Calculator

Box Fill Calculator

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Volume allowances for conductors, grounds, devices, and clamps.

Field aid only. This is an estimate, not the NEC, not a design, and not a permit. Verify it against the current code adopted in your area and the local AHJ before you build. Read the limits.

Inputs

Box volume (in³)

Stamped or catalog volume of the box. Do not guess a trade size.

Conductor size

Every allowance on this page uses this one size.

Insulated conductors that count

Count conductors that enter the box. Skip pigtails that never leave it.

Equipment grounding conductors

Any number above zero counts as one allowance, at this wire size.

Device yokes

Each yoke or strap counts as two allowances.

Internal cable clamps

One or more clamps count as a single allowance.

Fixture stud or hickey

Counts as one allowance when the box has one.

Result

Fits. 24 in³ required, 30 in³ in the box, 6 in³ spare.

Required: 24 in³
Allowances: 12

Fits. Each allowance is 2 in³ for 14 AWG.

Volume allowance breakdown
Item	Count	Volume
Insulated conductors	8	16 in³
Equipment grounds (all of them count once)	1	2 in³
Device yokes (two allowances each)	2	4 in³
Internal clamps	1	2 in³

Same-size estimate only. Mixed conductors, plaster rings, and domed covers are not in this version.

How it works

Box fill compares the volume of the box with the volume the parts inside it are allowed to take. Each counted item uses a cubic-inch allowance that depends on wire size. This page uses one size for every allowance.

Each insulated conductor that enters the box counts as one allowance. A pigtail that never leaves the box does not.
All equipment grounding conductors together count as one allowance, based on this size.
Each device yoke or strap counts as two allowances.
Internal cable clamps, if the box has any, count as one allowance.
A fixture stud or hickey counts as one allowance.

Required volume is the number of allowances times the cubic inches for the size you picked. If the box volume you entered is at least that number, the page says it fits.

Worked example

The form opens on a 30 in³ box, 14 AWG, 8 insulated conductors, 2 equipment grounds, 1 device yoke, and internal clamps. No fixture stud.

Allowances: 8 conductors + 1 for all the grounds + 2 for the yoke + 1 for the clamps = 12. Fourteen AWG uses 2 in³ each, so the box needs 24 in³. A 30 in³ box has 6 in³ spare.

Limits

Every allowance uses the single size you picked. If the box mixes sizes, this result is not reliable. The real method counts each conductor at its own size, and counts devices, clamps, and grounds from the largest conductor tied to that item.
You have to know the box volume. The page does not store a catalog of boxes.
Plaster rings, extension rings, and domed covers can add or remove volume. Add them into the volume you type, or leave them out on purpose and say so in your notes.
Equipment grounding conductors are counted once, not once per conductor, and only at the selected size.
This is not Article 314, and it is not a pass from the inspector. Check the code edition you are building under.

FAQ

Do pigtails count?

Not if no part of that conductor leaves the box. Count the conductors that come in from the cables or raceway. Do not add the short jumpers that stay inside.

Why do four grounds count as one?

The usual box-fill method counts all of the equipment grounding conductors in a box as a single volume allowance, based on the largest of them. This page follows that idea and assumes they are the size you selected.

What if the wires are different sizes?

Don't trust this result. Use it only when the counted conductors are the same size. For a mix, do the count from the code book, conductor by conductor.

Where do I get the box volume?

From the marking in the box or the manufacturer's catalog, including any ring you are actually installing. Do not assume a trade size has one volume.