Box Fill Calculator
Count your conductors and get the required box volume per NEC 314.16(B) — with the worked steps and the smallest compliant box, the same way you'd solve it on the exam.
Each conductor counts at its Table 314.16(B) free volume. Add 2× the largest conductor for each device (yoke), and 1× for cable clamps.
14 AWG
2.00 in³ each
12 AWG
2.25 in³ each
10 AWG
2.50 in³ each
8 AWG
3.00 in³ each
6 AWG
5.00 in³ each
Required box volume
18.50in³
Smallest compliant box: 4" sq × 1½" deep (21 in³).
How to calculate box fill (NEC 314.16)
- 1
Assign each conductor a volume
From NEC Table 314.16(B): #14 = 2.00 in³, #12 = 2.25 in³, #10 = 2.50 in³, #8 = 3.00 in³, #6 = 5.00 in³. Multiply by how many of each you have.
- 2
Count devices and clamps
Each device (a switch or receptacle yoke) counts as two conductors of the largest size connected to it (314.16(B)(4)). One or more cable clamps together count as one of the largest size (314.16(B)(2)). All grounds together count as one (314.16(B)(5)).
- 3
Add it up and pick the box
Sum the volumes. The total is the minimum box capacity in cubic inches — choose a box whose capacity meets or exceeds it.
Box fill — frequently asked questions
How do you calculate box fill?
Add the free volume of every conductor (NEC Table 314.16(B)), count each device as two of the largest conductor and clamps as one, add all equipment grounds as one, and total it. That total is the minimum box volume in cubic inches.
What counts toward box fill?
Conductors that originate outside and terminate or splice inside (each = 1), each device yoke (= 2 of the largest conductor), one or more internal cable clamps together (= 1), support fittings (= 1 each), and all equipment grounding conductors together (= 1). Conductors that just pass through unbroken count as one.
What NEC section covers box fill?
NEC 314.16 — the per-conductor volumes are in Table 314.16(B), and the counting rules for devices, clamps, and grounds are in 314.16(B)(1) through (5).
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