Box Fill Calculator

Box fill compares the volume that everything inside an electrical box requires with the box’s usable volume, under NEC 314.16. You add up the volume allowances for the conductors, devices, clamps, grounds and fittings, then multiply by the per-conductor volume for the wire size.

Each current-carrying conductor counts as one allowance, each device yoke as two, all the grounds together as one, all the internal clamps together as one, and each support fitting as one. Multiply the total by the volume allowance for the wire size to get the required box volume.

For example, a box with four 14 AWG conductors, two grounds and one device has seven allowances, and at 2.00 cubic inches each that is 14 cubic inches required. The box passes when its volume is at least that. This calculator does the full count and the pass/fail check.

Each wire size has a volume allowance from NEC Table 314.16(B), in cubic inches: 18 AWG is 1.50, 16 AWG is 1.75, 14 AWG is 2.00, 12 AWG is 2.25, 10 AWG is 2.50, 8 AWG is 3.00, and 6 AWG is 5.00.

Every conductor of that size in the box uses one allowance, and devices and other items are counted in multiples of the same per-conductor volume. A larger wire needs more room, so the same box holds fewer of them.

When a box has more than one wire size, the allowance for each item uses the largest conductor in its category. This calculator assumes a single wire size; pick the size and it applies the right volume allowance.

Under NEC 314.16(B), each item is counted as a number of conductor volume allowances. Each device on a yoke or strap — a switch or receptacle — counts as two allowances, based on the largest conductor connected to it.

All of the internal cable clamps together count as one allowance, not one each. Each fixture stud or hickey (a support fitting) counts as one allowance each. All of the equipment grounding conductors together count as just one allowance, no matter how many there are.

So you add the conductor count, plus two per device, plus one if there are any clamps, plus one per support fitting, plus one if there are any grounds. This calculator applies each of these rules for you.

Divide the box’s usable volume by the volume allowance for the wire size to get the maximum number of conductor allowances it can hold. Then subtract the allowances used by devices, clamps, grounds and fittings to see how many actual wires are left.

For example, an 18-cubic-inch box at 14 AWG (2.00 cubic inches each) holds nine allowances. A device uses two and the grounds use one, leaving six allowances for conductors.

So the number of wires depends not just on the box size but on everything else inside it. This calculator shows the maximum allowances for your box and wire size, and how many are left after the devices and fittings.

For box fill, a conductor is any wire that enters the box and is spliced, terminated, or passes straight through — each counts as one volume allowance. A wire running through the box without a splice still counts once.

Pigtails — short jumper wires that begin and end inside the box — do not count at all. Neither the box’s own volume allowance for grounds nor the device allowance is a conductor count; those are handled separately.

So count every hot and neutral that comes in from a cable or conduit, including pass-throughs, but not internal pigtails. This calculator takes your conductor count and applies the per-wire volume.

Yes, but all of the equipment grounding conductors together count as just one volume allowance, not one each, under NEC 314.16(B)(5). So whether you have two grounds or six, they add a single allowance.

The one exception is a separate isolated-grounding conductor, which adds one more allowance of its own. The allowance uses the largest grounding conductor in the box.

So enter the actual number of grounds and the rule applies the single allowance automatically. This calculator counts all the grounds as one allowance, as the code requires.

NEC 314.16 is the National Electrical Code rule that limits how much can go into an outlet, device or junction box, to prevent overcrowding that can damage insulation or cause overheating. Table 314.16(A) lists standard box volumes, and 314.16(B) gives the volume allowance each item requires.

The rule works by volume, in cubic inches, not by a simple wire count. You total the allowances for conductors, devices, clamps, grounds and fittings, multiply by the per-conductor volume for the wire size, and check that the box is at least that big.

This calculator follows 314.16(B): conductors at one allowance each, devices at two, clamps and grounds at one each as a group, and support fittings at one each. Always confirm against the current code edition and your local inspector.

An overfilled box violates NEC 314.16 and will fail inspection, but the real risk is physical: too many conductors crammed in makes it hard to fold the wires back without nicking or pinching insulation, which can lead to shorts, arcing or overheating over time.

The fix is to use a larger box, add a listed box extension or plaster ring that adds volume, or reduce what is in the box — for example by moving a splice to another box.

This calculator shows the fill percentage and the spare volume, and flags a pass or fail against the box volume, so you can catch an overfilled box before it is wired. Choose a bigger box or add volume if it fails.