Box Fill Calculator checks required box volume for wires, devices, clamps, fittings, terminal blocks, and grounds, showing compliance, capacity used, and remaining space instantly.
Select intended box to check fill capacity.
Count wires entering/leaving. Pigtails count as 0.
Switches and receptacles. 2x allowance per yoke.
NEC 2023 Rules: Clamps, studs, blocks, and EGCs.
How This Calculator Totals Box Fill Under NEC 314.16(B)
Box fill isn’t one number you look up. It’s five separate tallies added together, and NEC 314.16(B) assigns a fixed volume allowance to each conductor size, then tells you how to count devices, clamps, supports, and grounds differently from plain wires. The formula this calculator runs is:
$$V_{total} = V_{conductors} + V_{devices} + V_{fittings} + V_{grounds}$$
Each piece pulls from the same base lookup: the cubic-inch allowance assigned to a given AWG size under Table 314.16(B).
| Wire Size (AWG) | Volume Allowance per Conductor |
|---|---|
| 18 | 1.50 in³ |
| 16 | 1.75 in³ |
| 14 | 2.00 in³ |
| 12 | 2.25 in³ |
| 10 | 2.50 in³ |
| 8 | 3.00 in³ |
| 6 | 5.00 in³ |
Conductor volume is straightforward: $V_{conductors} = \sum q_i \times v(awg_i)$, where $q_i$ is how many wires of a given gauge you entered and $v(awg_i)$ is its table allowance.
Device volume works differently. Each strap or yoke — a receptacle, a switch — counts as two conductors sized to the wire actually connected to it: $V_{devices} = \sum q_j \times 2 \times v(awg_j)$. A single 12 AWG receptacle isn’t 2.25 in³, it’s 4.50 in³, because the device itself displaces as much space as two wires would.
Fittings get their own logic. One or more internal cable clamps together cost a single allowance, sized to whichever conductor in the box is largest — not one allowance per clamp. Support fittings like studs or hickeys are counted per fitting, each charged at that same largest-conductor size.
Terminal blocks are different again: each one is charged individually, at the volume of whatever wire size you’ve assigned to it, regardless of what else is in the box.
Grounding conductors follow their own rule too. Up to four equipment grounding conductors of the same gauge, combined, only cost one allowance — the volume of a single conductor at that gauge. Every ground beyond the fourth adds just a quarter of that base volume, not a full one.
Worked Example: A 12 AWG Duplex Receptacle Box
Picture a standard 4-inch square box holding one duplex receptacle. Two 12 AWG NM cables run through it — one feed, one continuing to the next box — plus an internal cable clamp and two bare grounds bonded together. Here’s how the volume stacks up.
Conductors: two cables each carrying a hot and a neutral gives four 12 AWG wires. $4 \times 2.25 = 9.00$ in³.
Device: one duplex receptacle wired with 12 AWG. $1 \times 2 \times 2.25 = 4.50$ in³.
Clamp: one internal cable clamp is present, so it’s charged at the largest conductor size in the box — 12 AWG, or 2.25 in³. It stays at 2.25 in³ even if there were three clamps instead of one.
Grounds: two 12 AWG grounding conductors. Because the count is four or fewer, they’re treated as a single allowance rather than two: 2.25 in³, not 4.50 in³.
Add it up: $9.00 + 4.50 + 2.25 + 2.25 = 18.00$ in³. That’s the number the calculator returns as your total required volume, before it’s compared against anything.
Box Volume Used: 18.00 in³ of 24.00 in³ Example Capacity Conductors 9.00 in³ Device 4.50 in³ Clamp 2.25 in³ Grounds 2.25 in³ Remaining 6.00 in³ (unused box capacity)
Why “Over Capacity” on This Calculator Isn’t a Judgment Call
Box fill has a hard ceiling. Once your total volume exceeds the box’s rated capacity, the calculator switches to an “OVER CAPACITY (FAIL)” state — there’s no partial credit, and unlike a voltage drop guideline, this isn’t a target you’re free to exceed if the circuit still works. Cramming too many conductors into too little space leaves no room for heat to dissipate and makes every splice harder to seat correctly, which is exactly what Table 314.16(B) exists to prevent.
