Box fill calculation doesn't have to be difficult. There are four primary things to add together to get the total volume in cubic inches needed for all wire and devices:
#1 A Conductor counts as 1 conductor volume (unless its +12" then it counts as two)
#2 Grounds count for 1 conductor volume until you reach 4 then each additional ground = .25
#3 All internal cable clamps count as 1 conductor volume (there is debate as to which field added clamps count into the equation so it is safer to add any that protrude into the box)
#4 Each Yoke (switch or receptacle) counts as 2 conductor volumes unless the yoke takes up two gangs, which then makes it count as 4.
A less common situation would be a stud or hickey needing to be calculated into the box fill. A stud is a threaded rod which will support a luminaire like a Chandelier. A hickey is a type of support nut that would generally thread onto a stud and act as a coupling. They both require a single conductor volume to be added for each one in the box. You only add them if they are attached to the strap and enter into the junction box.
Another thing to consider would be if you have 14/2 NM cables coming into a junction box and one 12/2 NM cable coming in, you would have to rate all of the 14 gauge conductors as 12 gauge and multiply by the 12 gauge volume allowance. If you had a single 12 gauge ground then only the grounds would be multiplied by the 12 gauge column, and not the conductors. Conductors and grounds are figured separately.
Box Fill Volume multiplier for each conductor size:
18 gauge = 1.5 cu.in.
16 gauge = 1.75 cu.in.
14 gauge = 2.0 cu.in.
12 gauge = 2.25 cu.in.
10 gauge = 2.5 cu.in.
8 gauge = 3.0 cu.in.
6 gauge = 5.0 cu.in.
0:00 Introduction
0:19 Box Fill For Beginners
4:21 Summary of Basic Box Fill Calculation
5:01 1st Practice Box
6:47 2nd Practice Box
7:21 3rd and 4th Practice Boxes
8:50 5th Practice Box
10:12 6th Practice Box
11:03 FINAL BOX
15:03 Summary
15:20 Rant Dance