Pipe Head Loss Calculator (Hazen-Williams)

How it works

h_f = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.8704) (SI: m, m³/s, m); v = Q ÷ (π D²/4); 1 m head = 0.0981 bar = 1.422 psi

Water loses pressure to friction as it flows, and the Hazen-Williams equation is the plumber’s and irrigation designer’s standard for estimating that loss without iterating on Reynolds numbers. Loss grows almost with the square of flow and — crucially — with the diameter to the power of nearly five, so going one pipe size up cuts friction by two thirds or more. The roughness coefficient C ranges from about 150 for new plastic to under 100 for old, tuberculated iron. Fittings add losses that are usually folded in as an equivalent extra length of 10–30%. Add the static lift to get the head a pump must generate, and keep velocity in the 0.5–1.5 m/s band in buildings to avoid noise and hammer.

Worked example

40 L/min through 30 m of 25 mm PVC (C 150) with 20% for fittings: 2.98 m of head loss (0.29 bar) at 1.36 m/s; with a 3 m rise the pump needs about 6 m plus outlet pressure.

Frequently asked questions