Box fill is a counting exercise with unusual rules. Some things in an outlet box count once, some count twice, some count as a group no matter how many there are, and some do not count at all. The electrical box fill calculator above applies the counting rules of Article 314.16 of the National Electrical Code — NFPA 70 — to the items you tell it are in the box, multiplies by the volume allowances you look up in the code table, and compares the total with the volume marked on the box itself. It is a preliminary checking and teaching tool. It is not a design, it is not an inspection, and a licensed electrician signs off the installation.
Arb Digital publishes it alongside a free electrical set that includes the wire size calculator and the voltage drop calculator. Those two sit on the circuit; this one sits at the box, which is where crowded work turns into damaged insulation and loose terminations. Every figure that could be wrong if typed from memory — box volume, per-conductor allowance — is an input on this page rather than a built-in table, and the reason is explained below.
Jurisdiction: Which Code This Follows
The counting rules here follow NEC Article 314.16 as adopted in the United States. The NEC is published by the National Fire Protection Association on a three-year cycle, and it has legal force only where a state or local authority adopts it — frequently with amendments, and frequently an edition or two behind the current one. Canada works to the CSA C22.1 Canadian Electrical Code, the UK and Ireland to BS 7671, Australia and New Zealand to AS/NZS 3000, and the counting conventions are not identical. Adopted building and electrical codes for many US jurisdictions are searchable through the ICC digital codes library, and the NFPA publishes plain-language background on electrical safety for homes and workplaces in its electrical information hub.
Use this page to understand and check the arithmetic. Use your adopted code, and an electrician licensed in your jurisdiction, to decide what actually gets installed.
What This Box Fill Calculator Does
It converts the contents of a box into a count of volume allowances, then into cubic inches. Insulated conductors that originate outside the box each count once, whether they terminate inside it, are spliced inside it, or pass straight through. Conductors that both begin and end inside the box — pigtails and jumpers — count as zero, which is why the pigtail field exists and contributes nothing. One or more internal cable clamps count as a single allowance together. Each fixture stud or hickey counts as one. Each device yoke or strap counts as two, for every gang it occupies. All equipment grounding conductors together count as one, and an additional isolated grounding set adds one more.
The allowances for clamps, fittings, devices and grounds are all based on the largest conductor in the box, which is why that is a separate input. In a box where a heavier conductor passes through a run of lighter ones, that single detail changes several lines at once.
How to Use It
- Read the volume off the box. Boxes are required to be durably marked with their cubic-inch capacity. For a box built up from sections, add the marked volumes of the sections and any listed extension ring.
- Look up the two volume allowances in Table 314.16(B) of your adopted edition and type them in. The page will not fill them in for you on purpose.
- Count conductors that come from outside. Two cables into a box each carrying a hot and a neutral is four conductors, not two cables.
- Add clamps, studs, devices and grounds separately. Each has its own rule, and the tool applies it.
- Compare required volume with box volume. The fill percentage and spare volume tell you the margin, not just whether it squeezes in.
The Formula / How It's Calculated
Writing Vc for the per-conductor allowance and VL for the largest-conductor allowance:
Required = (terminating + through) × Vc + clamps × VL + studs × VL + 2 × yokes × gangs × VL + ground sets × VL, where clamps is 1 if any internal clamp is present and 0 otherwise, and ground sets is 1 if any equipment grounding conductor is present plus one for each additional isolated set.
Take the default box: two 14 AWG cables enter, each with a hot and a neutral, so four conductors terminate; two equipment grounding conductors arrive with them; there are internal clamps; one duplex receptacle occupies one gang; and two pigtails are made up inside. The counting gives 4 conductor allowances, 1 clamp allowance, 2 device allowances and 1 ground allowance — eight allowances in total, and the pigtails add nothing. If your table gives 2.00 cubic inches for 14 AWG, the required volume is 16.0 cubic inches. An 18 cubic inch box would leave 2.0 cubic inches spare, an 89 percent fill.
That worked example is deliberately close to the limit, because it is the most common box in a house and it is the one people overfill. Add a second device to make it a two-gang, or run a heavier conductor through, and the arithmetic stops working long before the wires stop physically fitting.
Why This Page Has No Built-In Tables
Two figures in a box fill calculation are commonly hard-coded into web tools: the volume of standard boxes, and the volume allowance for each conductor size. Both are published, both are revised, and both are exactly the kind of number a reader will trust without checking. A device box sold as a "single gang" comes in several depths with materially different volumes; a listed extension ring changes the total; and the allowance table has been amended across editions.
So this page asks for them. It is the same choice made by our breaker size calculator, which publishes no ampacity table and explains why: an ampacity read off a free web page and applied to a real circuit is a fire risk, not a convenience. Where a figure carries safety consequences and can only be right for one edition in one jurisdiction, the honest design is to make it an input and tell the reader where to find it.
The Counting Rules That Catch People Out
Four rules produce most of the disagreements between an estimate and an inspection. First, a device counts double, per gang. A duplex receptacle in a single-gang box is two allowances, and a device that spans two gangs is four. People consistently count it as one. Second, all grounds count as one. Three cables bringing three equipment grounding conductors is a single allowance, not three — this is the one rule that works in your favour, and it is routinely applied wrongly in the conservative direction.
