Sprinkler Spacing Calculator
Sprinkler precipitation rate and minutes from flow, area, and target depth.
What it does
Sprinkler zones perform when heads overlap head to head and every head in the zone rains the same inches per hour. The precipitation rate ties flow to area, and runtime follows from the target depth.
The tool above takes zone flow with area, target, and days. It returns inches per hour with session minutes and the weekly total.
The default 12 GPM over 1,500 sq ft rains 0.77 in/hr, so 0.5 in needs 39 min, 117 a week over 3 days. The metric mirror of 45.4 L/min over 139.4 sq m reads 19.6 mm/hr.
Formulas follow UF IFAS turf irrigation with NMSU Circular 660 behind them. Both state PR = 96.3 x GPM / area used here.
How to use
- Pick gallons or metric flow.
- Enter total zone GPM.
- Enter watered area.
- Enter target inches.
- Enter days per week.
- Read rate, runtime, weekly.
Formula
- GPM
- Zone flow in gallons per minute.
- area
- Watered area in square feet.
- target
- Target depth per session.
- 96.3
- Gal to in/hr constant.
The 96.3 constant converts gallons over square feet to inches per hour (231 / 144 x 60). Metric converts to imperial first, then displays mm/hr (x 25.4). Runtime rounds up to whole minutes.
Worked example
Example: 12 GPM zone
Imperial defaults: 12 GPM, 1,500 sq ft, 0.5 in, 3 days.
- Zone: 12 GPM over 1,500 sq ft.
- Numerator: 96.3 x 12 = 1,155.6.
- Rate: 1,155.6 / 1,500 = 0.77 in/hr.
- Target 0.5 in: 0.5 / 0.7704 = 0.649 hr.
- Minutes: 0.649 x 60 = 39 min.
- Weekly at 3 days: 39 x 3 = 117 min.
- Tool shows 0.77 in/hr, 39 min, 117 min/week.
Reference table
| Head | Spacing | Note |
|---|---|---|
| Impact | 35 to 45 ft | At maker pressure |
| Gear driven | 25 to 35 ft | At maker pressure |
| Stationary spray | 8 to 15 ft | Short strips |
| Any | Head to head | Past 60% fails |
Limits
- Total area method; audits refine.
- Clay needs split cycles.
- Wind and pressure shift real rates.
- Narrow strips need matched nozzles.
Common mistakes
- Mixing rotors with sprays.
- Spacing past 60% of throw.
- Sizing by pump, not zone flow.
- Watering daily shallow.
- Skipping catch can audits.
Try it yourself
Quick checks. Answers sit tucked under each question.
8 GPM, 1,000 ft2, 0.5 in, 2 days?
PR 96.3 x 8 / 1,000 = 0.77 in/hr, 39 min, 78 min/week over 2 days.
Metric defaults?
Same math: 19.6 mm/hr, 39 min, 117 min/week.
4 GPM, 800 ft2, 0.5 in?
PR 96.3 x 4 / 800 = 0.48 in/hr: 0.5 in needs 62 min.
Questions
What is the precipitation formula?
96.3 x GPM / area sq ft, in inches per hour. Metric: 1,000 x m3/hr / sq m for mm/hr.
How close should heads sit?
Head to head: each head throws to its neighbor. Past 60% of throw diameter, uniformity fails.
Can I mix rotors and sprays?
Never in one zone. Rotors and sprays cannot match rates; corners need quarter flows.
How do arcs change flow?
Full circle flow is double half circle and quadruple quarter: 50% edges, 25% corners.
What if water runs off?
Slow clay near 0.12 to 0.25 in/hr ponds under typical rates: split into cycles with soak gaps.
Should I audit with cans?
Yes: 15 min catch cans, average depth x 4 gives inches per hour to check the math.