Pipe Head Loss Calculator

Head loss is the energy a fluid loses as it flows through piping, from friction with the pipe walls and from disturbances at bends, valves, and reducers. It is measured in meters of liquid column. It splits into distributed loss (along the straight pipe) and localized loss (at the fittings). It is unavoidable — there is no frictionless piping — but it is minimized with correct diameter and routing design.

Distributed loss is calculated by the Darcy-Weisbach equation: h_f = f · (L/D) · v²/(2g), where f is the friction factor, L the length, D the internal diameter, v the velocity, and g gravity. The factor f comes from 64/Re in laminar flow and from the Colebrook-White equation (approximated explicitly by Swamee-Jain) in turbulent flow. Localized loss from fittings is h = ΣK · v²/(2g). The calculator sums both.

Darcy-Weisbach is physical and universal: it holds for any fluid and any regime (laminar or turbulent), because it accounts for viscosity through the Reynolds number — it is the correct method for oils and viscous fluids. Hazen-Williams is empirical and simpler (no need to iterate the friction factor), but valid only for water at ordinary temperature, in turbulent flow, using the material’s C coefficient. For critical suction design, use Darcy-Weisbach.

FB Bombas recommends a maximum velocity of 1.5 m/s at the suction of FBCN centrifugal pumps with water, and 0.5 m/s at the suction of FBE gear pumps with viscous fluids. High suction velocity increases head loss (which grows with the square of velocity), reduces available NPSH, and promotes cavitation. On the discharge, the typical economic range is 1 to 3 m/s for water. The rule of thumb: always keep the suction generous.

Head loss in the suction line enters the available NPSH formula as a direct subtraction: NPSHa = P_abs/γ + Z_s − h_f − P_v/γ. Each meter of suction head loss is one meter less of NPSHa. When NPSHa falls below the pump’s NPSHr, cavitation begins — vapor bubbles that collapse and erode the impeller. That is why poorly sized suction is the #1 cause of cavitation in industrial installations.

The absolute roughness ε (which controls the friction factor in turbulent flow) varies widely with the material: PVC and plastics ~0.0015 mm, stainless steel ~0.015 mm, new commercial steel ~0.045 mm, galvanized steel ~0.15 mm, asphalted cast iron ~0.12 mm, and cast iron ~0.26 mm (Crane TP-410 values, new pipe). Aged and scaled pipes have roughness many times higher — cast iron can reach 0.6–1.2 mm over its service life, raising head loss.