Electrical boxes with the same face dimensions can have substantially different usable volumes. A common 4-inch square steel box provides 18.0, 21.0, or 30.3 cubic inches depending on its depth and construction. Use the chart to narrow your selection, then verify the actual box capacity and compare it with the required box-fill volume; dimensions alone do not establish how many conductors and devices a box can contain.
Box Sizes and Volumes Chart for Common Steel Boxes
| Box type | Nominal face dimensions | Nominal depth | Reference capacity (in³) |
|---|---|---|---|
| Square | 4 × 4 in. | 1-1/4 in. | 18.0 |
| Square | 4 × 4 in. | 1-1/2 in. | 21.0 |
| Square, common welded construction | 4 × 4 in. | 2-1/8 in. | 30.3 |
| Square | 4-11/16 × 4-11/16 in. | 1-1/2 in. | 29.5 |
| Square | 4-11/16 × 4-11/16 in. | 2-1/8 in. | 42.0 |
| Octagon | 4 in. | 1-1/2 in. | 15.5 |
| Octagon | 4 in. | 2-1/8 in. | 21.5 |
| Single-gang switch/device | 3 × 2 in. | 2-1/2 in. | 12.5 |
| Single-gang switch/device | 3 × 2 in. | 2-3/4 in. | 14.0 |
| Single-gang switch/device | 3 × 2 in. | 3-1/2 in. | 18.0 |
These U.S. reference capacities are documented in Hubbell/RACO’s manufacturer catalog and product specifications. They identify common steel-box configurations, not every product with similar dimensions. The manufacturer catalog used here is dated December 2025; this article’s 2026 update does not change that source date.
The distinction matters even within one box family. RACO lists a drawn 4-inch square × 2-1/8-inch box, catalog number 232D, at 30.0 in³, while common welded versions are listed at 30.3 in³. Check the exact product rather than automatically assigning 30.3 in³ to every deep 4-inch square box.
Applicable NEC Edition and Scope
NEC 314.16(A) addresses available box volume; 314.16(B) addresses the volume required by conductors, clamps, support fittings, devices, and grounding conductors. The basic comparison is:
[
V_{\text{available}} \geq V_{\text{required}}
]
This chart supports ordinary small-conductor box-fill checks. It is not a pull-box sizing table for larger conductors; boxes enclosing 4 AWG or larger conductors also require consideration of NEC 314.28.
As of October 11, 2026, NFPA identifies the 2026 NEC as its current published edition. That does not mean your jurisdiction has adopted it: states and municipalities may enforce earlier editions and local amendments. Confirm the applicable edition with the authority having jurisdiction, or AHJ, before using a calculation for an installation.
Where to Find the Box Volume
Look for the manufacturer’s capacity marking, commonly expressed as “cu. in.” or “in³.” For an exact product, the manufacturer’s specification should also identify its cubic capacity; RACO’s product pages, for example, list this under “Dimensions.”
NEC 314.16(A) distinguishes standard metal boxes from other boxes:
- Standard metal boxes without a capacity marking use the applicable volume in Table 314.16(A).
- Nonmetallic boxes must carry a durable, legible manufacturer volume marking.
- Other boxes of 100 in³ or less that are not described in the table must also be marked.
- A standard box with a larger capacity than the table value may carry a manufacturer marking identifying that greater capacity.
This is why a plastic single-gang box should not inherit the capacity of a similarly sized steel switch box. Identify its marked volume instead. Likewise, do not calculate allowable capacity by multiplying exterior length, width, and depth: that calculation does not establish the code-recognized volume of the actual enclosure.
For reference, 1 in³ equals 16.387064 cm³. Keep available capacity and required fill in the same units throughout the comparison.
Why 4-Inch Square Boxes Come in Different Depths
Depth changes capacity without necessarily changing the box’s wall footprint. Moving from a common 4-inch square × 1-1/2-inch box to a welded 2-1/8-inch-deep version increases reference capacity from 21.0 to 30.3 in³—a difference of 9.3 in³. That additional space can accommodate a larger calculated fill while preserving the same nominal face dimensions.
The available choices solve different constraints:
| Selection approach | What it changes | What still needs verification |
|---|---|---|
| Use a deeper box | Increases capacity while retaining the nominal face size | Wall-cavity depth, actual product capacity, and device clearance |
| Use a larger-face box | Provides another capacity option, such as a 4-11/16-inch square box | Mounting space and compatible covers |
| Add a compatible extension ring or raised cover | May increase the assembled enclosure’s recognized volume | Whether the added volume qualifies under 314.16(A) |
| Use a different device-box configuration | Changes the enclosure arrangement | Exact marked capacity and suitability for the intended device |
RACO documents both the capacity differences and the use of qualifying rings and covers in assembled-volume calculations. It also identifies separate box-depth and device-clearance requirements. A satisfactory cubic-inch total therefore does not, by itself, establish that the device fits appropriately.
Counting Raised Covers and Extension Rings
NEC 314.16(A) permits qualifying additional volume from assembled sections, including plaster rings, domed covers, and extension rings. Added space must be supported by the applicable marking or standard-box provisions—not estimated from its appearance.
For a hypothetical assembly with a verified 21.0 in³ box and a compatible ring marked 5.0 in³:
[
V_{\text{available}} = 21.0 + 5.0 = 26.0\text{ in}^3
]
The 5.0 in³ ring is an assumed example input, not a claim about a particular product. Do not credit that amount unless the actual component qualifies.
