Box Fill Calculator

Electrical box fill under NEC 314.16: wires, devices, clamps and grounds in cubic inches, checked against standard metal boxes or your own.

NEC 314.16 says how many wires an electrical box can hold, by volume. Each wire gets an allowance by size: 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG and 2.50 in³ for 10 AWG. Each device on a yoke counts as two wires, internal clamps together count as one, and all the ground wires together count as one. A receptacle fed by two 12/2 cables, with internal clamps, needs 4 wires × 2.25 + 2.25 for clamps + 4.5 for the device + 2.25 for the grounds = 18 in³, so it fits a 4 in square box 1½ in deep (21 in³). This calculator adds up the allowances for any mix of wires, devices, clamps, studs and grounds, and checks them against standard metal boxes from Table 314.16(A) or a volume you enter. It also suggests the smallest standard box that's big enough.

in³
Insulated wires entering the box

Count the hot and neutral wires (not the grounds) that come into the box. A wire passing straight through counts once, or twice if it's an unbroken loop of 12 in or more. Pigtails that start and end inside the box don't count.

Volume needed
18 in³

8 allowances

Box volume
21 in³
Result
Fits

3 in³ spare

Smallest standard box
18 in³

4 in square × 1¼ in

How it was worked out
  1. Conductors: 4 × 2.25 in³ (12 AWG) = 9 in³
  2. Internal cable clamps: 1 × 2.25 in³ (12 AWG) = 2.25 in³
  3. Devices (2 per yoke): 2 × 2.25 in³ (12 AWG) = 4.5 in³
  4. Equipment grounding conductors: 1 × 2.25 in³ (12 AWG) = 2.25 in³
  5. Total needed: 18 in³; box 21 in³ = 21 in³

From NEC 314.16. Each device counts as two wires of the biggest size connected to it (two per gang for a device wider than one gang). One or more internal clamps count as one wire, and each luminaire stud or hickey as one, of the biggest wire in the box. Up to four ground wires count as one, plus a quarter for each extra one. Box volumes are from Table 314.16(A) for standard metal boxes; plastic boxes and rings are marked with their own volume. Boxes with 4 AWG or larger wires are sized under 314.28 instead.

Common use cases

  • Check that a box isn't overfilled before wiring a switch or outlet
  • Choose the box size for a new junction, switch or light
  • See whether an extension ring or a deeper box is needed
  • Practise box fill calculations for an exam

How to use

  1. Choose the box, or enter the volume marked on it, plus any marked volume on a plaster ring, extension ring or raised cover.
  2. Enter the insulated wires that come into the box, by size.
  3. Enter the devices, ground wires and any luminaire studs or hickeys, and tick internal clamps if the box has them.
  4. Read the volume needed against the box's volume.
  5. If it's overfilled, use the smallest standard box suggested, or add a ring.

FAQ

How many 14 AWG wires can go in a box?

Divide the box volume by 2.00 in³, then allow for devices, clamps and grounds. A 3 × 2 × 2½ in device box (12.5 in³) with one receptacle on two 14/2 cables needs 4 × 2 + 4 for the device + 2 for the grounds = 14 in³, so it's too small; a 3 × 2 × 2¾ in box (14 in³) is just enough.

How do ground wires count in box fill?

All the equipment grounding conductors together count as one wire of the largest ground size. Since the 2020 NEC, each ground beyond four adds a quarter allowance.

How do switches and outlets count?

Each yoke or strap counts as two wires of the largest wire connected to it. A device too wide for a single-gang box counts as two for each gang it takes up.

Do pigtails count in box fill?

No. Wires that start and end inside the box, such as pigtails and short jumpers, don't count. A wire that passes through without a splice counts once, or twice if it's a loop of 12 in or more.

What about plastic boxes?

Plastic boxes aren't in Table 314.16(A); they're marked with their volume in cubic inches. Choose "Other" and enter that number.

Related tools

Results are estimates worked out from the figures you enter and published tables, for planning only. They are not electrical, engineering or professional advice. Mains electricity can kill and start fires. Check cable and circuit sizes against the wiring rules where you live (the NEC and local amendments in the US, BS 7671 in the UK, AS/NZS 3000 and 3008 in Australia and New Zealand), and use a licensed electrician where the law requires one.