Sprinkler System Calculator: Heads, Zones and Run Time

Enter the size of the lawn, how far a head throws, the flow of one head and the flow your water supply gives. You get the heads in a head-to-head grid, the total flow, the zones, how fast water falls and the minutes to put on an inch a week.

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Example job shown. Change the numbers to match yours.

Sprinkler heads

20 heads

5 along the length and 4 across, 15.0 ft apart

Heads counted as full circlesThe four corners are quarter circles and the edges are half circles: 12.0 heads
Total flow with all heads onEach head uses about its share of a full circle: 24.0 gpm
Zones for your supplyEach zone must stay under 12.0 gpm. Each zone needs a valve.: 2 zones
Flow in each zoneIf the zones are an equal size: 12.0 gpm

Watering

Precipitation rate96.25 × gpm ÷ the area one head covers in sq ft: 0.86 in/hr
Minutes a week for the water wantedFor each zone, in as many cycles as the soil can take without runoff: 70 min

The heads are in a square grid with head-to-head coverage, which means the spacing is the radius of the throw. Each head covers a square of side equal to the radius, so the rate is 96.25 × gpm divided by that square in sq ft, where 96.25 is 60 minutes times 231 cubic inches a gallon divided by 144 square inches. A real layout has to avoid the house and the paths, uses full, half and quarter circle heads to suit, and may use a triangle layout. Pressure and flow decide the throw and the gpm: take them from the maker's chart for your nozzle, and measure your supply. Lower-flow zones work better, and slopes and clay need short cycles.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

Worked example: a lawn 60 × 40 ft with heads that throw 15 ft needs 5 heads along the length and 4 across, which is 20 heads. The four corners are quarter circles and the edge heads are half circles, so they count as 12.0 full circles, which at 2 gpm is 24.0 gpm. With a supply of 12 gpm that is 2 zones. A head of 2 gpm over a 15 ft square puts down 96.25 × 2 ÷ 225 = 0.86 in/hr, so an inch a week takes 70 minutes. A supply of 24 gpm would need 1 zone. Rotors with a 30 ft throw at 5 gpm give 9 heads and a rate of 0.53 in/hr. A lawn of 30 × 20 ft takes 9 heads.

Heads and zones for common lawns (15 ft throw, 2 gpm head, 12 gpm supply)

The grid of heads, the flow and the zones.

Lawn	Heads	Flow with all on	Zones
30 × 30 ft	9	8.0 gpm	1
40 × 30 ft	12	12.0 gpm	1
50 × 40 ft	20	24.0 gpm	2
60 × 40 ft	20	24.0 gpm	2
80 × 50 ft	35	48.0 gpm	4
100 × 60 ft	40	56.0 gpm	5

Tips before you buy

Measure your flow with a bucket and a timer, and your pressure with a gauge on a hose bib, before you choose heads.
Do not mix spray heads and rotors in one zone: they put the water down at very different rates.
Match the precipitation rate in each zone, and use half and quarter heads at the edges.
Water in several short cycles on a slope or clay soil, so that it soaks in instead of running off.
Run the system in the early morning, and adjust for rain and the season.

Tools & materials for this project

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Rotor sprinkler head Spray sprinkler head Zone valve Sprinkler controller Pressure gauge for hose bib

Frequently asked questions

How many sprinkler heads do I need?

In a head-to-head layout the heads are one throw apart: for a 60 × 40 ft lawn and a 15 ft throw, 20 heads.

How many zones do I need?

Add up the flow of the heads and divide by the flow of your supply, rounded up: 24.0 gpm with a 12 gpm supply is 2 zones.

How do I find the flow of my water supply?

Time how long it takes to fill a 5 gallon bucket from the hose bib, and divide the gallons by the minutes. Use a figure a little lower for safety.

How long should I run each zone?

The inches wanted divided by the precipitation rate, times 60: 70 minutes a week for 1 inch at 0.86 in/hr, split into several cycles.

What is head-to-head coverage?

Each head throws water as far as the next head, so the spray from every head reaches the heads around it. It gives an even cover.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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