How to use the irrigation zone flow calculator
- Enter the flow your water supply can give in gallons per minute. Measure it with the Flow Rate Calculator if you do not know it.
- Set a safety margin. The 10% shown is an example that keeps each zone a little under the supply.
- List each type of head with how many you have and the flow of one head from its data sheet.
- Choose whether to keep each head type in its own zones, then read the zones and the heads in each.
Formula
flow allowed in a zone = supply flow x (1 - margin)
total head flow = sum of (number of heads x flow of one head)
fewest possible zones = total head flow / flow allowed, rounded up
heads per zone for one type = flow allowed / flow of one head, rounded down
When head types are mixed, heads are placed largest first in the first zone with room. That plan is valid but not always the smallest possible.
A yard with 26 heads and 12 gallons a minute
With a 10% margin each zone may use 10.8 gallons a minute. The yard has 16 spray heads at 1.5, 6 rotors at 3 and 4 drip lines at 1, for 24 + 18 + 4 = 46 gallons a minute. No plan can use fewer than 46 / 10.8 = 4.26, so 5 zones.
Keeping each type apart, sprays fit 7 per zone, so 16 sprays need 3 zones of 6, 5 and 5 heads. Rotors fit 3 per zone, so 2 zones of 3. The 4 drip lines fit in one zone. That is 6 zones. If you let the types mix, the heads pack into 5 zones of 10.5, 10.5, 10.5, 10.5 and 4 gallons a minute.
Why heads are kept together
Spray heads, rotors and drip lines put down water at very different rates. A zone that mixes them waters some parts of the lawn much more than others. Keeping each type in its own zones is the safer design, even if it uses a zone more than a mixed plan would.
You can turn the option off to see how much a mixed plan saves, then decide whether the saving is worth the uneven watering.
Flow is not the only limit
A zone also needs enough pressure at the farthest head, and long pipes and small valves lose pressure. The flow you enter should be what your supply gives at working pressure, and head flows should come from the maker data at that pressure. Check pipe size and valve ratings before you lay out the system.
Related garden and water tools
Set the run time with the Sprinkler Run Time Calculator, plan feeding with the Lawn Fertilizer Calculator, and see how much water pipe holds with the Pipe Volume Calculator.
Assumptions and limits
- Every head of a type has the same flow, taken from its data sheet.
- The supply flow is steady and is not shared with other use during watering.
- The margin is an example, and you can set it to 0.
- Pressure loss, pipe sizing and the layout of the heads in the yard are not modeled.
Frequently asked questions
How many sprinkler heads can I put on one zone?
Divide the flow you can use in a zone by the flow of one head and round down. At 10.8 gallons a minute and 1.5 per spray head, a zone holds 7 spray heads.
How do I find out how much flow my water supply has?
Time how long a bucket takes to fill at the tap or hose bib the system will use, then divide the volume by the time. The Flow Rate Calculator on this site does the arithmetic.
Why does the plan have more zones than the minimum?
The minimum assumes heads can be split in any way. Keeping each head type in its own zone, and whole heads only, can need one or more extra zones.
What safety margin should I use?
The page uses 10% as an example. A larger margin gives more room when pressure drops, and 0 uses the full supply. Choose the one that suits your supply and equipment.