Box Fill Calculator

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Add the conductors, clamps, devices, and grounds in an electrical box to find the required volume and check it against NEC 314.16.Learn more ▾Show less ▴
This calculator follows NEC 314.16, the section that governs how much can go into an electrical box. It applies the per-conductor volume allowance from Table 314.16(B) and the counting rules for clamps, support fittings, devices, and grounding conductors, then compares the total to your box's volume. Choose a standard box from the NEC Table 314.16(A) list or enter a custom volume, and it tells you whether everything fits — or recommends the smallest box that does.
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Volume breakdown

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About This Tool

The Box Fill Calculator totals the volume that the contents of an electrical box require under NEC 314.16 and compares it to the box's capacity. It covers all five fill components — conductors, clamps, support fittings, devices, and grounds — so electricians, inspectors, and DIYers can confirm a box is legal before it is wired and closed up.

Sources: NEC (NFPA 70)

How to Use

  1. Choose a standard box or enter its cubic-inch volume — or switch to "find minimum box" to size it automatically.
  2. Add the conductors by size and quantity, then set the devices, clamps, support fittings, and grounding conductors in the box.
  3. Read the required volume, how much the box has, and the pass/fail result — or the recommended minimum box.

Methodology

Calculations follow NEC (NFPA 70) Section 314.16. Each conductor uses the volume allowance from Table 314.16(B) — for example 2.00 in³ for 14 AWG and 2.25 in³ for 12 AWG. Internal clamps add one allowance and each support fitting one, both at the largest conductor; each device yoke adds two allowances per gang. Grounding conductors are counted with the 2020/2023 rule (one allowance up to four, plus a quarter each beyond) or the 2017 rule. The total is compared to the box volume from Table 314.16(A) or your custom entry.

Sources: NEC (NFPA 70) 314.16 · NEC 314.16 Box Fill (UpCodes) · Box Fill worked examples (EC&M)

Understanding Your Results

The required volume is the sum of every allowance in the box. If it is at or below the box's volume, the box passes; if it is greater, the box is overfilled and you need a larger box, a box-extender ring, or fewer contents. The breakdown shows where the volume goes. Devices and the largest conductor often dominate, because device and clamp allowances are taken at the largest conductor present. Device fill uses the largest conductor in the box. In a multi-gang box where different yokes connect different conductor sizes, this is slightly conservative — it can report a larger required volume than NEC 314.16(B)(4) strictly requires, but never smaller. Box fill is a North-American (NEC) concept. Other standards size boxes differently — for example, IEC and BS 7671 require only that a box have adequate space, without a cubic-inch calculation.

Sources: IEC 60670-1 — Boxes & enclosures · BS 7671 — IET Wiring Regulations

Sources

Practical Examples

Example 1: A 3×2×3½ device box (18.0 in³) with four 14 AWG conductors, internal clamps, one receptacle, and two grounds. That is 4×2.00 + 2.00 (clamps) + 2×2.00 (device) + 2.00 (grounds) = 16.0 in³ — it fits, with 2.0 in³ to spare. Example 2: The same layout in 12 AWG needs 4×2.25 + 2.25 + 4.50 + 2.25 = 18.0 in³, exactly filling the 18.0 in³ box. One more conductor would require a larger box.

Box fill tips

• Count conductors that enter and terminate in the box, but not pigtails that begin and end inside it. • Clamps count once total, not once per clamp — and only internal clamps. External cable connectors do not count. • Each device gang is two allowances at the largest conductor, so larger conductors raise device fill quickly. • When in doubt, size up. A box-extender (plaster) ring adds marked cubic inches and is an easy way to gain room. • Match the NEC edition to your jurisdiction — the grounding rule is the one that changed in 2020.

All calculations are performed locally in your browser. No data is sent to any server.

Frequently Asked Questions
What is box fill and why does it matter?
Box fill is the total volume, in cubic inches, that the contents of an electrical box require under NEC 314.16. Conductors, clamps, support fittings, devices, and grounds each take a defined amount of space. If their total exceeds the box's volume, the box is overfilled — a common code violation that makes wiring hard to fold in safely and can damage insulation.
How do I calculate box fill?
Add up the volume allowance for five things, per NEC 314.16(B): each conductor (by its size), cable clamps, support fittings such as luminaire studs or hickeys, devices, and equipment grounding conductors. This calculator does all five automatically — enter what's in the box and it returns the required cubic inches, compares it to your box volume, and shows a pass or fail.
Which conductors count toward box fill?
Under 314.16(B)(1), each conductor that runs through the box or terminates in it counts as one volume allowance, based on its size. A conductor that both originates and ends inside the box — such as a short pigtail or jumper — is not counted. Conductors that come from outside and end on a device are counted normally.
How are devices counted in box fill?
Each yoke or strap that holds a device — a receptacle, switch, or similar — counts as two volume allowances, based on the largest conductor connected to that device (314.16(B)(4)). A device wider than a single gang counts two allowances for each gang it occupies. Enter the number of device gangs and the calculator applies the doubling.
Do cable clamps and connectors count?
Internal cable clamps count as a single volume allowance total — not one per clamp — based on the largest conductor in the box (314.16(B)(2)). Cable connectors whose clamping mechanism is outside the box, and locknuts and bushings, do not count. Check the clamps option only when the clamping mechanism is inside the box.
How is the grounding conductor counted?
All equipment grounding conductors together count based on the largest one present. Under the 2020 and 2023 NEC, that is one allowance for up to four grounds, plus a quarter allowance for each additional ground beyond four (314.16(B)(5)). The calculator lets you set how many grounds are in the box and which edition to apply.
What changed between the 2017 and 2020 NEC for grounds?
In the 2017 NEC and earlier, all equipment grounding conductors counted as a single volume allowance (with one more allowance if a separate isolated-ground set was present). The 2020 NEC replaced that with one allowance for up to four grounds plus a quarter allowance per additional ground. Use the edition selector to match the code your jurisdiction enforces.
What box volume should I use?
Standard steel and nonmetallic boxes have their volume stamped inside or printed on the box. You can also pick a standard box from the NEC Table 314.16(A) list built into this tool, which covers common round, octagonal, square, and device boxes. For boxes not on the list, enter the marked cubic-inch volume directly.
Is my data private?
Yes. The calculator works entirely in your browser. Nothing you enter about the box or its contents is uploaded, stored on a server, or shared. It keeps working offline once loaded, and a shared link only includes your inputs if you choose to copy it.