Box Fill Calculator

Box Fill Calculator tells you in seconds whether the conductors, clamps, fittings and devices in an outlet or device box legally fit — based on Article 314.16 of the National Electrical Code. Enter what’s in the box and get the required volume in cubic inches plus a clear pass/fail verdict. Works fully offline and nothing you enter leaves your phone.

HOW IT WORKS

The app follows NEC 314.16(B) using Table 314.16(B) volume allowances and standard box volumes from Table 314.16(A) (NFPA 70, 2020):

1. Everything becomes “volume allowances.”
Each allowance equals:

* 14 AWG: 2.00 in³
* 12 AWG: 2.25 in³
* 10 AWG: 2.50 in³
* 8 AWG: 3.00 in³
* 6 AWG: 5.00 in³
(Also 1.75 and 1.50 in³ for 16 and 18 AWG.)

2. Each conductor entering and terminating counts 1. Pass-through conductors also count 1. Pigtails entirely inside the box count 0.

3. All grounding conductors together count as 1 allowance, sized by the largest — a common mistake is counting each separately.

4. Internal clamps count as 1 total; external clamps count 0. Each stud or hickey counts 1. Each device yoke counts 2, and multi-gang devices count double per gang (e.g., 2-gang = 4).

5. Total allowances are multiplied by the volume per conductor and compared to the box volume (Table value or manufacturer marking).

Example: two 12/2 cables + one receptacle →
4 conductors + 1 ground + 1 clamp + 2 (yoke) = 8 allowances
8 × 2.25 = 18.0 in³ → fits exactly in a 3×2×3½ box.

WHAT YOU CAN DO

* Enter box volume (table or marked capacity)
* Count conductors, grounds, clamps, fittings, and devices
* Select conductor size (18–6 AWG)
* Get required volume in in³ and cm³, % used, and spare/excess
* See how many more conductors fit and the minimum box size needed
* View full breakdown (314.16(B)(1)–(5)) and assumptions
* Learn the rules in clear, simple language

BUILT FOR CLARITY

Clean indigo and copper design with the verdict front and center. Light/dark themes, small download, no clutter, no account.

IMPORTANT

This is an educational tool based on the 2020 NEC. Local codes and amendments may differ and are legally binding. This app covers box fill (314.16) only — not conduit fill (Chapter 9) or large conductor box sizing (314.28).

For simplicity, the app uses the largest conductor size for all allowances, which is conservative for mixed-wire boxes. Electrical work is hazardous: have installations verified by a licensed electrician and inspected by the authority having jurisdiction. This is not professional advice.

PRIVACY

No personal data is collected. The app works fully offline. Ads are served by Google AdMob, which may use an advertising identifier per Google policies.

Download Box Fill Calculator and stop guessing whether the box is big enough.