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Box Fill Calculator

Count conductors, devices, clamps, and grounds the NEC 314.16 way — get the cubic inches you need and whether the box you grabbed actually passes.

Conductors — how many enter the box
14 AWG
12 AWG
10 AWG
8 AWG
6 AWG
What else is in the box
Volume required
—
Add the conductors in the box
NEC 314.16(B): 14 AWG = 2.00 in³, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 = 5.00. A device = 2×, clamps = 1×, grounds = 1×, at the largest conductor.
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The counting rules trip people up

Box fill isn't just counting wires — it's counting the right way. Each conductor that enters the box counts once at its own gauge. But a device (a receptacle or switch) counts as two conductors, based on the largest wire landing on it. All the cable clamps together count as one. Each fixture stud or hickey counts as one. And all the equipment grounds — however many — count as one. Total those cubic inches and compare to the volume of the box, either from Table 314.16(A) or stamped inside. Cram a box and you're fighting the devices in, overheating the splices, and failing the inspection. This does the count the code way so you grab the right box the first time.

Questions

How do you calculate box fill?
Add a volume allowance for each item per 314.16(B): each conductor at its gauge (14 = 2.00, 12 = 2.25, 10 = 2.50 in³), all clamps as one, each support fitting as one, each device yoke as two, and all grounds as one — the multiplier items at the largest conductor. Compare the total to the box volume.
How many conductors in a box?
Depends on the box and what's inside. An 18 in³ device box with a device (2 × 12 AWG = 4.5 in³), a clamp, and a ground leaves room for about five more 12 AWG. Enter your box above for the exact count.
Does a device count?
Yes — each yoke counts as two conductors at the largest wire connected to it. A device on 12 AWG adds 2 × 2.25 = 4.50 in³.
Is this free and accurate?
Free, in your browser, NEC 2023 314.16(B) allowances and counting rules. A reference tool — verify against your adopted code.

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For reference and estimating only. Based on NEC (NFPA 70) 2023 — 314.16(B) volume allowances and the standard counting rules, with box volumes from Table 314.16(A) or the box marking. Not a substitute for the National Electrical Code, a licensed electrician's judgment, or your authority having jurisdiction. Verify against your locally adopted code before installing.