Box Fill Calculator
Check a device or junction box against NEC 314.16. Enter the conductors (mixed sizes supported), devices, clamps, support fittings, and grounds, and the calculator totals the required cubic-inch volume and compares it to your box — pass or fail, with the spare volume left over.
Box & Contents
Box Fill — NEC 314.16
NEC 314.16(A) box volumes; 314.16(B) allowances — each conductor at its own size, devices 2× / clamps & fittings 1× the largest conductor, all EGCs = 1×.
How this calculator works
NEC 314.16 turns everything in the box into conductor-equivalent volume allowances, each taken from Table 314.16(B) at the size of the conductor it represents:
required in³ = Σ conductors (each at own size) + devices × 2 + clamps × 1 + fittings × 1 + EGCs × 1
The count rules are where boxes get blown: every conductor entering and terminating or spliced counts once at its own size (314.16(B)(1)); each strap-mounted device — receptacle or switch — counts as two allowances at the largest conductor connected to it (314.16(B)(4)); all internal cable clamps together count once at the largest conductor in the box (314.16(B)(2)); each support fitting (fixture stud or hickey) counts once at the largest conductor (314.16(B)(3)); and all equipment grounding conductors together count as a single allowance at the largest EGC (314.16(B)(5)). The total must not exceed the box volume — the value stamped in the box or listed in Table 314.16(A) for standard metal boxes, which the calculator’s box picker carries.
Worked example
Three 12/2 NM cables land in a box: six 12 AWG circuit conductors, one receptacle, internal clamps, and the ground wires (12 AWG). The 12 AWG allowance is 2.25 in³:
- Conductors: 6 × 2.25 = 13.50 in³.
- Device: 1 receptacle × 2 × 2.25 = 4.50 in³.
- Clamps (all together): 1 × 2.25 = 2.25 in³.
- Grounds (all together): 1 × 2.25 = 2.25 in³.
- Required volume: 13.50 + 4.50 + 2.25 + 2.25 = 22.50 in³. An 18.0 in³ device box fails by 4.5 in³; a 4 × 2⅛ square (30.3 in³, per Table 314.16(A)) passes with 7.8 in³ spare.
These are the calculator’s default inputs — swap the 18 in³ box for the 4 × 2⅛ square in the box picker and watch the check flip to green.
Reference: volume allowance per conductor
NEC Table 314.16(B) — the volume allowance for each conductor size, from the same data the calculator computes with:
| Conductor size | Allowance |
|---|---|
| 18 AWG | 1.50 in³ |
| 16 AWG | 1.75 in³ |
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
Frequently asked questions
How many 12 AWG wires can go in a 4-inch square box?
A 4×1½ square box is 21.0 in³ (NEC Table 314.16(A)) and each 12 AWG conductor takes 2.25 in³ (Table 314.16(B)), so nine conductors fit if the box holds nothing else. Real boxes rarely hold nothing else: add one device (two allowances, 4.5 in³) and the grounds (one allowance, 2.25 in³) and you're down to six. Enter the actual contents in the calculator instead of leaning on a rule of thumb.
Do ground wires count toward box fill?
Yes, but all equipment grounding conductors in the box together take a single volume allowance, based on the largest EGC present — NEC 314.16(B)(5). This calculator sizes that allowance from the EGC size you select. Recent NEC editions add a further allowance when more than four EGCs enter a box, so verify against the code edition adopted in your jurisdiction.
Do wirenuts and pigtails count toward box fill?
No. Wire connectors take no allowance, and neither does a pigtail or any conductor that originates and terminates entirely inside the box (NEC 314.16(B)(1)). What counts is each conductor entering the box that terminates or is spliced there. A conductor passing through unbroken counts once — unless it's looped at least twice the minimum free-conductor length of NEC 300.14, in which case it counts twice.
How much volume does a receptacle or switch take up?
Each strap holding a device counts as two conductor allowances (NEC 314.16(B)(4)) — the code bases them on the largest conductor connected to the device, and this calculator conservatively uses the largest conductor in the box. On a 12 AWG circuit that's 4.5 in³ per device, which is why a switch box that 'looks fine' is often over fill on paper.
Do cable clamps count in the box fill calculation?
Only internal clamps. One or more cable clamps inside the box together take a single allowance based on the largest conductor present (NEC 314.16(B)(2)); connectors that clamp the cable at the knockout, outside the box wall, take none. If your box has factory internal NM clamps, answer yes in the calculator.
What do I do if the box is over fill?
Use a box with more volume — a deeper box, the 4-11/16 size, or an extension ring (its marked volume adds to the total). You can also reduce the count: splice through-runs in a different enclosure or drop a device to a separate box. Never just cram it; overfilled boxes are a common inspection failure and a real heat and insulation-damage issue.
Method: per-conductor volume allowances from NEC Table 314.16(B), counted per 314.16(B)(1) through (B)(5) — conductors at their own size; devices at 2× and clamps/support fittings at 1× the largest conductor; all EGCs together at 1× the largest EGC. Box volumes from the stamped rating or Table 314.16(A). Conductors that pass through unbroken and short pigtails that never leave the box have their own counting rules — when in doubt, count. Verify against the governing NEC edition.
Conduit bodies have their own fill rules (314.16(C)) — see the conduit bodies chart. Pair with the Conduit Fill Calculator for the raceways in and out of the box.