What this is for
This answers one question: roughly how much pipe should go on the truck. It is a loading decision, not a takeoff. Enter your fixture runs, the average run length, the main line length, and a waste factor, and it returns an estimate with the waste already folded in.
The value is in not making the second trip. Running short at 2pm on a Tuesday costs more in lost hours than the extra fifty feet ever costs in material.
Trunk-and-branch vs. home-run
Distribution layout changes the number substantially. A manifold home-run system, where every fixture gets a dedicated line back to a central manifold, uses roughly 30% more pipe than a trunk-and-branch system serving the same fixtures.
If you are running home-run, estimate on the high side of whatever this returns. The tradeoff is real but so is the material draw.
Waste factors that hold up
Ten to fifteen percent covers most residential work. Push toward the top of that range on retrofits, older homes, and anything with unknown wall cavities, because obstructions force reroutes you cannot see from the basement.
New construction with open framing and a clean plan sits comfortably at the bottom of the range.