Electrical Box Fill Calculator

Add up conductors, devices, clamps and grounds to find the box volume to check against.

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
14 AWG conductors4 x 2.00 = 8.00 cu in
Devices, 2 allowances each2 x 2.00 = 4.00 cu in
Equipment grounds1 x 2.00 = 2.00 cu in
Minimum box volume14.00 cu in
Smallest common box at or above this4" octagon x 1-1/2" metal, 15.5 cu in

Turn these numbers into a build plan.

  • 14.0 cu in box volume
  • 14 AWG largest wire
Materials List
Recommended materials and quantities
4" octagon x 1-1/2" metal1

Check the volume stamped in your box. Raised covers and extension rings add their marked volume. Turn the power off and test before opening a box.

Checked against NEC (NFPA 70) 2023, section 314.16. These rule values have not been reviewed by a licensed professional. Your local code and inspector decide, so verify with your building department.

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Project plan · built from your numbers

Replace an electrical box, 14.0 cu in needed

Fit a box checked against the 14.0 cu in this wiring needs. Verify with your building department before you start.

Materials
$15
Time
1 afternoon
Level
Advanced
5 steps 4 tools 2 materials
  1. 1

    Check permit needs

    Ask your building department whether this work needs a permit or a licensed electrician.

    • Permit question answered
  2. 2

    Shut off and test

    Turn off the breaker and test each conductor before touching anything.

    • Breaker off
    • Tested dead
  3. 3

    Remove the old box

    Disconnect the device, label the wires and remove the box.

    • Wires labeled
01 · Your numbers
Box volume
14.0 cu in
Largest wire
14 AWG
02 · The plan

5 steps · 4 tools · 2 materials

2 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Replace an electrical box, 14.0 cu in needed

Finished in 1 afternoon · $15 DIY

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How to calculate electrical box fill

Counting rules follow NEC (NFPA 70) 2023, section 314.16. Check your local amendments and the volume stamped in your box.

  1. Count conductors. Count each insulated wire that enters and ends or is spliced in the box.
  2. Add device allowances. Two allowances per switch or receptacle.
  3. Add clamps and studs. One allowance for all internal clamps, and one per fixture stud or hickey.
  4. Add grounds. One allowance for up to four grounds, then a quarter each.
  5. Total the volume. Multiply each count by the volume for its wire size and add them up.

Volume per conductor

WireVolume
6 AWG5 cu in
8 AWG3 cu in
10 AWG2.5 cu in
12 AWG2.25 cu in
14 AWG2 cu in
16 AWG1.75 cu in
18 AWG1.5 cu in

Common box sizes

BoxVolume
3 x 2 x 2-1/2" metal device box12.5 cu in
4" octagon x 1-1/2" metal15.5 cu in
Single-gang plastic, 18 cu in18 cu in
3 x 2 x 3-1/2" metal device box18 cu in
Single-gang plastic, 20.3 cu in20.3 cu in
4" square x 1-1/2" metal21 cu in
4" octagon x 2-1/8" metal21.5 cu in
Single-gang plastic, 22.5 cu in22.5 cu in
4-11/16" square x 1-1/2" metal29.5 cu in
4" square x 2-1/8" metal30.3 cu in
Two-gang plastic, 34 cu in34 cu in
4-11/16" square x 2-1/8" metal42 cu in
Three-gang plastic, 54 cu in54 cu in
Four-gang plastic, 68 cu in68 cu in

Volumes are nominal. Check the marking inside your box. See the electrical cheat sheet and the voltage drop calculator.

Frequently asked questions

How do I count wires in a box?

Each insulated conductor counts once. Wires that pass through without a splice still count once. Grounds and devices have their own allowances.

What size box for a single receptacle?

It depends on the wire count. Two 14-2 cables with a receptacle need 14 cu in in this calculator, and a larger box makes work easier.

Do pigtails count?

Generally a pigtail that starts and ends inside the box does not add a count.

What if my fill is too high?

Use a deeper box, add a marked extension ring, or reroute some wiring through another box.

Does this mean my box will pass inspection?

No. It is a math check against a published section, and your inspector decides. Verify with your building department.

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