When your total stays at or under the box’s volume, the result reads “COMPLIANT” and shows remaining capacity — how much room you have left before you’d need a bigger box or an extension ring.
When it doesn’t, the remaining-capacity figure goes negative, showing you exactly how far over you are in cubic inches, not just a pass or fail flag.
If you haven’t picked a box size yet, the calculator shows “VOLUME ONLY” — it will still total your conductors, devices, fittings, and grounds, it just won’t judge them against anything until you tell it what box you’re using.
One quirk worth knowing: the visual fill bar stops filling at 100% even if your total badly exceeds the box’s rating, but the percentage text next to it keeps climbing past 100. Trust the number, not the bar, if you’re trying to gauge how far over you actually are.
What Actually Drives Your Box Fill Number Up
Devices move the number faster than almost anything else. Because each one counts double, swapping a blank conductor for a receptacle or switch on the same wire size adds as much volume as two extra wires would.
Wire gauge matters too, and not in a straight line — going from 14 AWG to 12 AWG adds 0.25 in³ per conductor, but jumping from 10 AWG to 8 AWG adds 0.50 in³, and 8 AWG to 6 AWG adds a full 2.00 in³ per wire. Larger gauges get expensive quickly.
Grounding conductors behave the opposite way from what most people expect. Adding a second, third, or fourth ground wire costs nothing extra past the first, since all four are bundled into one allowance.
Only the fifth ground and beyond start adding volume, and even then just a quarter of a full conductor’s worth each. Terminal blocks don’t get that same break — each one is charged individually at its own assigned wire size, so a handful of terminal connections can add up faster than an equivalent number of loose conductors would.
Clamps and support fittings sit in between: any number of clamps still costs a single allowance, but each individual support fitting (a stud, a hickey) is charged separately, both sized to whatever the largest conductor in the box happens to be.
Two range limits are built into the tool itself. The AWG table tops out at 6 AWG — conductors 4 AWG and larger aren’t part of this volume-allowance table and need to be handled separately. And on the device rows specifically, the smallest available wire size is 14 AWG; 18 and 16 AWG aren’t offered as device connections, only as standalone conductor or ground sizes.
Box Fill Questions People Actually Ask
Do multiple cable clamps count against box fill more than once?
No. One or more internal cable clamps together are charged a single volume allowance, sized to your largest conductor in the box. Ten clamps cost the same as one.
How much room does a duplex receptacle actually use compared to a bare wire?
Twice as much as a single conductor of the same gauge. A 12 AWG receptacle takes up 4.50 in³, because each device is counted as two conductors of its largest connected wire size, not one.
Do all my grounding conductors count separately toward box fill?
Not up to a point. Four or fewer equipment grounding conductors of the same gauge are combined into a single allowance. Only conductors past the fourth add extra volume, and each of those only adds a quarter of a full conductor’s worth.
What wire sizes does NEC box fill volume cover?
18 AWG through 6 AWG. Each size has its own fixed cubic-inch allowance under Table 314.16(B), running from 1.50 in³ at 18 AWG up to 5.00 in³ at 6 AWG. Sizes larger than 6 AWG fall outside this table.
Can devices be wired with 16 or 18 AWG?
Not in this calculator. Device rows only accept 14 AWG through 6 AWG, since those are the practical minimum gauges used for switches and receptacles.
What does “Over Capacity” mean and what should I do about it?
It means your total required volume is higher than the box’s rated capacity, and the shortfall is shown as a negative remaining-capacity figure. The fix is a larger box, an extension ring, or fewer conductors and devices in that box — not a smaller wire gauge chosen just to make the math work.
What does the “Volume Only” result mean?
It shows up when no box size has been selected yet. The calculator still totals your conductors, devices, fittings, and grounds, it just isn’t comparing that total against a capacity until you pick a standard box or enter a custom volume.
This calculator is for planning and estimation. Final conductor counts, box selection, and box fill compliance should be verified against your local electrical code and confirmed by a licensed electrician before any wiring is installed or inspected.