Use this box fill calculator to total NEC-style wire space and see if your marked electrical box volume is enough.
Advanced options
Table of contents
How to use our Box Fill Calculator
- Enter Wires that are not grounds (AWG=count) using pairs such as 14=4 or 12=2, 14=2.
- Set Ground wires present and, if yes, choose Largest ground wire size (AWG).
- Enter Switches or outlets on straps (count) and the marked Box volume (cubic inches).
- Open Advanced options only if you need to count Internal cable clamps, a Fixture stud or hickey inside the box, or a Device wire size override (AWG).
- After you calculate, sanity-check Fill breakdown against the actual box: grounds should count once, each strap should count as two spaces, and Space left or short should make sense for the box size.
Definitions
AWG: American Wire Gauge, the wire size number used in the calculator. Smaller AWG numbers mean larger wires and larger box-fill allowances.
Volume allowance: The cubic-inch space assigned to one conductor of a given AWG size for box-fill counting.
Wires that are not grounds (AWG=count): Hot, neutral, traveler, switched, and spliced conductors entered as size equals count, such as 14=4.
Ground wires present: Whether equipment grounding conductors are in the box. All grounds together count as one allowance based on the largest ground wire.
Switches or outlets on straps (count): The number of device yokes or straps in the box. Each strap counts as two conductor volume allowances.
Box volume (cubic inches): The marked or listed internal volume of the electrical box, not the outside dimensions.
Space left or short: Box volume minus the calculated required volume. Positive means extra space; negative means the box is too small.
Same-size wire spaces left: The margin converted to whole spaces of the largest entered non-ground wire size, rounded down.
Common mistakes and quick fixes
Mistake: Putting ground wires in Wires that are not grounds (AWG=count) .
Fix: Remove ground wires from that box and use Ground wires present plus Largest ground wire size (AWG) instead.
Mistake: Using outside box dimensions for Box volume (cubic inches) .
Fix: Use the cubic-inch volume marked on the box or listed by the box maker.
Mistake: Counting one duplex receptacle strap as 2 in Switches or outlets on straps (count) .
Fix: Count the strap, not the two plug openings; one duplex receptacle strap is usually 1.
Mistake: Leaving Internal cable clamps set to No when the clamps are inside the box.
Fix: Set Internal cable clamps to Yes for clamps inside the box, but keep it No for external clamps.
Mistake: Letting Device wire size override (AWG) stay on automatic when the device strap is connected only to a smaller wire size.
Fix: Pick the actual device wire size so Minimum box volume needed does not use the wrong allowance.
Mistake: Treating a negative Space left or short as extra room.
Fix: A negative value means the Box check is short, so use a larger listed box or reduce the fill.
Limitations & Key Assumptions / Boundary Conditions
- The calculator supports 18, 16, 14, 12, 10, 8, and 6 AWG conductor allowances only.
- For 4 AWG or larger conductors, box and pull-box sizing rules can be different, so this calculator should not be used as the final sizing method.
- The result depends on the counts you enter. Miscounted pigtails, travelers, cable assemblies, straps, internal clamps, or support fittings will change the answer.
- Box volume (cubic inches) must come from the box marking or manufacturer listing. Outside length, width, and depth are not enough.
- The calculation does not add separate fill for wire connectors, locknuts, bushings, or small fittings not counted by the inputs.
- Local code rules, product listings, and inspector interpretations can be stricter than the simplified inputs shown here.
- The calculator checks box-fill volume only. It does not check conductor ampacity, conduit fill, grounding method, box support, device ratings, or safe installation practice.
Methodology
How the volume is counted
The calculator parses Wires that are not grounds (AWG=count) as AWG and count pairs. It supports these cubic-inch allowances per conductor: 18 AWG = 1.50, 16 AWG = 1.75, 14 AWG = 2.00, 12 AWG = 2.25, 10 AWG = 2.50, 8 AWG = 3.00, and 6 AWG = 5.00.
wire_fill = sum(count_awg * volume_allowance_awg)
ground_fill = volume_allowance_ground_awg if grounds are present; otherwise 0
device_fill = device_count * 2 * volume_allowance_device_awg
clamp_fill = volume_allowance_largest_present_awg if internal cable clamps are yes; otherwise 0
support_fill = volume_allowance_largest_present_awg if fixture stud or hickey is yes; otherwise 0
required_volume = wire_fill + ground_fill + device_fill + clamp_fill + support_fill
How the box is checked
The box comparison keeps the margin signed. A positive margin means space left. A negative margin means the box is short.
space_margin = box_volume - required_volume
fill_percent = required_volume / box_volume * 100
same_size_wire_spaces = floor(space_margin / volume_allowance_largest_non_ground_awg)
The same-size wire spaces result rounds down to a whole count. For example, 4.75 cubic inches left with 12 AWG as the largest non-ground wire gives floor(4.75 / 2.25) = 2 spaces.
Worked example
For 14=4, ground wires present with 14 AWG ground, 1 device strap, 18.0 cubic inches of box volume, no internal clamps, and no support fitting:
wire fill is 4 * 2.00 = 8.00 cubic inches, ground fill is 2.00, and device fill is 1 * 2 * 2.00 = 4.00. The minimum box volume needed is 14.00 cubic inches. The space left is 18.00 - 14.00 = 4.00 cubic inches, box fill used is 77.8%, and same-size wire spaces left is floor(4.00 / 2.00) = 2.
Number display
The Number display setting changes only formatting. Auto uses regular decimals for normal box-fill values and scientific notation only when the absolute value is at least 1000000 or greater than 0 but less than 0.0001.