Box Fill Calculator
Count what actually enters the box and get the required cubic inches per NEC 314.16, with every allowance itemized: conductors, devices, clamps and grounds. If it does not fit, the calculator names the smallest standard box that does.
Take this math to the job site.
The JobVolt app runs the same verified engines offline, keeps every calculation in History, and turns them into inspector-ready PDF submittals on your letterhead.
How box fill is counted
Each conductor that originates outside and terminates or is spliced in the box counts once at the volume of Table 314.16(B). Each device yoke counts as two allowances at the largest conductor connected to it. All internal clamps together count as one allowance at the largest conductor in the box, and all equipment grounding conductors together count as one at the largest EGC. The calculator itemizes every line so the count is checkable.
Common mistake
Pigtails made up entirely inside the box do not count. Conductors passing through unbroken count once. When the total beats the box, the fix is a deeper box or a mud ring, and the calculator suggests the smallest standard size that clears the requirement.
Prefer the raw numbers? The box fill tables of 314.16 is on the site too, rendered from the same verified dataset this calculator reads.