When to use this: at rough-in to confirm the outlet box you picked has enough cubic-inch capacity for everything that will land in it. Use it together with the Conduit Fill Calculator for raceway sizing and the NEC Wire Size Calculator for ampacity.
Box Fill Calculator
Counts conductors, devices, clamps, supports, and EGCs per NEC 314.16 and reports required cubic inches against standard box volumes from Table 314.16(A).
Conductors entering the box
Count each current-carrying conductor as 1. A through-conductor (no splice) is 1. A loop with both ends terminated counts as 2. Do not count EGCs here – the EGC allowance is added below.
Devices, clamps, support, EGC
Per NEC 314.16(B): all internal clamps together = 1 conductor; each fixture stud / hickey = 1; each strap-mounted device = 2; all EGCs together = 1.
Box selection
Results
Largest conductor in box—
Conductor allowance—
Device allowance (2 x largest device conductor)—
Clamp + fixture-support + EGC allowance—
Total required volume—
Selected box volume—
Headroom—
Status—
Last reviewed: April 2026 (NEC 2023)
Tools we used or recommend for this work: a deep cut-in box for retrofit (Carlon B120A), a digital caliper for verifying old-work box depth, and a Klein non-contact voltage tester for safe inspection. See our Klein vs Milwaukee battery vs hand tools writeup for the rationale on each pick. Links may be affiliate-tagged where the brand has a program.
