You have to account for turns. A UV typically has two T-type
turns, so there's a good 20 feet of pipe in just a UV.

From:

http://www.aquaticeco.com/index.cfm/...Detail/ttid/54

:

Next find the friction loss caused by the fittings. Please note that
the friction loss depends on the fitting diameter. A standard 1.5"
elbow is equal to about 4' of pipe; long elbows, sweeps, and 45° elbows
are equal to about 2' of pipe; straight through a "T", about 3' of
pipe; and a 90° turn through a "T", about 9' of pipe.

:

Anyone interested should see the chart on that page since you need to
know flow rate and effective pipe length and its diameter.
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SLOTHEAD