The calculation
Max heads per zone = floor(available GPM × (1 − margin) ÷ flow per head)
Example: 10 GPM measured, 2.5 GPM spray heads, 20% margin: 10 × 0.8 = 8 usable GPM; 8 ÷ 2.5 = 3 heads per zone. Run the full version (including total head count and zone count) in the Sprinkler Zone Calculator.
Typical numbers
| Supply (bucket test) | Fixed spray heads (2.5 GPM) | MP rotators (0.7 GPM) | Rotors (2.5-5 GPM) |
|---|---|---|---|
| 8 GPM | 2-3 | 9 | 1-2 |
| 10 GPM | 3 | 11 | 2-3 |
| 12 GPM | 3-4 | 13 | 2-3 |
| 15 GPM | 4 | 17 | 3-4 |
Low-flow rotary nozzles are the practical way to raise head count without touching your plumbing - three heads' worth of spray flow runs nine rotators.
Three rules that override the math
- Hydrozoning first. Turf, shrubs and vegetables need different water - they cannot share a zone even if the flow fits.
- Never plan at 100%. Pressure drops when the valve opens; the 10-30% margin is what keeps the far head at full radius.
- Mixed nozzles break zones. Sprays (1.5-2 in/hr) and rotors (0.4-0.8 in/hr) apply water at different rates - keep families together.
Where the flow number comes from
Measure it with the bucket test - do not copy a neighbor's number. Then total your planned heads with the Irrigation Flow Calculator and check the pipe side with the Pipe Flow Calculator.