Box Fill Calculator - NEC

NEC box fill calculator for electricians. Article 314.16, all standard boxes.

NEC box fill calculator for electricians, apprentices, and electrical engineers. Your go-to junction box calculator for outlet box fill and device box volume calculations per NEC 314.

Calculate box fill volumes instantly using official NEC 2023 Article 314.16 data (Tables 314.16(A) and 314.16(B)). No internet required — works 100% offline on the job site.

FEATURES:
• All standard metal box types: Device, Round/Octagonal, Square 4", Square 4-11/16", Masonry, FS, FD
• Custom volume entry for non-standard boxes
• Conductor sizes: 18 AWG to 6 AWG per Table 314.16(B)
• Internal cable clamps, fixture studs, and hickeys
• Multi-gang device fill calculation
• Equipment grounding conductors (EGC) with NEC 2023 rules
• Isolated grounding conductors — separate allowance
• Visual PASS/FAIL indicator with fill percentage
• Shows minimum box volume needed when over-filled
• Suggests a standard box that will pass

PRO FEATURES (one-time purchase):
• No ads
• Save calculation history
• Export calculations as PDF reports
• Dark theme support

BUILT FOR THE JOB SITE:
This app is designed by electricians, for electricians. The interface is clean, fast, and works with one hand. Add conductors, check your fill, and move on — no complicated menus or unnecessary features.

ACCURATE NEC DATA:
All box volumes are sourced from NEC 2023 Table 314.16(A). Conductor volume allowances follow Table 314.16(B) exactly. Fill rules follow Article 314.16 for conductors, clamps, support fittings, devices, and grounding conductors. Always verify calculations against the official NEC code book before installation.

NEC 314.16 FILL RULES:
• Each conductor: volume from Table 314.16(B) based on AWG size
• Internal clamps: 1× largest conductor volume (all clamps combined)
• Support fittings: 1× largest conductor per fitting type
• Devices: 2× largest conductor per gang
• EGC (≤4): 1× largest EGC volume
• EGC (>4, NEC 2023): additional allowance per conductor
• Isolated grounds: counted separately

PERFECT FOR:
• Licensed electricians doing box fill calculations daily
• Electrical apprentices studying for journeyman exams
• Electrical engineers and designers
• Electrical contractors bidding residential and commercial jobs
• NEC code exam preparation (Journeyman & Master)
• Anyone who needs a fast, accurate junction box calculator

OFFLINE & PRIVACY:
• Works 100% offline — no internet needed
• No account required
• No data collection
• Your calculations stay on your device

Simplify your box fill calculations on the job site.