Irrigation System Pressure Loss Calculator
Calculate total pressure loss in an irrigation system using the Hazen-Williams equation, including pipe friction losses, elevation head, and minor losses from fittings.
Formula
Hazen-Williams Friction Loss (ft per 100 ft of pipe):
hf = 0.2083 × (100/C)1.852 × Q1.852 / D4.8704
Where: Q = flow rate (GPM), D = inside pipe diameter (inches), C = Hazen-Williams roughness coefficient
Total Friction Head Loss: Hf = hf × L / 100 (feet)
Fittings Loss: Hfit = Hf × (fittings factor / 100)
Elevation Head: Helev = elevation change (ft) — positive = uphill (loss), negative = downhill (gain)
Total Head Loss: Htotal = Hf + Hfit + Helev
Pressure (PSI): PSI = Htotal × 0.4335 (1 ft of water = 0.4335 PSI)
Assumptions & References
- Uses the Hazen-Williams equation, the industry standard for irrigation and water distribution design (ASABE, NRCS).
- Assumes full-flow, turbulent conditions in a circular pipe — valid for typical irrigation velocities (1–5 ft/s).
- Water temperature assumed at 60°F (15.6°C), standard for irrigation design.
- Fittings factor (minor losses) is expressed as a percentage of pipe friction loss — a common approximation; 10–15% is typical for simple systems.
- Elevation head uses the static head formula: 1 ft of elevation = 0.4335 PSI (based on water density of 62.4 lb/ft³).
- Recommended maximum pipe velocity: 5 ft/s for mainlines, 7 ft/s for laterals (Irrigation Association guidelines).
- C values: PVC 150, Copper 130, Galvanized Steel 120, Cast Iron 100 (Hydraulic Institute / Hazen-Williams tables).
- Does not account for emitter/sprinkler head pressure losses, water hammer, or pressure regulators — add those separately.
- Reference: Hazen, A. & Williams, G.S. (1920). Hydraulic Tables. NRCS National Engineering Handbook, Part 623.