The arithmetic is addition. The rules are the hard part

When to use this: deciding what box to buy, checking whether an existing box can take another cable, or working out why a device will not push back into the wall.

Every conductor, clamp and device in a box is assigned a volume from Table 314.16(B), and the total must fit the box. Nobody gets the addition wrong. Three counting rules produce nearly every wrong answer:

1. Pigtails count zero. A conductor that both originates and terminates inside the box adds nothing — 314.16(B)(1).
2. All grounds count one, together. Not one each. Six grounds are a single allowance — 314.16(B)(5).
3. Each device counts two. A yoke or strap is two allowances, not one — 314.16(B)(4).

The first two make people buy boxes larger than they need. The third makes them buy boxes too small. They do not cancel out.

A worked example — the most common box in a house

Two 14/2 NM cables into a device box, feeding one switch, with internal clamps:

A 3 × 2 × 2½″ device box holds 12.5 in³. This arrangement needs 16.00, so it does not fit — by 3.50 in³. The 3½″ deep box at 18 in³ does, with 2.00 in³ spare.

That surprises people, because the 2½″ box is everywhere. It is a perfectly legal box — just not for four conductors, a ground, clamps and a switch.

What the common mis-count gives

Count the two grounds separately and the device as one allowance instead of two, and you get 24.00 in³. That is only 8.00 in³ away from the right answer here — close enough to look plausible and wrong in a way that varies with the box. On a two-gang box with four grounds the same errors diverge much further.

Table 314.16(B) — volume per conductor

The largest conductor in the box sets the price of everything else. Clamps, device yokes and support fittings are all charged at the largest conductor present, not at their own size. Running a single 10 AWG through a box of 14 AWG lifts every one of those allowances from 2.00 to 2.50 in³ — with two devices and clamps, an extra 2.50 in³ on top of the conductor itself.

Table 314.16(A) — box volumes

Metal boxes only. Non-metallic boxes are required to be marked with their volume and that marking governs — read the number moulded into the box rather than measuring it.

The last two columns are the numbers everybody remembers — and they assume the box contains nothing else at all. A 4″ square × 1½″ takes 9 bare 12 AWG conductors; add a device, the grounds and clamps and it realistically holds six.

The rules in full

314.16(B)(1) — conductor fill

One allowance for each conductor that originates outside the box and terminates or is spliced inside. One for each conductor passing through unbroken. Zero for a conductor with both ends inside the box. Fixture wires smaller than 14 AWG entering from a domed luminaire canopy are also excluded, up to four of them.

314.16(B)(2) — clamp fill

One allowance in total where one or more internal cable clamps are present, at the largest conductor in the box. Clamps or connectors on the outside count nothing — which is the normal arrangement for NM cable into a metal box, and a common source of unnecessary volume in hand calculations.

314.16(B)(3) — support fittings

One allowance for each luminaire stud or hickey, at the largest conductor present.

314.16(B)(4) — device fill

Two allowances for each yoke or strap containing one or more devices, at the largest conductor connected to that yoke. In a device box this is usually the single largest item on the list. A device wider than a single 2-inch gang counts two allowances for each gang it occupies.

314.16(B)(5) — grounding conductor fill

One allowance for all of them, at the largest. Where an additional set of isolated grounding conductors is present under 250.146(D), that set gets one more allowance — the only circumstance in which grounds are counted twice.

When it does not fit

In order of cost:

• Add an extension or plaster ring. They are marked with their own volume, which adds to the box. This fixes most calculations that come up two or three cubic inches short, and it is why a box that fails on paper may be compliant as actually installed.
• Use a deeper box of the same footprint. A 3 × 2 device box goes from 7.5 in³ at 1½″ to 18 in³ at 3½″ for the same wall opening.
• Move the splices. Put the through-conductors and joints in a separate accessible junction box and run a single cable to the device.

Box fill is a minimum, not a target. A box filled to 98% is legal, unpleasant to work in, and leaves nothing for the next change. Crowded conductors are also how insulation gets damaged when a device is pushed back into the wall — which is the failure the rule exists to prevent.

Conduit bodies are different

314.16(C) handles them separately. A conduit body enclosing 6 AWG or smaller must have a cross-sectional area at least twice that of the largest conduit entering it. Where it contains splices or devices it must be durably marked with its volume and then calculated like a box. A conduit body with no splices, only unbroken conductors passing through, has no fill calculation at all.

For the raceway feeding the box, our conduit fill calculator works to NEC Chapter 9, and ground wire size covers the equipment grounding conductor that costs you one allowance here however many of them arrive.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.