MoonLeadGet Started
Free Estimating Tool

Box Fill Calculator

Calculate minimum electrical box volumes instantly based on NEC 314.16. Includes automated deductions for devices, clamps, and grounds.

Calculator Inputs

3 Wires
1 Device
1 Wire
Real-Time Results

Recommended Box Size

Calculation ReadyBased on your inputs, here is the minimum required box capacity.

3" x 2" x 3 1/2" Device

Required Box Volume:15.75 cu in
Selected Box Capacity:18.00 cu in
Volume Deduction Breakdown
Conductors (3x):6.75 cu in
Devices (1x @ 2 allowances each):4.50 cu in
Internal Clamps (1 allowance):2.25 cu in
Equipment Grounds (1x):2.25 cu in
Disclaimer: Box volume sizing is based on NEC 314.16. Actual internal volume varies by manufacturer. Plaster rings and extension rings may be used to increase volume if marked with their cubic-inch capacity.

Email this calculation to yourself

Keep a record of your 3" x 2" x 3 1/2" Device sizing for your current estimate.

Looking to automate your full estimating process?

Explore the MoonLead Estimator

Voltage Drop Calculator

Calculate voltage drop instantly for single and 3-phase circuits.

Conduit Fill Calculator

Calculate minimum conduit sizes per NEC Chapter 9.

Frequently Asked Questions

How many volume allowances does a receptacle take?

Under NEC 314.16(B)(4), a typical device like a receptacle or switch counts as a double volume allowance based on the largest wire connected to it.

Do all ground wires count separately for box fill?

No — up to four equipment grounding conductors in a box are counted together as a single volume allowance, based on the largest grounding conductor present. Only the fifth and each additional EGC beyond that adds a 1/4 volume allowance, based on the largest conductor size.