The volume coefficients are used by the length constructor. The internal areas are passed to the air velocity formula. The existing source does not identify the external standard behind these values, so they are documented here as implementation tables.

Nominal pipe size	Volume coefficient \([\unit{\cubicFoot\per\foot}]\)	Internal area \([\unit{\inch\squared}]\)
1/2 in	0.0021	0.3
3/4 in	0.0037	0.53
1 in	0.006	0.86
1-1/4 in	0.0104	1.5
1-1/2 in	0.0141	2.04
2 in	0.0233	3.36
2-1/2 in	0.0333	4.79
3 in	0.0513	7.39
3-1/2 in	0.0687	9.89
4 in	0.0884	12.73
5 in	0.1389	20
6 in	0.2006	28.89
8 in	0.3442	50.02
10 in	0.5476	78.85
12 in	0.7763	111.9
14 in	0.9354	135.3
16 in	1.223	176.7
18 in	1.555	224
20 in	1.926	278
24 in	2.793	402.10