Key takeaways
- Precipitation rate = GPM × 96.25 ÷ (head spacing × row spacing), so 6 GPM on a 30 × 25 ft layout gives 0.77 in/hr.
- Minutes per week = target inches × 60 ÷ precipitation rate, so 1 inch at 0.77 in/hr takes about 78 minutes.
- Illinois and Iowa State extension both put lawns at about 1 to 1.5 inches of water a week, but your local office sets the figure for your area.
Enter one head's flow in gallons per minute, the spacing between heads and between rows, and whether the layout is square or triangular. Then add how many inches of water you want a week and how many days you water. The tool gives the zone's precipitation rate and the minutes to run it.
If you'd rather measure than calculate, use the catch-can option and enter what your cans collected.
How the calculator works
These are the formulas on Toro's formula and conversion sheet, with spacing in feet:
- Square or rectangular layout: rate (in/hr) = GPM × 96.25 ÷ (head spacing × row spacing)
- Triangular layout: rate (in/hr) = GPM × 96.25 ÷ (head spacing² × 0.866)
- Minutes per week = target inches per week × 60 ÷ rate
- Minutes per watering = minutes per week ÷ waterings per week
In a triangular (equilateral) layout, each row sits 0.866 × the head spacing from the next. So the tool only needs the head spacing there, and ignores the row spacing.
The GPM is for a full-circle head. If your zone uses half- or quarter-circle heads with matched nozzles, enter the flow a full-circle head would use: a half-circle head's GPM × 2, or a quarter-circle head's × 4. Rain Bird's rotor guide does the same thing by multiplying by 360 ÷ the arc.
Where 96.25 comes from
A gallon is 231 cubic inches. One gallon a minute for an hour is 60 gallons, or 13,860 cubic inches. Spread over 1 square foot (144 square inches), that's 96.25 inches of water. Rain Bird's guide rounds the constant to 96.3, and Toro's own catalog uses both. The difference is about 0.05 percent, so it won't change your run time.
Worked example
A zone of heads flowing 6 GPM each, 30 feet apart in rows 25 feet apart:
- Flow term: 6 × 96.25 = 577.5
- Area per head: 30 × 25 = 750 square feet
- Rate: 577.5 ÷ 750 = 0.77 inches per hour
- For 1 inch a week: 1 × 60 ÷ 0.77 = 78 minutes a week
- Watering twice a week: 78 ÷ 2 = 39 minutes each time
The same heads in a triangular layout, 30 feet apart:
- Area per head: 30² × 0.866 = 900 × 0.866 = 779.4 square feet
- Rate: 577.5 ÷ 779.4 = 0.74 inches per hour
- For 1 inch a week: 60 ÷ 0.741 = 81 minutes a week
Minutes per week by rate
| Precipitation rate | 1 inch a week | 1.5 inches a week |
|---|---|---|
| 0.5 in/hr | 120 min | 180 min |
| 0.77 in/hr | 78 min | 117 min |
| 1.0 in/hr | 60 min | 90 min |
| 1.5 in/hr | 40 min | 60 min |
| 2.0 in/hr | 30 min | 45 min |
Rain Bird's technical newsletter notes that rotors apply water more slowly than spray heads, which is why a spray zone and a rotor zone on the same lawn can need very different times. Your heads' published GPM, or a catch-can test, tells you where yours fall.
The catch-can test
Published GPM assumes the right pressure and clean nozzles. A test measures what your zone actually does.
- Set out several identical, straight-sided containers across the zone. Illinois Extension suggests coffee cans or similar, and Iowa State suggests 2 or 3 rain gauges in the spray pattern.
- Run the zone for a set time, such as 15 minutes.
- Measure the depth in each container and average them.
- Rate (in/hr) = average inches collected × 60 ÷ test minutes
For example, an average of 0.2 inches in 15 minutes is 0.2 × 60 ÷ 15 = 0.8 inches per hour.
If the cans differ a lot from each other, some parts of the zone are getting much more water than others. Fix the heads before you lengthen the run time, or the wet spots get wetter.
How much water a week
The calculator leaves the weekly target to you, because it depends on your grass, soil, season and climate.
- Illinois Extension: "about one to 1.5 inches of water per week is needed to maintain green color and active growth."
- Iowa State: "Most lawns in Iowa require 1 to 1½ inches of water per week", applied in one watering or two, 3 or 4 days apart.
Both are Midwest figures. In a dry or hot climate the need can be quite different, and many water utilities set watering days. Ask your state's extension office or your water utility for a local figure, and cut back after rain.
If water puddles or runs off before the time is up, split the run into shorter cycles with a break between them.
Common mistakes
- Using a part-circle head's GPM as is. The formula expects the full-circle figure, so a half-circle head's flow needs doubling.
- Entering row spacing for a triangular layout. The triangular formula uses head spacing only; the 0.866 already accounts for the rows.
- Grouping unmatched heads on one valve. Rain Bird's newsletter says not to combine sprinklers on one valve if their rates differ by more than 10 percent, since one run time can't suit both.
- Trusting the catalog over the lawn. Low pressure or worn nozzles lower the real rate, which a catch-can test shows.
- Watering a set time all year. The target changes with the season and the rain.
Frequently asked questions
How long should I run my sprinklers?
Divide your weekly target in inches by the zone's precipitation rate, then multiply by 60 for minutes. At 0.77 inches per hour, 1 inch a week takes about 78 minutes.
How do I calculate sprinkler precipitation rate?
Multiply one head's full-circle GPM by 96.25, then divide by the area each head covers: head spacing × row spacing for a square layout, or head spacing² × 0.866 for a triangular one.
How do I do a catch-can test?
Set several identical, straight-sided cans across the zone, run it for a set time, and average the depths. Multiply by 60 and divide by the minutes you ran it.
How many inches of water does a lawn need a week?
Illinois Extension and Iowa State both give about 1 to 1.5 inches a week. Your local extension office can give a figure for your climate and grass.
Should I water every day?
Iowa State suggests one watering a week, or two spaced 3 or 4 days apart, rather than frequent light watering.
Why does my spray zone need less time than my rotor zone?
Rain Bird notes that rotors apply water more slowly than spray heads. Work out each zone's rate separately and give each its own run time.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- Toro: Formulas and conversion factors
- Rain Bird: Rotor nozzle guide, triangular spacing
- Rain Bird: Technical newsletter on sprinkler spacing and precipitation rates
- Illinois Extension: Watering your lawn
- Iowa State University Extension: How often should I water my lawn?