Box Fill Calculator
Size your boxes right the first time. Built on NEC 314.16 and Table 314.16(B)—no overfilled boxes, no failed inspections.
Switch to CEC1Conductors Entering Box
Count each conductor that enters the box. Pigtails originating inside the box do NOT count.
Current-Carrying Conductors
Equipment Grounding Conductors (EGC)
All EGCs count as ONE allowance based on the largest EGC
2Devices (Yokes)
Each device yoke counts as 2× the largest conductor connected to it.
3Other Fill Items
4Select Box
Calculation breakdown
How We Calculate Box Fill
This calculator uses conductor volumes from NEC Table 314.16(B) and box volumes from Table 314.16(A). Fill requirements come from NEC 314.16(B)(1) through (B)(5).
The calculation is straightforward:
- Sum conductor volumes — Each entering conductor × its Table 314.16(B) value
- Add allowances — For clamps, fittings, devices, and grounding conductors
- Select box — Choose a box with volume ≥ total required
Box Fill: What Every Electrician Should Know
Do all equipment grounding conductors count separately?
No. Per NEC 314.16(B)(5), ALL equipment grounding conductors (EGCs) in a box count as a single conductor allowance based on the largest EGC present.
For example, if you have four 14 AWG and two 12 AWG grounding conductors, you only count 2.25 cubic inches total (based on the 12 AWG), not 6 separate conductors.
This is one of the most common mistakes electricians make on box fill calculations.
Do pigtails count toward box fill?
Pigtails that originate inside the box do NOT count. Per NEC 314.16(B)(1), only conductors that originate outside the box and enter it are counted.
A 6-inch pigtail from a wire nut to a device terminal doesn't count because it starts inside the box. However, the incoming conductor that the pigtail splices to DOES count.
How do I count conductors that pass through the box?
A conductor that passes straight through the box without a splice or termination counts as ONE conductor, not two.
Per NEC 314.16(B)(1), "each conductor that runs through a box without splice or termination shall be counted once."
So 4 conductors entering the left side and exiting the right side unbroken = 4 conductors for fill purposes, not 8.
Do internal clamps add to box fill?
Yes, but only one allowance for all clamps combined. Per NEC 314.16(B)(2), one or more internal cable clamps count as a single conductor volume based on the largest conductor in the box.
External clamps (like NM connectors on the outside) don't count toward fill.
Common mistake: counting each clamp separately—don't do that.
How do devices (switches/receptacles) affect box fill?
Each device yoke (mounting strap) counts as DOUBLE the volume of the largest conductor connected to that device.
Per NEC 314.16(B)(4), a receptacle connected to 12 AWG wire adds 4.5 cubic inches (2 × 2.25 cu in).
- A multi-gang yoke with multiple devices on a single strap counts as ONE device
- Two separate single-gang devices count separately
What's the volume allowance for each wire size?
NEC Table 314.16(B) provides the volume allowance per conductor:
- 18 AWG = 1.50 cu in
- 16 AWG = 1.75 cu in
- 14 AWG = 2.00 cu in
- 12 AWG = 2.25 cu in
- 10 AWG = 2.50 cu in
- 8 AWG = 3.00 cu in
- 6 AWG = 5.00 cu in
Conductors larger than 6 AWG typically require junction boxes per NEC 314.28.