Electrical Box Fill Calculator
Calculate how many additional wires fit in an electrical junction box after accounting for installed devices. Critical for NEC compliance when adding wires or devices to an existing box.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
NEC 314.16 limits how many conductors a box may hold. Each conductor counts as the volume allowance in NEC Table 314.16(B): 18 AWG 1.50 in³, 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 in³. Each device (switch or receptacle) on a yoke counts as two conductors of the largest size connected to it (314.16(B)(4)), and all equipment grounding conductors together count as one conductor (314.16(B)(5)). The formula is: max conductors = floor((boxVolume − devices × 2 × v) / v) − 1, where v is the allowance for your wire size and the − 1 reserves the grounding-conductor allowance (conservative if the box has no grounds). Count every insulated conductor that enters and leaves the box, plus each wire that starts and ends inside it; pigtails and wire nuts do not count. If the box has internal cable clamps, subtract one more conductor; fixture studs and hickeys also count one each. A result of zero means the box is too small.
How to use
Example: a 22 cubic inch box, 14 AWG wire and 1 device. Step 1 — allowance for 14 AWG: 2.00 in³. Step 2 — device: 1 × 2 × 2.00 = 4.00 in³, leaving 22 − 4 = 18 in³. Step 3 — 18 / 2.00 = 9 conductor allowances. Step 4 — reserve one for the grounding conductors: 9 − 1 = 8 insulated conductors. With 12 AWG in an 18 in³ box and one device: (18 − 4.5) / 2.25 = 6, minus 1 = 5 conductors. If the box has internal clamps, subtract one more.
Frequently asked questions
How do I find the volume of my electrical junction box?
Most modern electrical boxes have their cubic inch volume stamped or printed inside the box, often near the mounting ears or on the back wall. If you cannot find a marking, measure the interior length, width, and depth in inches and multiply them together (length × width × depth = volume in in³). Gangable boxes may list volume per section. If you are working with a very old box that has no markings and cannot be measured accurately, the safest option is to replace it with a clearly rated new box.
What counts as a 'device' in the NEC box fill calculation?
Under NEC 314.16(B)(4), a device is a switch, receptacle or similar component on a single yoke or strap. Each one counts as two conductors of the largest size connected to it, so 4.0 in³ with 14 AWG or 4.5 in³ with 12 AWG; this calculator uses that rule for the gauge you enter. GFCI receptacles and combination devices still count as one device. All equipment grounding conductors together count as one conductor (included in the result), internal cable clamps count as one more, and wire nuts and pigtails that do not leave the box do not count.
When is an electrical box considered overfilled and what should I do about it?
A box is overfilled when the total volume of conductors, devices, clamps, and other fill items exceeds the box's rated cubic inch capacity per NEC 314.16. Signs of an overfilled box include insulation cracked from being jammed in, difficulty closing the cover, or wires visibly pinched. If your calculation shows zero remaining capacity, you must either remove conductors/devices or replace the box with a larger-rated enclosure. Shallow retrofit boxes and extension rings are common solutions when wall depth allows. Never simply force a cover onto an overfilled box—it is a code violation and a fire risk.