Use the cubic-inch volume the manufacturer stamps or prints inside the box — every listed box is marked. For a box with a plaster ring, extension ring, or domed cover that is itself marked with a volume, add that volume to the box. For ganged device boxes, add the volume of each section.