A question often asked is what size pipe to buy.
From Table 4, Appendix X, 1-1/4 in. plastic would
have a friction loss of 2.31 psi (1.0 x 2.31 ft.) instead
of 8.8 psi (3.8 x 2.31 ft.) for the 1 in. pipe. Since the
outlet pressure difference between the two pipe sizes