How to calculate box fill
The fetched OmniCalculator box fill page follows 2020 NEC Article 314.16(B). The main idea is to count volume allowances for everything entering or mounted in the box, then multiply those allowances by the required free space for the applicable wire size.
Allowance rules from the source page
Omni lists five contributors to box fill: conductors, clamps, support fittings, devices or equipment, and equipment grounding conductors.
A_w = number of conductors
A_c = 1 if internal clamps are used, otherwise 0
A_s = 1 if support fittings are present, otherwise 0
A_d = 2 x number of devices
A_g = 1 for 1-4 grounds, then +0.25 for each extra ground
Conductor volume table used by Omni
The fetched table includes these conductor allowances in cubic inches: 18 AWG = 1.50, 16 AWG = 1.75, 14 AWG = 2.00, 12 AWG = 2.25, 10 AWG = 2.50, 8 AWG = 3.00, and 6 AWG = 5.00.