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Sprinkler Calculator
Use this free sprinkler calculator to plan a sprinkler system for your lawn before you buy heads, pipe, and valves. Enter your lawn area, pick spray or rotor heads, and give your available flow rate and runtime. The calculator works out how many heads you need, how many zones to split them into, and how many gallons of water each session uses.
Measure flow by timing how long it takes to fill a 5-gallon bucket from an outdoor tap: 5 gallons in 25 seconds is 12 GPM.
| Amount | Value |
|---|---|
| Coverage per head | 900 sq ft |
| Sprinkler heads needed | 6 |
| Heads per zone | 4 |
| Zones needed | 2 |
| Flow used per zone | 12 GPM |
| Water per session | 360 gal |
| Water per week | 1,080 gal |
| Total runtime per session | 40 min |
How it works
A sprinkler plan has three numbers to get right: how many heads cover the lawn, how many of those heads can run at once, and how much water the whole system uses. The first number comes from spacing. Sprinkler heads are laid out on a grid so that each head sprays all the way to the next one, which is called head-to-head coverage. A spray head is spaced 15 feet apart and covers about 225 square feet, while a rotor head is spaced 30 feet apart and covers about 900 square feet. Divide the lawn area by the coverage of one head and round up to get the head count.
The second number comes from your water supply. Every head uses a known flow: about 1.5 gallons per minute for a spray head and about 3 gallons per minute for a rotor. Your available flow is the most water your tap and pipe can deliver, and you find it by timing a bucket fill, as the hint above explains. Divide the available flow by the flow of one head and round down: with 12 gallons per minute and rotor heads at 3 gallons per minute, you can run 4 heads on one zone. Then divide the total heads by heads per zone and round up to get the number of zones. Six heads at 4 per zone need 2 zones.
Heads needed = lawn area ÷ coverage per head, rounded up
Heads per zone = available flow ÷ flow per head, rounded down
Zones = heads ÷ heads per zone, rounded up
Water per session = heads × flow per head × runtime
Water per week = water per session × watering days
The third number is water use. Multiply the number of heads by the flow per head and the runtime in minutes to get gallons per session. Six rotor heads at 3 gallons per minute running 20 minutes use 360 gallons. Multiply by watering days per week for the weekly total: 360 gallons times 3 days is 1,080 gallons a week. Zones run one after another, so the total runtime for a session is the runtime per zone times the number of zones: 20 minutes times 2 zones is 40 minutes.
Two rules keep a system working well. First, never mix spray heads and rotor heads on the same zone, because sprays put down water about four times faster than rotors, and a mixed zone would drown one part of the lawn while the other stays dry. Second, treat the head count as a starting point and sketch the layout: corners, curves, narrow strips, and beds all need heads placed by eye, and a lawn shaped like an L needs more heads than a square lawn of the same area.
Runtime is the setting you will adjust most. Aim to put down about half an inch of water per session, which usually takes 15 to 30 minutes for spray heads and 30 to 60 minutes for rotors. Set a few straight-sided cans on the lawn and run a zone: when they hold about half an inch, that is your runtime. Watering deeply two or three times a week grows deeper roots than a short sprinkle every day.
Worked example
You have a 5,000 square foot lawn, rotor heads, 12 gallons per minute of available flow, a 20 minute runtime, and 3 watering days a week.
Step 1: Coverage per head — a rotor spaced 30 feet apart covers 30 × 30 = 900 sq ft.
Step 2: Heads needed — 5,000 ÷ 900 = 5.6, rounded up to 6 heads.
Step 3: Heads per zone — 12 ÷ 3 = 4 heads per zone.
Step 4: Zones — 6 ÷ 4 = 1.5, rounded up to 2 zones.
Step 5: Water per session — 6 × 3 × 20 = 360 gallons.
Step 6: Water per week — 360 × 3 = 1,080 gallons, with a total session runtime of 20 × 2 = 40 minutes.
The result is 6 rotor heads in 2 zones, using 360 gallons per watering session and 1,080 gallons per week.
FAQ
How many sprinkler heads do I need for my lawn?
Divide your lawn area by the coverage of one head. A spray head spaced 15 feet apart covers about 225 square feet, so a 5,000 square foot lawn needs about 23 spray heads. A rotor head spaced 30 feet apart covers about 900 square feet, so the same lawn needs about 6 rotor heads. This calculator does the division for you, but always sketch the layout on paper too, because corners, curves, and narrow strips need extra heads that a simple area calculation misses.
Should I use spray heads or rotor heads?
Use spray heads for small lawns and narrow areas under about 15 feet wide: they pop up, throw water 10 to 15 feet, and water evenly over short distances. Use rotor heads for large open lawns: they throw water 25 to 40 feet, use less water per square foot, and need far fewer heads. A lawn with both a small front strip and a big back yard often needs spray heads in front and rotors in back, on separate zones.
How many sprinkler zones do I need?
A zone is a group of heads that run together, and its size is limited by your water flow. Divide your available flow in gallons per minute by the flow of one head: with 12 gallons per minute and rotor heads using 3 gallons per minute each, you can run 4 heads per zone. Divide your total heads by heads per zone and round up. Six heads at 4 per zone need 2 zones. Never mix spray heads and rotor heads on one zone, because they apply water at very different rates.
How long should I run each sprinkler zone?
Run each zone long enough to put down about half an inch of water, which usually takes 15 to 30 minutes for spray heads and 30 to 60 minutes for rotors. The exact time depends on your pressure and nozzle size, so check with a few tuna cans set on the lawn: when they hold about half an inch of water, that is your runtime. Watering deeply two or three times a week grows deeper roots than a short daily watering.
How much water does a sprinkler system use?
Multiply the number of heads by the flow per head and the runtime in minutes. Six rotor heads at 3 gallons per minute running 20 minutes use 360 gallons per session, or 1,080 gallons a week at three sessions. Spray systems use a similar total for the same lawn, because they have more heads at a lower flow each. A rain sensor or smart controller that skips watering after rain is the easiest way to cut that number without hurting the lawn.