NEC 2026

Box Fill Calculator

Check that a box is big enough for its conductors, devices, clamps and grounds, with each rule and clause shown. The result updates as you type.

Updated

Conductors (314.16(B)(1))

Count each conductor that comes from outside the box and ends or is spliced in it once, and each conductor that passes through the box without a splice or termination once. Count each loop or coil of unbroken conductor that is at least twice the free length required in 300.16 twice. A conductor that never leaves the box is not counted.

Devices (314.16(B)(4))
Equipment grounding conductors (314.16(B)(5))
Terminal blocks (314.16(B)(6))

Pick a box and add at least one conductor, device or grounding conductor to see the fill.

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For reference only. Always verify with the NEC edition adopted by your AHJ.

How box fill is counted

Box fill is a volume check. Every conductor, device and fitting in the box takes up a volume allowance, and the total must fit within the box volume. The allowance depends on the conductor size (Table 314.16(B)(4)): a 14 AWG conductor takes 2.00 in³, a 12 AWG takes 2.25 in³ and a 10 AWG takes 2.50 in³.

  • Conductors (314.16(B)(1)): Count each conductor that comes from outside the box and ends or is spliced in it once, and each conductor that passes through the box without a splice or termination once. Count each loop or coil of unbroken conductor that is at least twice the free length required in 300.16 twice. A conductor that never leaves the box is not counted.
  • Clamps (314.16(B)(2)): all internal cable clamps together count once, at the largest conductor in the box.
  • Support fittings (314.16(B)(3)): each type of fixture stud or hickey counts once at the largest conductor.
  • Devices (314.16(B)(4)): each yoke or strap counts as two conductors at the largest conductor connected to it.
  • Grounds (314.16(B)(5)): all equipment grounding conductors together count as one conductor, with a fraction added for each extra one beyond a small group.
  • Terminal blocks (314.16(B)(6)): each block counts as one conductor at its largest conductor.

Pick a box from Table 314.16(A) or switch to the volume marked on the box. When the total is too big, the calculator names the smallest standard box of the same type that holds it. Conductor fill in a conduit is a different calculation: use the conduit fill calculator for that, and the wire size calculator for conductor sizing.

Box fill questions

How do you calculate box fill?
Add up the volume allowance of everything in the box and compare the total with the box volume. Each conductor counts at the allowance for its size from Table 314.16(B)(4), for example 2.00 in³ for a 14 AWG and 2.25 in³ for a 12 AWG conductor. Clamps, fittings, devices and grounds add their own allowances (314.16(B)(1) to (B)(6)). Under 314.16(B)(1) a conductor that comes from outside and ends or is spliced in the box counts once, a conductor that passes through the box without a splice counts once, and a loop or coil of unbroken conductor at least twice the free length required in 300.16 counts twice. The total must not exceed the volume marked on the box, or the volume in Table 314.16(A) for a standard metal box.
Do ground wires count in box fill?
Yes, but not one by one. Under 314.16(B)(5), all the equipment grounding conductors in the box together count as a single allowance, based on the largest of them, with a smaller added allowance for each additional ground beyond a small group. This calculator takes the number of grounds and the largest size and applies the rule.
How do you count a device in box fill?
A device (a switch or receptacle on one yoke or strap) counts as two conductor allowances, sized by the largest conductor connected to it (314.16(B)(4)). A device wider than the usual single gang counts that double allowance for each gang it needs. Add one row to the device list for each yoke.
Do cable clamps and fixture studs count?
Yes. All internal cable clamps in the box together count as one allowance, sized by the largest conductor in the box (314.16(B)(2)). Each type of fixture stud or hickey adds one allowance at the largest conductor (314.16(B)(3)). Clamps that sit outside the box do not count.
What if the box has its volume marked on it?
Use the marked volume. Switch the box field to "Marked volume" and enter the cubic inches stamped on the box, which is what the inspector will compare against. The standard boxes list is the minimum volume of common metal boxes from Table 314.16(A); nonmetallic boxes carry their own marking.
What does the calculator not cover?
It does not model the exception that lets certain grounds and small fixture wires from a luminaire canopy be left out (leave them out of the inputs yourself), or boxes with barriers unless you subtract the barrier volume from the marked volume. Always check the box marking and the edition your AHJ enforces.

You can read the NEC 2026 for free at NFPA free access.