WireGaugeCalc Free calculator · NEC cited · works offline
Box Fill Calculator App
Box fill calculator app is fast with WireGaugeCalc, since you enter the conductors and devices and it returns the required cubic inches.
The app counts each conductor, clamp, support fitting, and device per NEC 314.16(B) and compares the total to the box volume.
So you can confirm a box is large enough before you make up the splices.
Open the calculatorFill against NEC Chapter 9, Table 1 (40% for 3+ conductors) with Table 4 and Table 5 areas. Field aid only, verify against the current NEC and your local authority having jurisdiction.
How the box fill calculation works
Each current-carrying conductor counts as one volume allowance based on its wire size.
Internal clamps add one allowance, and each yoke or strap of a device adds two allowances at the largest conductor connected to it.
The tool adds those allowances and tells you the minimum cubic inches the box must hold.
What the conductor volume tool checks
Enter what is inside the box and the calculator handles the count:
- Conductor count and size in AWG
- Internal cable clamps present or not
- Device yokes such as switches and receptacles
- Equipment grounding conductors as a single allowance
- Listed box volume in cubic inches
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
Do all the ground wires count separately in box fill?
No. All equipment grounding conductors together count as a single volume allowance based on the largest ground present.
How is a receptacle counted in the box fill total?
Each device on a yoke or strap counts as two volume allowances using the largest conductor connected to that device. Verify the count against the current NEC and your local authority having jurisdiction.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.