Calculator Inputs
Example data table
These examples show how flow and spacing change precipitation rate. Use your manufacturer nozzle charts for precise flows.
| Scenario | Layout | Spacing | Flow per head | Calculated PR | Matched? |
|---|---|---|---|---|---|
| Spray zone, consistent nozzles | Rectangular | 10 × 10 ft | 1.5 gpm | 1.44 in/hr | Yes |
| Mixed spray nozzles | Rectangular | 10 × 10 ft | 1.5 gpm and 2.0 gpm | 1.44 vs 1.93 in/hr | No |
| Triangular grid, same flow | Triangular | 10 × 10 ft | 1.5 gpm | 1.66 in/hr | Yes |
| Metric example, evenly matched | Rectangular | 3 × 3 m | 6.0 L/min | 40.0 mm/hr | Yes |
| Runoff risk check | Rectangular | 3 × 3 m | 7.5 L/min | 50.0 mm/hr | Depends on infiltration |
Formula used
Triangular: A = 0.866 × Sx × Sy
Metric: PR(mm/hr) = Flow(L/min) × 60 ÷ Area(m²)
Deviation% = |PR(type) − Target PR| ÷ Target PR × 100
How to use this calculator
- Choose units and the sprinkler layout that matches your design.
- Pick spacing mode for grid zones, or enter total zone area.
- Enter head counts and flow per head for each nozzle type.
- Set a mismatch tolerance, then calculate to see match status.
- Optionally add soil infiltration rate to flag runoff risk.
- Use the runtime estimate to apply your desired water depth.
Matched precipitation in practical garden irrigation
Matched precipitation means every head in a zone applies water at nearly the same rate. When rates differ, plants receive uneven moisture, causing stressed turf, dry edges, or soggy corners. This calculator compares head types against a zone-wide target precipitation rate and flags mismatch beyond your tolerance. Use it early in design, and again when you retrofit nozzles or change spacing.
Spacing and layout effects you can quantify
Spacing defines coverage area per head, so it directly controls precipitation rate. A tighter grid increases application rate; a wider grid reduces it. Triangular spacing typically improves distribution, but the effective area factor changes the calculated rate. Enter the same spacing your installer will stake out, not the radius printed on a box. Small spacing errors compound quickly across many heads.
Mixing nozzle types without creating wet and dry zones
Mixing sprays, rotors, and different nozzle families is the most common cause of mismatch. Even within sprays, different arcs and pressure-regulated bodies can change flow. Keep one zone to one “rate family,” or use nozzles that are explicitly rated for matched precipitation. If you must mix, split the zone or adjust flows toward the calculated target.
Reading deviation, tolerance, and what to change first
Deviation shows how far each head type sits from the weighted target rate. A 5–10% tolerance is a strong target for high-uniformity beds, while 10–15% may be acceptable for mixed landscapes. Start corrections with the head type showing the largest deviation. The “flow to match target” value gives a practical nozzle selection goal for each head.
Runtime and infiltration as a scheduling checkpoint
Runtime is estimated from the target precipitation rate and your desired applied depth. Pair that with soil infiltration to reduce runoff and puddling. If precipitation exceeds infiltration, consider cycle-and-soak scheduling or reducing nozzle flow. Use the runtime as a starting point, then verify with catch-cans and seasonal adjustments.
- Layout: Rectangular · Spacing: 10 × 10 ft · Tolerance: 10%
- Head A: 6 heads @ 1.50 gpm → PR ≈ 1.44 in/hr (deviation ≈ 7.7%)
- Head B: 2 heads @ 2.00 gpm → PR ≈ 1.93 in/hr (deviation ≈ 23.1%)
- Target PR ≈ 1.565 in/hr · Max deviation ≈ 23.1% → Matched: No
- Desired depth: 0.50 in → Runtime ≈ 19.2 minutes
FAQs
1) What does “matched precipitation” mean?
It means heads in the same zone apply water at nearly the same rate, so the whole area receives uniform depth during one runtime.
2) What tolerance should I use?
Start with 10%. Use 5–8% for high-uniformity beds and 12–15% for mixed landscapes where perfect matching is difficult.
3) Can I mix sprays and rotors in one zone?
It’s not recommended. Sprays and rotors often have very different precipitation rates, which creates wet and dry areas. Split the zone whenever possible.
4) Why does triangular layout change results?
Triangular spacing changes effective coverage area per head. Because precipitation rate depends on flow divided by area, the layout factor changes the computed rate.
5) What if my spacing varies across the zone?
Use the dominant spacing or redesign into smaller zones. Large spacing variation reduces uniformity even if nozzle rates are matched on paper.
6) How should I use infiltration rate?
Compare infiltration to the calculated precipitation rate. If precipitation is higher, use cycle-and-soak or reduce nozzle flow to limit runoff.
7) Is the runtime estimate exact?
No. It’s a planning value based on ideal distribution. Confirm with catch-cans, observe plant response, and adjust by season and wind exposure.