Third, clamps count once regardless of quantity, and only internal clamps count. A cable connector whose clamping mechanism sits outside the box does not add to fill. Fourth, pigtails count zero. A conductor that originates and terminates inside the box adds no allowance, which is why making up grounds and neutrals with pigtails costs you nothing on the fill calculation even though it visibly fills the box with copper. That last point is worth pausing on: box fill is a code counting exercise, not a measurement of how much space the wires occupy.
Fill Is a Minimum, Not a Target
Passing the calculation means the box is large enough to be legal. It does not mean the work will be good. A box at 95 percent of its permitted fill is genuinely difficult to work in — conductors get forced back against terminal screws, insulation gets nicked on box edges, and a device that has to be shoved into place can loosen a termination as it goes. Loose terminations are a leading cause of heating at outlets, and they usually start at installation rather than developing later.
The practical response is to build in margin at the design stage, when it is free. Choosing a deeper box, a four-inch square box with a mud ring, or a two-gang box for a location that might later take a second device costs a few dollars during rough-in and nothing in labour. Doing it after drywall costs a wall repair. If your result lands above roughly 85 percent, treat that as a prompt to specify a larger box rather than a pass.
Box Fill, Conduit Fill and Derating Are Different Limits
Three separate limits get conflated because they all involve counting conductors. Box fill governs volume inside an outlet or junction box, and it is what this page computes. Conduit fill governs the cross-sectional area conductors may occupy inside a raceway, expressed as a percentage of the raceway's internal area and set out in the code's chapter 9 tables — you can get the raw geometry for that from our cross-sectional area calculator, though the permitted percentages come from the code. Ampacity adjustment, sometimes called derating, reduces the allowable current when several current-carrying conductors are bundled together, because they cannot shed heat independently.
A run can satisfy one and fail another. A box can be legally sized while the raceway feeding it is overfilled, and a raceway can be within its fill percentage while the conductors inside it need their ampacity adjusted. Each check has to be made on its own terms, and none of the three substitutes for the others.
Arb Digital builds free, fast, search-friendly tools that bring electrical contractors and suppliers steady organic traffic. Browse the library or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Counting a device as one allowance — each yoke counts double, and doubles again for every extra gang it occupies.
- Counting each ground wire separately — all equipment grounding conductors together are a single allowance.
- Counting cables instead of conductors — a two-conductor-with-ground cable brings two counted conductors plus a share of the single ground allowance.
- Guessing the box volume — boxes are marked with their capacity, and nominally identical boxes vary by depth and by maker.
- Treating a bare pass as good practice — a box at the limit is hard to work in, and forced conductors damage insulation and loosen terminations.
Related Free Tools From Arb Digital
Size the circuit with the wire size calculator, check the run with the voltage drop calculator, work out the protective device with the breaker size calculator, get load figures from the electrical power calculator, and price consumption with the electricity bill calculator. For raceway geometry, the cross-sectional area calculator handles the areas. Everything else is on the free online tools hub.
Frequently Asked Questions
Count each insulated conductor entering the box as one allowance, one allowance in total for internal clamps, one for each fixture stud or hickey, two for each device yoke per gang it occupies, and one for all equipment grounding conductors together. Multiply the counts by the volume allowances in Table 314.16(B) and compare the total with the volume marked on the box.
Two. Each device yoke or strap counts as a double volume allowance based on the largest conductor connected to it, and a device occupying more than one gang counts double for every gang. This is the single most commonly missed rule in a box fill calculation.
All of them together count as one single allowance, however many arrive. An additional set of isolated equipment grounding conductors adds one further allowance. This is the one counting rule that reduces the total rather than increasing it.
No. A conductor that both originates and terminates inside the box adds nothing, because the code counts conductors entering the box rather than measuring the copper inside it. Pigtails still take up real space, so a box that passes on paper can still be awkward to work in.
Because boxes of the same nominal size differ by depth and by manufacturer, and a wrong volume produces a confidently wrong answer. Boxes are required to be durably marked with their cubic-inch capacity, so the correct figure is on the part in your hand. Our breaker size calculator omits its ampacity table for the same reason.
No. Box fill limits the volume of an outlet or junction box in cubic inches. Conduit fill limits the cross-sectional area conductors may occupy inside a raceway, as a percentage set by the code. Ampacity adjustment for bundled conductors is a third, separate limit, and a run can pass one while failing another.
No. It reproduces the counting arithmetic so you can check your own work and understand the rules. Your adopted code edition and its local amendments govern, the inspector decides compliance, and a licensed electrician signs off the installation.
This tool reproduces the box fill counting arithmetic of NEC Article 314.16 for teaching and preliminary checking only. It is not an electrical design, it is not an inspection, and it carries no professional certification. Volume allowances and box capacities must be taken from the code edition and product data that apply to your jurisdiction, and a licensed electrician must design, install and sign off the work.
