Box Fill Calculator
Add up conductors, devices, clamps and grounds and compare the total with the volume of the box.
Step-by-step
- Enter valid values to begin.
Volumes are from NEC Table 314.16(A) and (B). Conductors that originate and end in the box and are shorter than 12 in. are not counted; a conductor that loops through unbroken and is at least twice the minimum length counts twice (314.16(B)(1)). This tool assumes all conductors share the size selected.
Box Fill: Counting Wires, Devices and Clamps Before They Cook
Core Engineering Principles
Boxes are sized by volume because a crowded box is a hot box, and a box you can’t close without bending the wires hard is how nicks and shorts happen. The Code assigns each item a volume allowance based on the largest conductor involved, adds it all up, and checks that the box is bigger than the total. The unit is the cubic inch, and the allowance for a conductor goes up as it gets bigger: 14 AWG gets 2.00 in³, 12 AWG gets 2.25, 10 AWG gets 2.50, 8 AWG gets 3.00, and 6 AWG gets 5.00.
The counting rules are what trip people up. Each conductor that enters the box and ends there counts once. A conductor that runs through unbroken counts once too (twice if it’s at least twice the length of the minimum free conductor). Jumpers and pigtails that start and end inside the box, with no part leaving it, don’t count at all. A yoke with a device counts as two conductors, the equipment grounds together count as one, internal clamps count as one, and each fixture stud or hickey counts as one.
Required volume = Equivalents × volume per conductor (Table 314.16(B)) ≤ box volume (Table 314.16(A))
NEC & Standard References
314.16(A) and its table give the volume of standard metal boxes. A 4 in. square box that is 1-1/2 in. deep is 21 in³, a 4-11/16 in. square, 2-1/8 in. deep box is 42 in³, and a 3 × 2 × 2-3/4 in. device box is 14 in³. For non-standard or nonmetallic boxes, use the volume marked on the box. 314.16(B)(1) through (B)(5) set how each item is counted. 314.16(B)(4) says each yoke counts as two based on the largest conductor connected to it. 314.16(B)(5) says all equipment grounding conductors together count as one, based on the largest. 314.16(C) covers conduit bodies, and 314.28 sets pull-box dimensions.1. Conductors: 6 × 1 = 6.
2. Device: 1 yoke × 2 = 2.
3. Grounds: all together = 1.
4. Total equivalents: 6 + 2 + 1 = 9.
5. Volume: 9 × 2.00 = 18 in³ — the 21 in³ box passes with 3 in³ to spare.
Add two more conductors and you’re at 22 in³ and the box fails.
- Use the box marking. A cheap plastic box often holds less than the table says. Read the stamped volume inside it.
- Mixed sizes. When you have 12 and 14 AWG in the same box, count each at its own allowance. This page assumes one size, so do a second pass for the other.
- Extension rings and boxes with covers. Raised covers and extensions add volume you may count.
- Tight boxes run hot. If you have to force the device in, go bigger.
- Pull boxes and conduit bodies are different. Over 6 AWG the dimensions come from 314.28, not volume.