Each conductor that enters and leaves = 1. Conductor terminated inside = 1. All conductors of the same size sharing the same status can be grouped.
Other allowances
Volume used
— cu-in
of — cu-in available
Breakdown
Item
Cu-in
Per-conductor volume
AWG
Cu-in each
#18
1.50
#16
1.75
#14
2.00
#12
2.25
#10
2.50
#8
3.00
#6
5.00
⚡ US / NEC 314.16 only. This calculator uses the NEC 314.16(B) volume-per-conductor method (US National Electrical Code). It is not valid for Canadian installations — CEC 12-3034 uses a different method with different conductor allowances and different device-fill rules, and the numbers do not interchange. Canadian apprentices: use CEC 12-3034 Table 22/23 directly, by hand, for now.
Within US scope: volume is per conductor of a given AWG. Confirm against the box's stamped/marked interior volume — that overrides everything if larger. Boxes for domed covers, plaster rings, and extension rings can add volume — check the manufacturer marking. When in doubt, size up.