Box Fill Calculator

Is this electrical box big enough?

Largest wire
wires
Ground wires
Internal cable clamps
Devices (switches or receptacles)
Fixture stud or hickey
cu in

Your box needs at least

18cubic inches needed

Section 314.16 allowances for 6 AWG and smaller. Ground counting follows the 2020 and later rule. Earlier editions and local amendments differ. Planning figure only. Confirm against the electrical code your area has adopted and have a licensed electrician check the work.

Box: 20 cu inNeeded: 18 cu in8 conductor allowances#12 wire: 2.25 cu in eachFits this box
Volume per wire2.25 cu in
Conductor allowances8
Grounds count as1
Spare volume2 cu in

Count conductors, grounds, clamps and devices against your box volume using Section 314.16 allowances.

How it works

Each conductor counts once at its Table 314.16(B) volume. All internal clamps count once together. Each device yoke counts twice, grounds count once for the first four and a quarter each after that, and a fixture stud counts once.

Before you order

  • Use the volume stamped inside the box, or the listed volume for plastic boxes.
  • Twist-on connectors and short pigtails do not add allowance.
  • GFCI receptacles are bulky even though they count the same, so go a size up.

Common questions

Why does a device count as two wires?

Code gives each yoke a double allowance based on the largest wire connected to it.

Do all grounds count as one?

Under the 2020 and later rule, up to four count as one and each additional ground counts as a quarter. Older editions and some local rules count differently.