Box Fill Calculator

How to use this tool

Pick a mode. “Fill count for a box” takes conductors by AWG size, devices, grounds, clamps and fittings, plus the box, and gives the volume needed, the percent full and the room left. “Find a box” names the smallest standard box for the same count. “Typical rooms” and “Reference tables” show common layouts and the volume tables. If the box is marked with its volume, or you add a marked ring, choose the marked option and enter it in cubic inches or cubic centimetres.

The input people most often get wrong is the conductor count. Count wires, not cables: a 12/2 cable brings two insulated conductors, and its bare ground is counted with the other grounds. A cable in and a cable out at a receptacle is 4 conductors, not 2. Set clamps to none if the box has no internal clamps, as many plastic boxes do not.

What the calculation is doing

Box fill turns everything that takes room in a box into a number of conductors at one wire size, then multiplies by the volume for that size. Table 314.16(B) gives 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG, 2.50 in³ for 10 AWG, 3.00 in³ for 8 AWG and 5.00 in³ for 6 AWG, which is 32.8, 36.9, 41.0, 49.2 and 81.9 cm³.

Each conductor that ends or is spliced in the box counts once, and so does each conductor that runs through unbroken. A loop of unbroken conductor 12 in or longer counts twice. Each device yoke counts twice, at the size of the largest wire connected to it. All the grounds together count once at the largest ground size, though from the 2020 edition only the first four count as one and each extra adds a quarter. All internal clamps together count once at the largest conductor in the box, and each type of luminaire stud or hickey counts once. Pigtails, locknuts, bushings and wire connectors count as nothing.

The box side is a volume: the listed volume of a standard metal box from Table 314.16(A), or the volume marked on the box, plus the marked volume of any plaster ring or extension ring. Percent full is the volume needed divided by the box volume. The room left divided by the volume for a wire size, rounded down, is how many more conductors of that size fit. One cubic inch is exactly 16.387064 cm³ because one inch is 2.54 cm.

Why box fill is about heat and elbow room, not amps

Box fill is not a current rating. It exists so a box has room for the conductors, the splices and the device, with space for heat to dissipate and for insulation to survive when the device is pushed back in. The failure it prevents is wires forced into a box that is too small, with damaged insulation and terminations loosened inside the wire nuts.

That is why the volume rises with wire size: a 6 AWG conductor takes 5.00 in³ against 2.00 in³ for 14 AWG, two and a half times the room, because heavier wire takes more space to bend back. A device counts as two conductors for the same reason, since a receptacle or switch fills a real part of the box. A device wider than one gang counts two for every gang it needs.

Grounds and clamps count once because they share space. The 2020 edition changed that for crowded boxes: the first four grounds count as one and each further ground adds a quarter of an allowance. Six 12 AWG grounds cost 1.5 allowances, 3.375 in³, where the 2017 text charged one, 2.25 in³. Seventeen grounds cost 4.25 allowances.

A box that looks large can still fail. Take a three way switch on 12 AWG, fed by a 12/3 cable with a 12/2 cable leaving and internal clamps. Its 5 conductors, 2 for the switch, 1 for the grounds and 1 for the clamps make 9 allowances of 2.25 in³, or 20.25 in³. A 4 in square box 1¼ in deep is listed at 18.0 in³, so it is over by 2.25 in³. The 1½ in deep box at 21.0 in³ passes at 96.4% full. A 3 x 2 x 2½ device box holds 12.5 in³, and a 12/2 receptacle with cable in and out needs 15.75 in³ even with no clamps.

When the count is over, there are three ways out. Use a deeper or larger box: a 4 in square box 2⅛ in deep is 30.3 in³, 44% more than the 1½ in box, and a 4 11/16 in square one of the same depth is 42.0 in³. Add a marked extension ring or plaster ring, whose volume adds to the box: 30.3 in³ plus a 3.60 in³ ring is 33.90 in³, and after ten 12 AWG conductors at 22.50 in³ that leaves 11.40 in³, room for five more 14 AWG. Or keep through wires whole: six 12 AWG wires passing through a 4 x 1½ square box count 13.50 in³, 64.3% full, but cut and spliced they count twelve, 27.00 in³, and are over by 6.00 in³.

Where this count stops being the answer

This tool is a planning aid. The rules above are the US National Electrical Code method, and what binds you is the edition your jurisdiction adopted, with local amendments, as read by your inspector or authority having jurisdiction. In Canada the Canadian Electrical Code has its own box rule, Rule 12 3034 with its tables, so Canadian installers should check that rule and not rely on this count. Conduit bodies, boxes with barriers and unusual equipment need the code text and the manufacturer’s marked volume.

The standard boxes and how many wires each holds

The standard metal boxes of Table 314.16(A), with the most conductors of one size each holds when nothing else is inside. A device, clamp or set of grounds removes allowances, so treat these as a ceiling.

A worked example you can check on paper

A duplex receptacle on 12 AWG with a 12/2 cable in and another out, internal clamps, in a 4 x 1½ square box listed at 21.0 in³. Conductors: 4 × 2.25 = 9.00 in³. Device yoke: 2 × 2.25 = 4.50 in³. The two grounds count once: 2.25 in³. The clamps count once: 2.25 in³. The total is 8 × 2.25 = 18.00 in³, or 295 cm³. The box is 18.00 ÷ 21.00 = 85.7% full with 3.00 in³ left, and 3.00 ÷ 2.25 = 1.33 means one more 12 AWG conductor fits.

