Sprinkler System Calculator
Design a lawn sprinkler system from your yard's shape. Draw the lawn, pick spray or rotor heads, and the calculator lays out the heads for full coverage, splits them into zones your water can actually run, sizes the underground pipe, and prices the parts.
How the sprinkler calculator works
A lawn sprinkler system has to satisfy two constraints at once: every square foot needs water (coverage) and the whole thing can't draw more water at once than your house delivers (flow). This calculator handles both. In the free Simple mode you drop your own heads on a photo of your yard and see the spray footprints and overlap — a fast way to check an existing system for dry spots. In Advanced mode you trace the lawn and the tool auto-places heads for head-to-head coverage, then does the design work.
Head-to-head coverage means every head's spray reaches the next head, so the overlap evens out the water. The calculator spaces heads as a percentage of their throw radius (tighter in wind), puts quarter-circle heads in corners, half-circle heads along edges, and full-circle heads in the interior — the standard matched-precipitation layout used by Rain Bird and Hunter design guides.
Because a home water service only delivers about 8–16 gallons per minute, the heads are grouped into zones (stations) that run one at a time. The calculator sizes those zones to your available water, tells you how many controller stations you need and how long each runs, and — the honest part — warns you when a lawn is simply too big for spray heads and rotors are the right tool. Switch the head type to rotor and a lawn that needed dozens of stations often collapses to a handful.
Finally, place your water source on the map and the calculator sizes the underground pipe (keeping flow velocity under 5 ft/s), checks that the farthest head still gets enough pressure — accounting for friction loss and any slope in the yard — and turns the whole design into a rough materials list with a cost estimate.
How to use this calculator
- Optional: upload an aerial photo of your yard (a screenshot from a maps program works) and set the scale by clicking two points a known distance apart. The image stays in your browser.
- Switch to Advanced and trace each lawn area as a closed shape. You can draw several separate patches.
- Pick a head type — spray for small lawns, rotor for big ones — then a nozzle size and spacing pattern. The heads are placed automatically for head-to-head coverage.
- Enter your available water in gallons per minute (a bucket test gives the real number: gpm = 300 ÷ seconds to fill a 5-gallon bucket). The calculator splits the heads into zones.
- Click the water source on the map and set your static supply pressure and backflow type. The calculator sizes each zone's pipe and confirms the farthest head has enough pressure.
- On a sloped yard, set the grade by marking the high and low points and entering the drop, so the pressure budget accounts for elevation.
- Read the results panel top to bottom: lawn area, head count and flow, zones and run times, pipe sizing, and a rough materials cost estimate.
Tips for a system that actually works
Measure your real GPM before anything else
The single number that decides your zone count is available flow. Guessing high gives you too few zones and weak, misting heads. Time how long a 5-gallon bucket takes to fill from an outside spigot — gpm = 300 ÷ seconds — and enter that.
Rotors for big lawns, spray for small
Spray heads throw 8–15 ft and suit tight or oddly shaped areas. Rotors throw 30–40+ ft at a much lower precipitation rate, so a large open lawn needs far fewer of them and far fewer zones. If the calculator flags your lawn as impractical for spray, switch the head type to rotor.
Match precipitation within a zone
Never mix spray and rotor heads on the same zone — sprays put down water 3–4× faster, so one group floods while the other stays dry. Keep each zone all-spray or all-rotor.
Account for slope
Every foot of elevation costs about 0.43 psi. Uphill heads can starve while downhill heads mist and drift. Set the grade in the calculator, and on steep zones consider check-valve heads so the low heads don't drain and puddle after each cycle.
Frequently asked questions
How many sprinkler zones do I need?
It depends on your available water, not your lawn size directly. Total the flow of all your heads and divide by about 75% of your available GPM (leaving headroom for friction and house demand). A typical home at 10–12 GPM runs 3–8 zones. The calculator computes this for you once you enter your flow.
How far apart should sprinkler heads be?
Head-to-head — each head's spray should reach the adjacent heads. In practice that's roughly 50–60% of the throw radius as spacing, tightened in windy areas. A 15 ft spray head spaces about 8–9 ft apart; a 30 ft rotor about 15–18 ft. The calculator places heads at the right spacing automatically.
What size pipe do I need for my sprinklers?
Size each pipe run so water velocity stays under about 5 ft/s to avoid water hammer and excess friction. For typical zone flows that's ¾" to 1" PVC on laterals and up to 1¼"–2" on the mainline. The calculator sizes every run from your zone flows and reports the total footage by size.
What water pressure do sprinklers need?
Spray heads are designed for about 30 psi at the nozzle; rotors want 40–45 psi. You need enough static supply pressure to cover that plus losses from the meter, backflow preventer, valves, pipe friction, and elevation. The calculator runs the full pressure budget out to the farthest head and tells you if it comes up short.
Do I need a backflow preventer?
Yes — code requires one to keep irrigation water from siphoning back into your drinking supply. A pressure vacuum breaker (PVB) or double-check valve costs a few psi; a reduced-pressure (RPZ) device is required in higher-hazard situations and costs about 12 psi. Pick the type in the calculator and it adjusts the pressure budget and the parts list.
Is this an exact engineering plan?
No — it's a planning-grade estimate to size a system and budget the materials, using standard irrigation formulas (head-to-head spacing, Hazen-Williams friction, matched precipitation). Confirm local code, backflow requirements, and final part counts before you buy or dig.