Undersized pipe is the number-one cause of poor pressure at the last sprinkler head. This guide walks you through sizing every pipe run from source to zone, using the simple velocity rule and flow-capacity tables.
The golden rule: velocity ≤ 1.5 m/s
Water in a pipe should never move faster than 1.5 m/s (5 ft/s). Exceeding this causes water hammer, noise, and accelerated fitting wear. The formula: v = Q / (π × (d/2)²), where Q is flow (m³/s) and d is internal diameter (m).
Pipe material comparison
| Material | Diameter range | Pressure rating | C factor | Flexibility | Cost |
|---|---|---|---|---|---|
| PVC Schedule 40 | 20–110 mm | 10–16 bar | 150 | Rigid | Low |
| PE/LDPE | 16–63 mm | 6–10 bar | 140 | Flexible | Medium |
| HDPE | 20–110 mm | 10–16 bar | 140 | Semi-flexible | Medium |
| Copper | 15–54 mm | 20+ bar | 130 | Rigid | High |
For residential systems, PE (polyethylene) is the best choice: lightweight, flexible, no glue required, joins with compression fittings. PVC is cheaper but rigid and requires solvent-weld connections.
PE pipe flow-capacity table
At a maximum velocity of 1.5 m/s:
- PE 25 mm (ID 21 mm) — up to 31 L/min
- PE 32 mm (ID 27 mm) — up to 52 L/min
- PE 40 mm (ID 35 mm) — up to 87 L/min
- PE 50 mm (ID 44 mm) — up to 137 L/min
Velocity calculation example
Flow 20 L/min through PE 25 mm (ID 21 mm): v = (20/60000) / (π × 0.0105²) = 0.96 m/s ✓ — within limits. Same flow through PE 20 mm (ID 16 mm): v = (20/60000) / (π × 0.008²) = 1.66 m/s ⚠️ — exceeds limit, need a larger pipe.
Mainline vs lateral sizing
The mainline carries water from the source to the valves — size it for the total flow of all simultaneous zones. The lateral runs from a valve to the sprinkler heads — size it for one zone's flow.
Rule: mainline should be at least one size larger than the largest lateral. Typical sizing: mainline PE 40–50, lateral PE 25–32. If the mainline exceeds 50 m at flows > 40 L/min, step up to PE 50.
Friction loss (Hazen-Williams)
Formula: hf = 10.67 × L × Q¹·⁸⁵² / (C¹·⁸⁵² × d⁴·⁸⁷). For PE pipe, C ≈ 150. Rule of thumb: mainline friction loss should stay under 1.5 bar (15 m head).
| Diameter | Flow 20 L/min | Flow 40 L/min | Flow 60 L/min |
|---|---|---|---|
| PE 25 | 0.8 bar/100 m | 2.7 bar/100 m | — |
| PE 32 | 0.3 bar/100 m | 0.9 bar/100 m | 1.8 bar/100 m |
| PE 40 | 0.1 bar/100 m | 0.3 bar/100 m | 0.7 bar/100 m |
Fitting losses (equivalent length)
Every fitting (elbow, tee, reducer) adds equivalent pipe length to the friction-loss calculation:
- 90° elbow (25 mm) ≈ 0.8 m equivalent length
- Tee (25 mm) ≈ 1.2 m
- Reducer ≈ 0.3 m
- Zone valve ≈ 2–4 m (varies by model)
Total equivalent length = actual pipe length + sum of all fitting equivalents. In practice, fittings add 10–20% to the actual run length.
Worked example: 12 sprinklers × 2 L/min = 24 L/min
A zone with 12 rotary nozzles, each flowing 2 L/min. Total: 24 L/min. The table shows PE 25 handles up to 31 L/min — technically sufficient, but friction loss is 1.2 bar/100 m. For a 30 m lateral that's acceptable (0.36 bar). If the lateral exceeds 50 m, step up to PE 32.
SmartPluvia auto-sizes pipes
In SmartPluvia, press P for pipe mode. The system automatically calculates velocity and friction loss using Hazen-Williams, factoring in fittings, elevation changes, and valve losses. Pipes that exceed the velocity limit are highlighted in red, and the MST algorithm builds the shortest route between heads. Standard Generic PE/PVC pipes and fittings are a budget-friendly option for most systems.