Cubic Inch to cm³ Converter for Electrical Boxes
Convert electrical box volume between cubic inches, cm³ and litres. 1 cu. in. = 16.387064 cm³. Includes every NEC 314.16(A) box size and worked box-fill examples.
By Jack Shi, updated October 11, 2026
Standard unit for NEC box fill calculations
Metric equivalent (1 cu.in. = 16.387 cm3)
1 L = 1,000 cm³
Conversion Formula: 1 cubic inch = 16.387064 cubic centimeters
The formulas
- cm³ = cubic inches x 16.387064
- cubic inches = cm³ / 16.387064
- litres = cm³ / 1,000 = cubic inches x 0.016387064
The factor is exact, not a rounded approximation: an inch is defined as 2.54 cm, and 2.54 cubed is 16.387064. Only your own rounding (how many digits you keep) introduces error.
Common Box Volume Conversions
Every standard box in the Table 314.16(A) data this site uses, converted with the exact factor. Volumes are the table values; litres are rounded to three decimals.
| Box | Cubic Inches (cu.in.) | Cubic Centimeters (cm3) | Litres |
|---|---|---|---|
| 4" x 1-1/4" Round/Octagonal | 12.5 | 204.8 | 0.205 |
| 4" x 1-1/2" Round/Octagonal | 15.5 | 254.0 | 0.254 |
| 4" x 2-1/8" Round/Octagonal | 21.5 | 352.3 | 0.352 |
| 4" x 1-1/4" Square | 18.0 | 295.0 | 0.295 |
| 4" x 1-1/2" Square | 21.0 | 344.1 | 0.344 |
| 4" x 2-1/8" Square | 30.3 | 496.5 | 0.497 |
| 4-11/16" x 1-1/4" Square | 25.5 | 417.9 | 0.418 |
| 4-11/16" x 1-1/2" Square | 29.5 | 483.4 | 0.483 |
| 4-11/16" x 2-1/8" Square | 42.0 | 688.3 | 0.688 |
| 3" x 2" x 1-1/2" Device Box | 7.5 | 122.9 | 0.123 |
| 3" x 2" x 2" Device Box | 10.0 | 163.9 | 0.164 |
| 3" x 2" x 2-1/4" Device Box | 10.5 | 172.1 | 0.172 |
| 3" x 2" x 2-1/2" Device Box | 12.5 | 204.8 | 0.205 |
| 3" x 2" x 2-3/4" Device Box | 14.0 | 229.4 | 0.229 |
| 3" x 2" x 3-1/2" Device Box | 18.0 | 295.0 | 0.295 |
| 2-Gang Masonry Box | 14.0 | 229.4 | 0.229 |
| FS Single Gang | 13.5 | 221.2 | 0.221 |
| FS Double Gang | 18.0 | 295.0 | 0.295 |
| FD Single Gang | 18.0 | 295.0 | 0.295 |
| FD Double Gang | 24.0 | 393.3 | 0.393 |
Conductor allowances in cm³
Table 314.16(B) allowances per conductor, converted. If a box is marked in cm³, compare it with these instead of converting the box.
| Conductor size | Cubic Inches (cu.in.) | Cubic Centimeters (cm3) |
|---|---|---|
| 6 AWG | 5.00 | 81.9 |
| 8 AWG | 3.00 | 49.2 |
| 10 AWG | 2.50 | 41.0 |
| 12 AWG | 2.25 | 36.9 |
| 14 AWG | 2.00 | 32.8 |
| 16 AWG | 1.75 | 28.7 |
| 18 AWG | 1.50 | 24.6 |
Worked examples
1. A box marked in cm³
A box is marked 295 cm³. 295 / 16.387064 = 18.00 cu. in., which matches the 18.0 cu. in. boxes in Table 314.16(A) (for example the 4 in square, 1-1/4 in deep box). At 2.00 cu. in. per 14 AWG conductor that is 9.00, so 9 counted conductors before any deductions.
2. A fit check with metric markings
Hypothetical load, for illustration: six 12 AWG conductors, one device yoke, one internal clamp and one equipment grounding conductor. That is 6 x 2.25 + 2 x 2.25 + 2.25 + 2.25 = 22.50 cu. in., or 368.7 cm³.
- Box marked 350 cm³ = 21.36 cu. in.: fails (1.14 cu. in. short).
- Box marked 400 cm³ = 24.41 cu. in.: passes (1.91 cu. in. spare).
Run your real configuration through the box fill calculator, which applies every 314.16(B) allowance for you.
3. Going from the code table to a metric spec sheet
The 4 in square, 2-1/8 in deep box is 30.3 cu. in. in Table 314.16(A). 30.3 x 16.387064 = 496.5 cm³ = 0.497 L. If a spec sheet lists a slightly different figure, check whether it is rounded or whether it is a different box size.
Frequently asked questions
How many cm³ are in a cubic inch?
Exactly 16.387064 cm³. An inch is defined as 2.54 cm, and 2.54 x 2.54 x 2.54 = 16.387064.
How do I convert an electrical box volume from cubic inches to cm³?
Multiply the cubic-inch figure by 16.387064. A 30.3 cu. in. box (4 in square, 2-1/8 in deep) is 30.3 x 16.387064 = 496.5 cm³.
How many litres is 18 cubic inches?
0.295 litres. First convert to cm³ (18 x 16.387064 = 295.0), then divide by 1,000.
How many 12 AWG conductors fit in a box marked 400 cm³?
400 cm³ is 24.41 cu. in. At 2.25 cu. in. per 12 AWG conductor that is 10.85, so 10 counted conductors, before any deductions for device yokes, internal clamps, support fittings or equipment grounding conductors.
Do I have to convert units to do a box-fill calculation?
Only when the box and the allowances are in different units. NEC Tables 314.16(A) and 314.16(B) print both cubic inches and cubic centimeters, but a box can be marked in just one of them. Convert the box marking to cubic inches (or the conductor allowances to cm³) so that every number in the total uses the same unit.
Why does my converted figure differ slightly from the one printed in a table?
Printed tables round their metric column. For example 2.25 cu. in. converts to 36.871 cm³ and is printed as 36.9. For code compliance use the value the table or the box marking gives; use the exact conversion here for everything else.