Box Fill Calculator

Six 14 AWG conductors, a cable clamp, a switch and a ground wire walk into a 3x2x3.5 box. Do they fit? Box Fill Calculator adds up the required volume the way the code counts it and shows you what is left over — before you start folding wire back into the box.

HOW IT WORKS
Pick a box from the preset list or type the volume stamped inside it. Add the conductors by gauge. Switch on the clamps, count the yokes and grounds. The remaining volume appears at the top, along with the percentage of the box you have used.

WHAT YOU GET
- Required, available and remaining volume, all in cubic inches
- A line-by-line breakdown showing what each item contributes
- Conductor sizes from 18 through 6 AWG
- Cable clamps counted once for the whole box
- Support fittings at one conductor each
- Device yokes at two conductors each
- Equipment grounds counted once as a group
- Ten common box presets, plus direct volume entry for anything else
- Saved records so you can work through a panel of boxes and compare them later

THE COUNTING RULES
Fittings are not all counted the same way, and that is where the mistakes happen. Clamps count once no matter how many are in the box. Every device yoke counts as two conductors. All the grounds together count as one. Each of those allowances uses the volume of the largest conductor in the box, not the one it is attached to. The app applies each rule and shows you the line it added, so the arithmetic is visible instead of hidden.

BUILT FOR THE FIELD
One screen, no account, no sign-in, and it works without a network connection. Results and saved records stay on your device.

WHO IT IS FOR
Electricians, solar installers, inspectors checking their own math, and apprentices who would rather understand the table than memorise it.

IMPORTANT
Volumes follow NEC Table 314.16(B). Your local inspector and the code edition adopted in your area always take precedence. This app is a calculation aid — it does not issue a compliance ruling, and it does not replace the judgement of the person doing the work. Box manufacturers vary, so use the volume marked inside the box whenever it differs from a preset.