Sprinkler Calculator
Enter your heads' flow and spacing to get the zone's precipitation rate, how long to water and how many heads a zone can run.
How to use the Sprinkler calculator
- Under Calculate, choose “Precipitation rate (in/h, mm/h)”, “Run time per week”, “Heads per zone” or “Water usage and cost”.
- For the rate, set Rate from to “Head flow and square spacing”, “Head flow and triangular spacing” or “Zone flow and area”.
- Enter Flow of one head in gpm or L/min. The help text says a half-circle head uses about half of its full-circle flow.
- Enter Spacing between heads, and Spacing between rows of heads for square spacing. With zone flow and area, enter Total flow of the zone (all heads) and Area watered by the zone.
- For run time, enter Precipitation rate of the zone, Weekly water target and Watering days per week.
- For heads per zone, enter Available flow and Flow of one head.
- For usage, enter Total flow of the zone (all heads), Minutes per run and Runs per week.
- Under More options, you can add Rain this week (subtract), a Safety margin (%), a Water price (optional) and Price is per.
- Read the results and warnings, then open How it was calculated.
The example buttons load a sample for each mode. Reset restores the defaults, Copy link stores every input in the page address, Copy result copies the result lines, and Print / save as PDF prints the inputs, results and steps.
How is sprinkler precipitation rate calculated?
Precipitation rate is the depth of water a zone puts on the ground per hour. In US units, inches per hour equals 96.25 × gpm ÷ ft², where the area is the ground one head covers. For square spacing that area is head spacing × row spacing. For triangular spacing it is spacing × spacing × 0.866. A 2 gpm head at 15 × 15 ft gives 96.25 × 2 ÷ 225 = 0.8556 in/h, or 21.73 mm/h. In metric, mm/h is 60 × L/min ÷ m².
How long should I run my sprinklers?
Divide the weekly water target by the precipitation rate to get hours per week, then divide by the watering days and multiply by 60 for minutes per day. With a rate of 1.5 in/h, a target of 1 in per week and 3 watering days, each run is 13.33 minutes. Rain you enter is subtracted from the target first. The tool starts with 1 in (25 mm) a week, a figure often quoted for lawns, but the right amount depends on your grass, soil and weather. You can change it.
How many sprinkler heads can I put on one zone?
Divide the available flow by the flow of one head and round down. A supply of 10 gpm with 1.5 gpm heads allows 6 heads, because 10 ÷ 1.5 = 6.67. They use 9 gpm and leave 1 gpm. A safety margin holds back part of the supply first. The tool warns if not even one head can run, or if more than half the flow is left over.
How much water does a sprinkler system use each week?
Multiply the zone flow by the minutes per run and the runs per week. A zone flowing 12 gpm for 20 minutes three times a week uses 12 × 20 × 3 = 720 gallons, or 2,725 liters. The tool also shows the monthly total, using 4.348 weeks per month, and the cost per week and per month if you enter a water price.
Sources
- NIST Handbook 44, Appendix C and NIST SP 811 — exact unit definitions (1 acre = 43,560 ft² = 4,046.8564224 m², 1 lb = 0.45359237 kg, 1 US gal = 231 in³); PR = 96.25 × gpm ÷ ft²
- University of Rhode Island, Water wisely — established lawns need about 1 in of water per week, rain included
Worked examples
| Input | Result |
|---|---|
| 2 gpm, 15 × 15 ft | 0.8556 in/h (21.73 mm/h) |
| 8 L/min, 4.5 m triangular | 1.078 in/h (27.37 mm/h) |
| 15 gpm, 2500 ft² | 0.5775 in/h (14.67 mm/h) |
| 1.5 in/h, 1 in/week, 3× | 13.33 min per watering day |
| 25 mm/h, 25 mm/week, 2× | 24 min per watering day |
| 10 gpm, 1.5 gpm | 6 heads |
| 12 gpm, 20 min × 3 | 720 gal (2,725 L) per week |
Calculated by the tool above with these values.
Frequently asked questions
Is the Sprinkler calculator free, and is anything I type uploaded?
Yes, it is free and needs no sign-up. It runs entirely in your browser, and nothing you type is uploaded.
Can I use the results in commercial work such as quotes and bids?
Yes. Results, steps and printouts can be used in any work, including quotes, bids and invoices. No attribution is needed.
Does the calculator recommend how much water or what rate to use?
No. The weekly target is a number you set. The figures are planning estimates, so check them against your sprinkler maker’s data, a catch-can test, or your local extension service before you set a schedule.
Why does the calculator show a warning?
Warnings flag results that may not work in practice. A precipitation rate above 3 in/h may run off, and a run longer than 60 minutes per day may need splitting into two shorter cycles. The tool also warns when rain already covers the weekly target.
Can it size sprinkler pipe?
No. This tool does not size pipe. For pipe velocity and the smallest inside diameter for a given flow, use the Pipe velocity and size mode of the irrigation calculator.
Related tools
- Irrigation calculator: use it for broader water and irrigation planning beyond a single sprinkler zone.
- Square foot to acre converter: use it to convert a lawn or zone area from square feet into acres.
- Hectare to acre converter: use it to convert a metric area into acres when your plan uses hectares.
- Acreage calculator: use it to work out the total acreage of a site before you size zones.
- Acre per hour calculator: use it to estimate how much area you can cover in an hour.
- Fertilizer calculator: use it to plan fertilizer amounts for a field or large area.
- Lawn fertilizer calculator: use it to plan fertilizer amounts for a lawn.
- Irrigation system design calculator: use it to size system flow, pipe friction and pump pressure.
- All calculators: use it to browse every calculator on the site.