Now mix sizes, the published EC&M example: a 14/3 cable to a three way switch and a 12/2 cable to a receptacle, with clamps. The switch yoke counts at 14 AWG, the wire connected to it: 3 + 2 = 5 allowances × 2.00 = 10.00 in³. At 12 AWG the receptacle adds 2 + 2 and the grounds and clamps add 1 + 1, sized at the largest wire in the box: 6 × 2.25 = 13.50 in³. The total is 23.50 in³, over a 21.0 in³ box by 2.50 in³ but 77.6% of a 4 x 2⅛ square box at 30.3 in³.

Mistakes that catch people out

Counting cables instead of conductors. A 12/2 cable is two insulated conductors plus a ground, so a cable in and a cable out is 4 conductors before any device. Count the wires, then let the grounds count once.

Forgetting that a device counts twice. One receptacle costs 4.50 in³ at 12 AWG. Enter one device per yoke, and one per gang of a wide device.

Cutting a through wire. An unbroken conductor counts once, but cut and spliced it counts twice. If the box is tight, leave the wire whole and tap it with a pigtail, which counts as nothing.

Using the table volume for a plastic box or a ring. Table 314.16(A) covers standard metal boxes. A plastic box has its own marked volume, and a ring adds volume only if it is marked or matches a listed box. Choose “Marked volume, any other box” and type the number.

Applying the wrong edition to grounds. From the 2020 edition a fifth ground adds a quarter allowance. Under the 2017 text the same grounds count as one. Set the edition your jurisdiction adopted.

Popular calculations

Each one opens the calculator with those figures filled in.

Fill counts for common jobs

Boxes that fail, and the fixes

Marked volumes and the grounds rule

Find the smallest box

Typical layouts

Reference tables

How this was checked

The five allowances of Table 314.16(B) agree across published lists from EC&M, Electrical Contractor magazine, IAEI Magazine, JADE Learning, ExpertCE and manufacturer catalogs, and the metric column equals 16.387064 times the inch values. All 18 box volumes equal Eaton Crouse Hinds, USESI and Grover Electric catalogs, and all 90 capacities for 14 to 6 AWG equal a transcription of Table 314.16(A).

The engine reproduces 15 published worked examples exactly, from EC&M, Electrical Contractor magazine, JADE Learning, ExpertCE and others, including a 3.60 in³ plaster ring and the 2020 ground rule with ten and seventeen grounds. A separately written implementation matched the engine on 200,000 random counts, with both ground editions, loops, fittings and mixed sizes. The smallest box matched a brute force search on 19,902 random counts.

Adding any item never lowered the volume needed (39,791 random additions), and a larger box never failed where a smaller one passed (344,443 box pairs). The Canadian point rests on the Canadian Electrical Code having Rule 12 3034 and tables of its own, seen in an Alberta government interpretation and a published summary of the rule; no Canadian numbers are used. Every address above was loaded and checked for a distinct title, the right canonical address and no error values.

Common questions

How do you calculate box fill?

Count each conductor once, add two for each device yoke, one for all the grounds, one for all internal clamps and one for each type of luminaire stud, then multiply by the volume for the wire size. A receptacle on 12/2 cable in and out, with clamps, is 4 + 2 + 1 + 1 = 8 allowances, and 8 × 2.25 = 18.00 in³.

How many wires fit in a 4 in square box?

A 4 in square box 1½ in deep is listed at 21.0 in³. With nothing else inside it holds 10 conductors of 14 AWG, 9 of 12 AWG, 8 of 10 AWG, 7 of 8 AWG or 4 of 6 AWG. Every device, clamp and set of grounds takes some of that away.

Do wire nuts and pigtails count in box fill?

No. Wire connectors, locknuts and bushings need no allowance, and a pigtail that starts and ends inside the box is not counted. The cable conductors it joins are counted, each once. What does add up is cutting a through wire, because each cut end then ends in the box.

How do grounds count in box fill?

From the 2020 NEC, the first four grounds and bonding jumpers together count as one conductor at the largest ground size, and each extra one adds a quarter allowance. Six 12 AWG grounds are 1.5 allowances, 3.375 in³. Before 2020 all grounds counted as one, so the same six were 2.25 in³, with one more allowance for a separate isolated set. The tool has a setting for the edition.

Does a GFCI, dimmer or switch count as one conductor or two?

Two, at the size of the largest wire connected to that device. A single gang yoke counts two allowances whatever its ampere rating. A switch on 14 AWG beside a receptacle on 12 AWG counts as two 14 AWG allowances and two 12 AWG allowances. A device wider than one gang counts two for each gang space it needs.

What can you do when the box is too full?

Use a deeper or larger box, add a marked extension ring or plaster ring, or keep through wires whole so they count once. The rule is a limit, not an estimate, so a count of 20.25 in³ in a 20.00 in³ box is still over.

Related Calculators and Reference

• Cable Size and Ampacity Estimator: pick the wire size for the load first, since it sets every allowance here.
• Voltage Drop Calculator: check the run length once the wire size and boxes are settled.
• Watts to Amps Calculator: the current a circuit carries, where the wire size starts.
• Ohm Law Calculator: voltage, current, resistance and power for the circuit in the box.
• Power Factor Correction Calculator: the next electrical planning step for motor and panel work.
• IAEI Magazine, Box Fill Calculations: the inspectors’ association article on the counting rules and box volumes.