Calculator
Electrical Box Fill Calculator
Check box fill before the inspector does — enter your conductors by AWG, cable clamps, device yokes, and equipment grounds to get required volume in cubic inches, pass or fail against NEC 314.16, and the smallest box that works. Free, no signup.
Free
Add a cableoptional
Adds one NM cable's insulated conductors plus its ground to the counts below. Tap once per cable entering the box — a feed plus a switch leg is two taps.
Conductors entering or leaving the box
Count every insulated conductor that originates outside the box and terminates or is spliced inside, plus every conductor that passes straight through — each counts once. A conductor looped through the box with at least 12 in. of free conductor counts twice, so enter it as 2. Grounds go in the section below.
Clamps, devices & grounds — 1 yoke · 2 grounds
Everything in the box that takes a volume allowance but is not a conductor. Pre-filled for a typical single-device rough-in.
Internal cable clamps
One or more clamps with parts inside the box take a single allowance at the largest conductor present. A connector whose clamping mechanism sits outside the box — the usual NM connector on a plastic box — takes none.
NEC 2023 · 314.16
Required box volume
14.00 in³
Custom box — 18 in³ marked holds 18.00 in³
Passes NEC 314.16 — room to spare
78% of box volume used100% = NEC limit
Smallest box that passes14 in³
3 x 2 x 2-3/4 device box
Box fill at a glance
Required volume
14.00 in³7 volume allowances
Free volume left
4.00 in³Spare capacity in the box
Fill
78%14.00 in³ of 18.00 in³
Conductors counted
4Largest in box: 14 AWG
Allowance breakdown
- 4 × 14 AWG conductorsNEC 314.16(B)(1) · 4 × 14 AWG allowance8.00 in³
- 1 device yoke / gang (double allowance each)NEC 314.16(B)(4) · 2 × 14 AWG allowance4.00 in³
- 2 equipment grounds (one allowance total)NEC 314.16(B)(5) · 1 × 14 AWG allowance2.00 in³
- Total required14.00 in³
NEC notes
- No clamp allowance added — cable connectors with the clamping mechanism outside the box take no allowance per 314.16(B)(2).
- Nonmetallic boxes must be marked with their volume — use the number moulded into the box, not Table 314.16(A).
Volume allowances — NEC Table 314.16(B)
| Conductor | Allowance | Typical use |
|---|---|---|
| 18 AWG | 1.50 in³ | Fixture whips, low-voltage |
| 16 AWG | 1.75 in³ | Fixture wire |
| 14 AWG | 2.00 in³ | 15 A lighting / receptacle |
| 12 AWG | 2.25 in³ | 20 A kitchen / bath / laundry |
| 10 AWG | 2.50 in³ | 30 A dryer, water heater |
| 8 AWG | 3.00 in³ | 40–50 A range, subfeed |
| 6 AWG | 5.00 in³ | 55–65 A feeder taps |
What you'll get
Pick the box
Choose a standard metal box from NEC Table 314.16(A), or enter the cubic-inch volume marked inside a plastic device box — the marking always governs for nonmetallic boxes.
Count what goes in
Tap a cable shortcut or enter conductors by AWG, then set internal clamps, device yokes and gangs, fixture studs or hickeys, equipment grounds, and terminal blocks.
Read the verdict
Get required volume in cubic inches versus box volume, fill percentage, remaining free space, a line-by-line allowance breakdown citing each NEC subsection, and the smallest standard box that passes.
How it works
- 1
Pick the box
Choose a standard metal box from NEC Table 314.16(A), or enter the cubic-inch volume marked inside a plastic device box — the marking always governs for nonmetallic boxes.
- 2
Count what goes in
Tap a cable shortcut or enter conductors by AWG, then set internal clamps, device yokes and gangs, fixture studs or hickeys, equipment grounds, and terminal blocks.
- 3
Read the verdict
Get required volume in cubic inches versus box volume, fill percentage, remaining free space, a line-by-line allowance breakdown citing each NEC subsection, and the smallest standard box that passes.
How NEC 314.16 box fill is actually counted
Box fill is not a conductor count — it is a volume calculation. NEC 314.16(B) assigns each item in the box a volume allowance from Table 314.16(B): 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG, 2.50 in³ for 10 AWG, and so on. Add the allowances together and the total must not exceed the box volume from Table 314.16(A) or the volume marked on a nonmetallic box. Overfilled boxes are one of the most common rough-in inspection failures, and the fix after drywall is expensive.
The counting rules are where most estimates go wrong. A conductor that originates outside the box and terminates inside counts once, and so does a conductor passing straight through — but a pigtail with no part leaving the box counts nothing. One or more internal cable clamps take a single allowance at the largest conductor present, while a connector clamping outside the box takes none. Each device yoke takes a double allowance, and a device wider than one gang takes a double allowance for every gang it occupies. All equipment grounding conductors share a single allowance based on the largest one, and since the 2020 NEC each ground beyond the fourth adds a quarter allowance. This calculator applies every one of those rules and shows you which subsection produced each line.
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View statesBox Fill Calculator FAQs
How do you calculate box fill?
Add up the volume allowances for everything in the box, then compare the total to the box volume. Each insulated conductor entering or leaving the box takes one allowance sized from NEC Table 314.16(B) — 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG. Internal cable clamps take one allowance total, each fixture stud or hickey type takes one, each device yoke takes two, and all equipment grounds share one. Pigtails that never leave the box take none. If the total exceeds the box volume, you need a bigger box or an extension ring.
Which NEC edition does this calculator use?
This calculator implements NEC (NFPA 70) 2023, Article 314.16, including the quarter-allowance rule for equipment grounding conductors beyond the fourth that was added in the 2020 edition and the terminal block fill rule added in 2023. Adopted code editions vary by jurisdiction and amendments are common, so always confirm the edition in force with your local authority having jurisdiction — the AHJ governs, not this tool.
Do ground wires count toward box fill?
Yes, but they are pooled rather than counted individually. Up to four equipment grounding conductors or bonding jumpers entering the box share a single volume allowance based on the largest one. Each additional ground beyond the fourth adds a quarter allowance. Six 14 AWG grounds, for example, come to 3.00 in³ — one full 2.00 in³ allowance plus two quarter allowances.
Does a receptacle or switch count toward box fill?
Each yoke or strap holding one or more devices takes a double volume allowance based on the largest conductor connected to that yoke. A duplex receptacle on 12 AWG takes 4.50 in³. A device wider than a single 2 in. device box takes a double allowance for every gang it occupies, so a two-gang-wide dimmer on 12 AWG takes 9.00 in³.
