Free calculator · 2026 and 2023 NEC

Box Fill Calculator

Enter the box and what goes in it, conductors, clamps, devices and grounds, and get the volume needed under NEC 314.16, whether the box is big enough, and the smallest standard box of each kind.

Box fill calculator

Box

Marked box volume (other boxes) (in³)

Plaster ring, extension or raised cover (in³)Volume marked on it, if any

Count each conductor that enters the box and is spliced or connected there, and each one that passes through. Count an unbroken loop long enough to be cut for two ends as two.

Conductors 1

Size

How many

Conductors 2

Size

How many

Internal cable clamps

Luminaire studs or hickeys

Devices (yokes or straps)

Largest conductor on the devices

Equipment grounding conductors

Largest grounding conductor

NEC edition

2026 NEC 2023 NEC

22.5 in³ needed, 30.3 in³ available

The box is large enough.

How it was worked out

6 × 12 AWG conductors: 6 × 2.25 in³ = 13.5 in³
Internal cable clamps (one allowance, largest conductor 12 AWG): 1 × 2.25 in³ = 2.25 in³
1 device yoke (two allowances each, 12 AWG): 2 × 2.25 in³ = 4.5 in³
2 equipment grounding conductors (one allowance for up to four, 12 AWG): 1 × 2.25 in³ = 2.25 in³
Each allowance is charged the volume for its conductor size in Table 314.16(B)(4). Total: 22.5 in³.
Box: 4 × 2-1/8 in. square = 30.3 in³. 22.5 fits within 30.3.

Smallest standard metal box of each kind that holds this (Table 314.16(A)):

Square, 4 in.: 4 × 2-1/8 in. square (30.3 in³)
Square, 4-11/16 in.: 4-11/16 × 1-1/4 in. square (25.5 in³)
FS or FD box: FD, multiple gang, at least 2-3/8 in. deep (24 in³)

Code sections (2026 NEC): 314.16(B); Table 314.16(A); Table 314.16(B)(4).

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What this calculator does not account for

Devices wider than a single 2-inch box, which count two for each gang they need.
Conductors larger than 6 AWG, where pull and junction box sizing (314.28) applies instead.
Boxes divided by barriers, which are figured one space at a time, and conduit bodies.
Fixture wires and a grounding conductor from a domed luminaire canopy, which may be left out of the count.

The installed design must follow the code edition your jurisdiction has adopted, its local amendments, and the authority having jurisdiction. Check every value in your own code book.

How to calculate box fill

Box fill is a count and a multiplication. Each conductor that enters the box and ends or is spliced there counts once; all the internal cable clamps together count once; each device yoke counts twice; and up to four equipment grounding conductors count once, with a quarter more for each one beyond four. Each count is charged the volume for its conductor size in Table 314.16(B)(4), such as 2.00 cubic inches for 14 AWG and 2.25 for 12 AWG. The box must have at least the total volume.

Volume allowance per conductor (Table 314.16(B)(4))

Box fill is measured in cubic inches. Each conductor, and each device, clamp or group of grounds, is charged a volume that depends on wire size.

Where to look
Article 314, 314.16(B) and Table 314.16(B)(4), "Volume Allowance Required per Conductor". Sizes run from 18 AWG to 6 AWG; read the in.³ column (the cm³ column is the metric value). The allowance grows with size: 14 AWG is 2.00 in.³ and each larger size takes more, up to 6 AWG at 5.00 in.³. Boxes with conductors 4 AWG and larger are sized by 314.28 instead.

Procedure

Find the conductor size.
Read its in.³ allowance in Table 314.16(B)(4).
Multiply by the number of allowances of that size.
For mixed sizes, total each size separately.

Worked example
Five 12 AWG conductors enter and are spliced in a box.

Table 314.16(B)(4): 12 AWG = 2.25 in.³.
5 × 2.25 = 11.25 in.³.

Most common mistake
Reading the cm³ column as cubic inches, or using the 14 AWG value for every conductor in a box that mixes 14 and 12 AWG.

Calculating box fill (314.16(B))

Box fill adds up allowances, each one a Table 314.16(B)(4) volume.