Conductor Volume Allowances for Box Fill
The conductor allowance converts each counted conductor—or equivalent allowance for another component—into required cubic inches. These are calculation allowances, not measurements of the wire’s physical displacement. RACO reproduces the following values in its box-fill reference.
| Conductor size | Volume per allowance (in³) |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
For mixed conductor sizes, calculate the conductor portion separately by size:
[
V_{\text{conductors}}=\sum n_i v_i
]
Here, (n_i) is the counted number of conductors of a particular size, and (v_i) is that size’s volume allowance. Add the applicable clamp, support-fitting, device, and grounding allowances afterward. Each category uses the conductor size specified by its own rule; using the largest wire everywhere is not the general calculation method.
Allowances Beyond Insulated Conductors
| Component | Treatment for an ordinary box-fill calculation |
|---|---|
| Conductors entering and terminating or splicing in the box | Count each applicable conductor once |
| Conductors passing through without a splice or termination | Count each applicable conductor once |
| Pigtails wholly contained within the box | Not counted as entering conductors |
| One or more internal cable clamps | One allowance, based on the largest conductor present |
| Cable connector with its clamping mechanism outside the box | No internal-clamp allowance |
| Ordinary single-gang device yoke or strap | Two allowances, based on the largest conductor connected to its supported device |
| Fixture studs or hickeys | One allowance for each fitting type, based on the largest conductor present |
| Up to four entering equipment grounding conductors or applicable bonding jumpers | One collective allowance, based on the largest entering grounding conductor or bonding jumper |
The conductor, clamp, support, and ordinary device treatments are documented in RACO’s reference. The grounding treatment shown here follows the modern rule rather than the older unlimited “all grounds count as one” shortcut. Each additional entering grounding conductor or applicable bonding jumper beyond four adds one-quarter of the relevant allowance; check the wording in the adopted NEC edition.
A box’s advertised conductor count is not a blanket allowance for an installed receptacle, internal clamps, and grounding conductors. Those components can consume additional calculated volume even when the insulated conductors alone appear to fit.
Worked Example: Selecting a Box for Two 12/2 Cables
This hypothetical example uses a straightforward configuration consistent with the 2023 NEC box-fill method:
- Two 12/2 cables with ground enter the box.
- Each cable contributes two insulated 12 AWG conductors and one 12 AWG equipment grounding conductor.
- One ordinary duplex receptacle occupies one mounting yoke.
- Internal cable clamps are present.
- No fixture studs, hickeys, barriers, or other equipment are present.
- Any pigtails remain entirely within the box.
- No additional cover or ring volume is credited.
RACO includes this same basic two-cable receptacle configuration as a box-selection example.
Calculate the Required Volume
1. Count the insulated conductors: two cables × two conductors = four.
2. Use 2.25 in³ per 12 AWG allowance.
3. Add one collective allowance for the two entering grounding conductors.
4. Add one allowance for the internal clamps.
5. Add two allowances for the receptacle’s single yoke.
6. Compare the total with the verified capacity of the proposed enclosure.
| Fill component | Allowances | Volume per allowance | Required volume |
|---|---|---|---|
| Insulated 12 AWG conductors | 4 | 2.25 in³ | 9.00 in³ |
| Two entering 12 AWG grounds, collectively | 1 | 2.25 in³ | 2.25 in³ |
| Internal clamps | 1 | 2.25 in³ | 2.25 in³ |
| One receptacle yoke | 2 | 2.25 in³ | 4.50 in³ |
| Total | 8 | 18.00 in³ |
Because every applicable allowance in this example uses 12 AWG:
[
V_{\text{required}}
=(4+1+1+2)\times2.25
=18.00\text{ in}^3
]
The calculation matches the 18.0 in³ minimum shown in RACO’s example.
Compare the Available Boxes
| Candidate from the reference chart | Available volume | Available minus required | Volume comparison |
|---|---|---|---|
| 3 × 2 × 2-1/2-in. device box | 12.5 in³ | −5.5 in³ | Insufficient |
| 3 × 2 × 3-1/2-in. device box | 18.0 in³ | 0.0 in³ | Meets the calculated volume exactly |
| 4 × 4 × 1-1/2-in. square box | 21.0 in³ | 3.0 in³ | Exceeds the calculated volume |
| Common welded 4 × 4 × 2-1/8-in. square box | 30.3 in³ | 12.3 in³ | Exceeds the calculated volume |
An 18.0 in³ enclosure meets this example’s volume calculation exactly, but leaves no calculated capacity for another entering conductor or additional counted component. The larger options provide more volume; their suitability still depends on the actual product, cover arrangement, device clearance, mounting, and applicable installation requirements.
Use the Box Fill Calculator to cross-check the same inputs. The hand-calculated target is 18.00 in³; if the output differs, reconcile the conductor counts, grounding treatment, clamp setting, and device-yoke allowance before selecting a box.
Box Selection Checklist
Use this checklist before accepting the volume comparison. It separates the chart lookup from the installation conditions that the chart cannot resolve.
- Verify the NEC edition and amendments enforced by the local AHJ.
- Identify the exact box type, depth, construction, and product number.
- Read the marked capacity or use the applicable standard-metal-box table entry.
- Record conductor sizes and count the applicable entering or passing conductors.
- Include internal clamps, device yokes, and support fittings where required.
- Apply the grounding allowance for the adopted code edition.
- Credit ring or cover volume only when supported by the applicable provisions.
- Confirm that available volume is at least the calculated required volume.
- Check device depth, mounting, environmental suitability, and any required fixture-support rating.
A box-fill result addresses space under the stated inputs. It does not constitute NEC approval, permit approval, or confirmation that the entire installation complies; manufacturer instructions, the adopted code, and site-specific professional judgment remain necessary.