Calculating GPM from City Water for Irrigation Design

Determining available GPM from municipal supply is essential for zone sizing. Use this quick reference based on service line and meter size:

Rule-of-Thumb GPM Table

Service Line	Meter Size	Available GPM
¾"	⅝" or ¾"	9–11
1"	⅝"	12–15
1"	1"	16–20

Note: ⅝" and ¾" meters have similar flow capacity.

Pressure Adjustment

If static pressure < 50 psi, deduct 20% from above numbers. Measure pressure at a hose bibb with a gauge (0–100 psi).

More Accurate Method: Bucket Test

1. Connect a known-length hose to the hose bibb closest to the irrigation tie-in.
2. Run full open into a 5-gal bucket. Time it.
3. GPM = (5 gal / seconds) × 60.
4. Repeat 3×, average. This is your dynamic flow at that pressure.

Design Safety Factor

Use 75–80% of measured GPM for zone design. Example: 12 GPM measured → design zones for 9–10 GPM max. Leaves margin for pressure fluctuations and future demand.

Next Steps

With GPM and pressure, select heads/nozzles where total zone GPM ≤ design GPM. Rotors: 2–5 GPM each. Sprays: 0.5–2 GPM each. Drip: 0.5–4 GPM per 100 ft.