Pushing fluid through a pipe costs energy, lost to friction along the walls. The Darcy–Weisbach equation ties that loss to four things: how fast the fluid moves, how long and wide the pipe is, and how rough its walls are. The friction factor comes off the Moody diagram above — the iconic log–log chart relating friction to the Reynolds number and relative roughness. Drag the operating point to see how your pipe sits on it.
Water at 20 °C through 100 mm commercial steel pipe at 20 L/s gives a velocity of about 2.55 m/s and a Reynolds number near 250,000 — firmly turbulent.
The friction factor lands around 0.018, and over 50 m of pipe that's roughly 30 kPa of pressure drop. Step up one pipe size and that figure falls sharply — pressure drop scales with diameter to the fifth power.
A lot. Below a Reynolds number of ~2,300 flow is laminar and friction follows the clean f = 64/Re law. Above ~4,000 it's turbulent and roughness starts to matter. Most pumped liquid systems are turbulent.
Because pressure drop scales with 1/D⁵. A small increase in bore dramatically cuts the loss — often the cheapest fix for an under-performing line is simply a larger pipe.
No — this is straight-pipe (major) loss only. Elbows, valves, tees, and entrance/exit effects add "minor losses" that can be significant in a real system and need to be added separately.
For water, very roughly 1–2.5 m/s is a common comfortable band — slow enough to limit erosion and noise, fast enough to avoid settling. The tool flags when you're outside typical ranges.