Where to look
Article 314, 314.16(B)(1) to (6):

(1) Each conductor from outside the box that ends or is spliced in it: one. Passing through unbroken: one. A conductor wholly inside the box (a pigtail): none.
(2) Internal cable clamps, any number: one, at the largest conductor in the box.
(3) Luminaire studs or hickeys: one for each type.
(4) Each yoke or strap with a device: two, at the largest conductor connected to that device.
(5) EGCs: up to four take one allowance, at the largest EGC; each EGC beyond four adds one quarter allowance.
(6) Each terminal block: one.

Procedure

Count each category.
Convert each to cubic inches at its size.
Add.

Worked example
Two 12-2 cables with ground enter a box with internal clamps and one receptacle.

Conductors: 4 × 2.25 = 9.00.
Clamps: 1 × 2.25 = 2.25.
Receptacle yoke: 2 × 2.25 = 4.50.
EGCs (two): 1 × 2.25 = 2.25.
Total: 18.00 in.³.

Most common mistake
Counting every bare ground, counting pigtails, or charging the yoke at the wrong size.

Sections to look up

All in the 2026 NEC, with the 2023 number where it is different. Open your own book to each one; this page explains the rules in its own words and does not reproduce the code.

314.16(B)
Table 314.16(B)(4) (Table 314.16(B)(1) in the 2023 NEC)
314.16(B)(1)–(6)

2023 NEC and 2026 NEC

The volume allowance table is Table 314.16(B)(4) in the 2026 NEC; it was Table 314.16(B)(1) in the 2023 NEC. The figures are the same.

Every exam-relevant change from the 2023 to the 2026 NEC

Practice questions

Work each one with your code book open, then check the answer.

What volume allowance does Table 314.16(B)(4) require for each 12 AWG conductor?
- A. 2.00 in.³
- B. 2.50 in.³
- C. 2.25 in.³
- D. 1.75 in.³
Show the answer to question 1
Answer: C. 2.25 in.³
Table 314.16(B)(4), 12 AWG: 2.25 in.³.
Reference: Table 314.16(B)(4) (2026 NEC)
Two 12-2 NM cables with ground enter a metal box through internal cable clamps. The box holds one duplex receptacle. What is the required box volume?
- A. 18.00 in.³
- B. 20.25 in.³
- C. 15.75 in.³
- D. 16.00 in.³
Show the answer to question 2
Answer: A. 18.00 in.³
Conductors 4, clamps 1, yoke 2, EGCs (two) 1, so 8 allowances × 2.25 = 18.00 in.³.
Reference: 314.16(B)(1), (2), (4), (5); Table 314.16(B)(4) (2026 NEC)
What volume allowance does Table 314.16(B)(4) require for each 6 AWG conductor?
- A. 5.00 in.³
- B. 3.00 in.³
- C. 2.50 in.³
- D. 81.9 in.³
Show the answer to question 3
Answer: A. 5.00 in.³
Table 314.16(B)(4), 6 AWG: 5.00 in.³ (81.9 cm³).
Reference: Table 314.16(B)(4) (2026 NEC)
A box with no clamps and no grounding conductors has six 14 AWG conductors entering it and two single-pole switches, each on its own yoke. What is the required volume?
- A. 16.00 in.³
- B. 22.50 in.³
- C. 20.00 in.³
- D. 12.00 in.³
Show the answer to question 4
Answer: C. 20.00 in.³
Conductors 6 + yokes 2 × 2 = 10 allowances × 2.00 = 20.00 in.³.
Reference: 314.16(B)(1), (4); Table 314.16(B)(4) (2026 NEC)

Common questions

How many 12 AWG wires can go in a 4-inch square box?

In a 4 × 2-1/8 inch square metal box (30.3 in³), 13 conductors of 12 AWG at 2.25 in³ each, if nothing else is in the box. A device, clamps or grounding conductors each take space from that count.

How many 14 AWG wires fit in a single-gang device box?

It depends on the depth. A 3 × 2 × 3-1/2 inch metal device box (18.0 in³) holds 9 conductors of 14 AWG at 2.00 in³ each, with no device or clamps. Plastic boxes are marked with their own volume.

Do ground wires count in box fill?

Yes, but together: up to four equipment grounding conductors count as one conductor of the largest grounding conductor's size, and each one beyond four adds a quarter of that volume.

Do wire nuts and pigtails count?

Wire connectors, locknuts and bushings count nothing. A pigtail that starts and ends inside the box does not count either; only conductors that come in from outside the box do.

How does a switch or receptacle count?

Each device yoke or strap counts as two conductors of the largest conductor connected to it. A device wider than a single 2-inch box counts two for each gang it needs.

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