Landscaping
Irrigation Calculator
Work out irrigation requirement from area, application rate and the available supply.
Result
Assumes even coverage and no rainfall. Measure your real precipitation rate with catch cans rather than trusting the head's rating. Actual output varies with pressure and spacing.
Get a Free Business AuditWhat moves the number
- Available flow and pressure, which cap how many heads can run at once.
- Precipitation rate of the chosen heads, which sets run time.
- Zoning, since the system is designed around what the supply can feed simultaneously.
Where this one goes wrong
Designing zones without confirming the supply. Available flow decides how many heads can run together, and a system designed beyond it delivers poor coverage on every zone rather than failing obviously.
Before you quote
- Measured static pressure and available flow at the point of connection.
- Whether backflow prevention is required, which it generally is.
- Whether the supply is metered and how that affects running cost.
Questions
About this calculator
What decides how many zones a system needs?
Available flow. Each zone can only contain as many heads as the supply can feed at their rated flow, so the supply measurement comes before the design.
Is backflow prevention required?
In most jurisdictions yes, for any irrigation connected to a potable supply. The device type and testing requirements are set locally.
How long should zones run?
Long enough to apply the required depth at the heads' precipitation rate, which differs by head type. Run times taken from another system rarely transfer.
The formula
run time = (weekly inches ÷ precipitation rate) ÷ watering days; gallons = sq ft × inches × 0.623
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How to use this calculator
- Area to water (sq ft) (sq ft). Enter your own figure; the page starts at 5,000 sq ft.
- Water needed (inches per week). Enter your own figure; the page starts at 1.
- Sprinkler output (inches per hour). Enter your own figure; the page starts at 0.6.
- Watering days per week. Enter your own figure; the page starts at 3.
- Water cost per 1,000 gallons ($). Enter your own figure; the page starts at $4.5.
The result updates as you type, so you can try a figure, see what it does to run time per session, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Area to water (sq ft) | 5,000 sq ft |
| Water needed (inches per week) | 1 |
| Sprinkler output (inches per hour) | 0.6 |
| Watering days per week | 3 |
| Water cost per 1,000 gallons | $4.5 |
What comes out
| Result | Value |
|---|---|
| Run time per session main result | 33.3 |
| Water used per week | 3,115 |
| Weekly water cost | $14.02 |
| Cost over a 26-week season | $364.46 |
Run each zone about 33.3 minutes, 3 days a week. That uses 3,115 gallons weekly, roughly $364.46 across a season.
How sensitive the answer is
Holding everything else steady and changing only water cost per 1,000 gallons, this is what happens to run time per session. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Water cost per 1,000 gallons ($) | Run time per session |
|---|---|
| $2.25 | 33.3 |
| $3.38 | 33.3 |
| $4.5 example above | 33.3 |
| $6.75 | 33.3 |
| $9 | 33.3 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Area to water (sq ft) (sq ft)
- Measured in sq ft, starting at 5,000 sq ft.
- Water needed (inches per week)
- A number, between 0 and 10, starting at 1.
- Sprinkler output (inches per hour)
- A number, between 0 and 10, starting at 0.6.
- Watering days per week
- A number, between 0 and 7, starting at 3.
- Water cost per 1,000 gallons ($)
- A money amount, starting at $4.5.
What the results tell you
| Result | What it is |
|---|---|
| Run time per session | The headline result. A decimal figure. |
| Water used per week | A number. |
| Weekly water cost | A money figure. |
| Cost over a 26-week season | A money figure. |
Seasonality is not in this
Coverage and material figures assume the area you enter. Slope, access for machinery, existing growth and the time of year all affect both quantity and labour, and none of them appear in a calculation like this.
Keep going
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The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.