NEC Box Fill Calculator
Check required versus available box volume for conductors, devices, grounding, clamps, supports and terminal-block assemblies.
Calculation basis: 2023 NEC 314.16. Verify the NEC edition and amendments adopted by your AHJ. Results are a checking aid, not universal code approval.
NEC Box Fill Calculator
Calculate required box volume for conductors, clamps, supports, devices, grounding conductors and terminal blocks.
Need a Complete Electrical Takeoff or Estimate?
Get drawing-based quantities and estimates from your plans, schedules and specifications.
Electrical Estimating ServicesHow to use this calculator
- Choose the box. Select a standard metal box or enter its marked volume.
- Enter wiring. Add counted conductors and qualifying long loops by AWG.
- Add applicable components. Enter clamps, supports, yokes, EGCs and terminal-block assemblies.
- Calculate and review. Compare required volume, available volume and spare capacity.
Conductor volume allowances
This V1 box fill calculation uses the following allowances under its 2023 NEC 314.16 basis. It supports #18 through #6 AWG; 4 AWG and larger are outside this workflow.
| Conductor size | Allowance (in³) |
|---|---|
| #18 | 1.50 |
| #16 | 1.75 |
| #14 | 2.00 |
| #12 | 2.25 |
| #10 | 2.50 |
| #8 | 3.00 |
| #6 | 5.00 |
Counted conductors and long loops
Under this calculator’s methodology, a conductor entering and terminating/splicing, or passing through, counts once. A qualifying long unbroken loop/coil counts twice. Ordinary pigtails entirely contained within the box are not entered as counted conductors.
Internal clamps, studs and hickeys
One or more applicable internal cable clamps collectively add one allowance based on the largest supported conductor present. External connectors do not automatically count as internal clamps. A luminaire stud and luminaire hickey are separate support-fitting types: each enabled type adds one largest-applicable-conductor allowance.
Devices and yokes
Create separate device/yoke groups when connected conductor sizes differ. One #12 yoke uses 4.50 in³; one #10 yoke uses 5.00 in³.
Equipment grounding conductor fill
For 1–4 entering EGCs, the locked 2023 methodology uses one equivalent allowance at the largest entering EGC size. Above four, Equivalent Allowances = 1 + 0.25 × (Count − 4). Zero entering EGCs adds no EGC fill. Unlimited grounding conductors do not always count as one allowance.
For #12 EGCs: 4 use 2.25 in³; 5 use 2.8125 in³; 6 use 3.375 in³; and 8 use 4.50 in³.
Terminal-block assemblies
Each entered terminal-block assembly adds one allowance based on the largest conductor terminated to that assembly under the locked V1 methodology. Enter assembly quantity, not the number of individual terminal poles.
Available box volume and result status
Choose from the built-in standard metal-box volume list or use Custom / Marked Volume. Enter eligible marked extension/ring capacity only as Eligible Marked Added Volume. Do not derive box capacity from outside dimensions.
Available Volume = base box + eligible marked added volume.
PASS means available volume exceeds calculated required volume; EXACT FIT means they are equal; OVERFILLED means required volume exceeds available volume. These are volume-check results, not universal code approval. Verify the NEC edition and amendments adopted by your AHJ.
Worked example: six #12 conductors
Use six counted #12 conductors, internal clamps ON, one #12 yoke and three entering #12 EGCs in the default 4 × 2-1/8 square box.
| Component | Volume (in³) |
|---|---|
| Conductors: 6 × 2.25 | 13.50 |
| Internal clamp | 2.25 |
| Device/yoke: 1 × 2 × 2.25 | 4.50 |
| Three #12 EGCs | 2.25 |
| Required | 22.50 |
| Available | 30.30 |
| Spare | 7.80 |
Fill is 74.257426% (displayed as 74.3%): PASS.
Mixed-conductor example
Three #14, two #12 and one #10 counted conductors use 13.000 in³. Add a 2.500-in³ clamp, 2.500-in³ stud, 2.500-in³ hickey, one #12 yoke at 4.500 in³, one #10 yoke at 5.000 in³, six #12 EGCs at 3.375 in³ and one #10 terminal assembly at 2.500 in³.
Required = 35.875 in³. With 42.000 in³ available, spare volume is 6.125 in³ and fill is 85.416667% (displayed as 85.4%): PASS.
V1 scope and limitations
This junction box fill checking aid does not handle 4 AWG+ sizing, NEC 314.28 pull/junction-box sizing, conduit-body sizing, automatic cable-to-conductor inference, automatic canopy exceptions, specialized clamp assemblies, barrier-space apportionment or manufacturer product databases. It does not calculate conductor ampacity, raceway/conduit fill, service/load calculations, pricing or labor. It provides neither universal compliance certification nor 2026 NEC methodology.
Box fill calculator FAQs
What is box fill?
Box fill compares the volume allowances for applicable wiring and components with the box’s available volume.
How is box fill calculated?
This tool sums conductor, clamp, support, device, EGC and terminal-block fill, then compares that required volume with the base box plus eligible marked added volume.
What NEC edition does this calculator use?
V1 uses 2023 NEC 314.16. Verify the edition and amendments adopted by the project’s AHJ.
How much volume does a #12 conductor require?
The V1 conductor volume allowance is 2.25 in³ for each counted #12 conductor equivalent.
Do pigtails count?
Ordinary pigtails entirely contained within the box are not entered as counted conductors in this workflow.
How are grounding conductors counted?
One through four entering EGCs use one allowance. Each entering EGC above four adds one-quarter allowance, based on the largest entering EGC size.
How do devices/yokes affect box fill?
Each yoke uses two allowances at the selected connected-conductor size. Separate groups allow different conductor sizes.
Do internal clamps count?
Applicable internal cable clamps collectively add one allowance at the largest supported conductor size. External connectors do not automatically count.
Can I use a marked/custom box volume?
Yes. Select Custom / Marked Volume and enter the marked capacity. Add only eligible marked extension/ring volume.
Does PASS guarantee NEC/AHJ compliance?
No. PASS reports this calculator’s volume comparison only. Project conditions, adopted edition, amendments and requirements still need review.