How Many Zones Does My Supply Need

If you are asking how many zones does my supply need, the answer comes from two numbers. Add up the flow of all your heads to get total flow, divide that total by the flow your supply can provide, and round up to the next whole number. That whole number is your zone count.

A zone is a group of heads that runs at the same time from one supply. When total flow is larger than supply flow, you split the heads into zones and run one zone at a time. The sprinkler calculator does this split after you enter lawn size, head throw and flow, and supply flow.

How Many Zones Does My Supply Need

The short answer is the number you get from one division and one rounding step. First work out total flow. Total flow is head count times flow per head. Then divide total flow by supply flow. Then round up. You always round up because a zone cannot run part of a head. If the division gives 3.2, you need 4 zones, not 3.

That rule stays the same for a small lawn and a large lawn. A larger lawn usually needs more heads, more heads mean more total flow, and more total flow means more zones on the same supply. A stronger supply means fewer zones for the same heads.

The Zones Formula in Plain Words

The calculator page gives four formulas, and they work in order. First, heads along each side equals ceil of side divided by radius, plus 1. Second, heads equals heads along length times heads along width. Third, total flow equals heads times flow per head. Fourth, zones equals ceil of total flow divided by supply. The word ceil means round up to the next whole number.

Start with the lawn. Measure length and width in metres. Choose a head type, or enter throw radius and flow from the nozzle chart. Sprinklers work properly only when each head throws water all the way to its neighbours, which is called head to head coverage. That means spacing equals throw radius. From spacing you get head count, and from head count you get total flow. Next, measure supply flow by timing a bucket fill at the outside tap with no other water running.

Zones Table for 20, 30 and 40 L per Minute Supplies

Read this table across a row to find zones for a given total flow. All values here are example inputs worked through the same page formula, apart from the worked example row for 96 L per minute total flow, which is shown in the next section.

Total flow L per minute	Zones on 20 L per minute supply	Zones on 30 L per minute supply	Zones on 40 L per minute supply
20	1	1	1
40	2	2	1
60	3	2	2
80	4	3	2
96	5	4	3
100	5	4	3
120	6	4	3
150	8	5	4
180	9	6	5

A stronger supply always needs the same number of zones or fewer for the same total flow. At 120 L per minute total flow, a 20 L per minute supply needs 6 zones, a 30 L per minute supply needs 4 zones, and a 40 L per minute supply needs 3 zones. Zones also rise in steps. At 100 L per minute on a 30 L per minute supply, the division gives 3.33, so you need 4 zones, even though total flow is lower than 120.

Use this table as a first check. Find the row nearest your total flow, or the row just above it, and read across to your supply.

Worked Example: 96 L per Minute Total on a 30 L per Minute Supply

The lawn is 10 by 6 m, with 4 m spray heads. Heads along the length are ceil of 10 divided by 4, plus 1, which is 4. Heads along the width are ceil of 6 divided by 4, plus 1, which is 3. Heads are 4 times 3, which is 12. The grid is 4 by 3, with 4 quarter heads in the corners, 6 half heads on the edges, and 2 full heads in the middle.

At 8 L per minute each, total flow is 12 times 8, which is 96 L per minute. Supply flow is 30 L per minute. Zones are ceil of 96 divided by 30, which is ceil of 3.2, which is 4 zones. Three zones would only carry 90 L per minute, which is less than the 96 L per minute total. Four zones carry 120 L per minute in total, which is enough.

The imperial version is kept separate, as a rounded equivalent, not an exact conversion. A 33 by 20 ft lawn with 13 ft heads is also 12 heads, with 24 gallons per minute total and 3 zones on an 8 gallon per minute supply. Do not mix metric and imperial figures in one calculation. The calculator gives no cost, pump, pressure or pipe sizing figure.

Total Flow Table: Head Count Times Flow per Head

Total flow is the bridge between head count and zones. Total flow is head count times flow per head. The table below shows that step for example head counts and example flow per head values. All values here are example inputs worked through the same page formula.

Heads	Total flow at 6 L per minute per head	Total flow at 8 L per minute per head	Total flow at 10 L per minute per head
6	36	48	60
8	48	64	80
10	60	80	100
12	72	96	120
16	96	128	160
20	120	160	200

The worked example sits in this table at 12 heads and 8 L per minute per head, giving 96 L per minute total. At 20 heads, moving from 6 to 10 L per minute per head moves total flow from 120 to 200 L per minute. Flow per head comes from the nozzle chart. Do not mix rotors and spray heads on one zone, because they apply water at very different rates.

Heads per Zone Table

Once you know supply flow and flow per head, you can ask how many heads one zone can run. Heads per zone equals available flow divided by flow per head. A 30 L per minute supply runs about 3 to 4 typical spray heads. The table below shows the whole number of heads that fit in one zone. All values here are example inputs.

Supply flow L per minute	Heads per zone at 6 L per minute per head	Heads per zone at 8 L per minute per head	Heads per zone at 10 L per minute per head
20	3	2	2
30	5	3	3
40	6	5	4

Read the middle row to check the page FAQ. At 30 L per minute supply and 8 L per minute per head, 30 divided by 8 is 3.75, so 3 whole heads fit in one zone. If you have 12 heads at 8 L per minute each, each zone can run 3 heads, so you need 4 zones of 3 heads each.

What Changes Your Zone Count

Three things change your zone count. More heads, higher flow per head, and lower supply flow all push zones up. Fewer heads, lower flow per head, and higher supply flow all push zones down. The table below starts from the worked example and changes one input at a time.

Change	Heads	Flow per head L per minute	Total flow L per minute	Supply L per minute	Zones
Worked example	12	8	96	30	4
More heads	16	8	128	30	5
Higher flow per head	12	10	120	30	4
Lower supply	12	8	96	20	5
Fewer heads	8	8	64	30	3
Higher supply	12	8	96	40	3

More heads from 12 to 16 raises total flow from 96 to 128 L per minute, and zones from 4 to 5, because 128 divided by 30 is 4.27, rounded up to 5. Higher flow per head raises total flow to 120 L per minute, but zones stay at 4, because 120 divided by 30 is exactly 4. A rise in total flow only raises zones when it crosses a whole number boundary. Lower supply raises zones to 5, fewer heads lowers zones to 3, and higher supply lowers zones to 3.

Why You Always Round Up

Rounding up is not a safety margin. It is a physical limit. You cannot run 0.2 of a head in one zone and the other 0.8 of the same head in the next zone. When the division gives a fraction, you need one more whole zone to carry the heads that do not fit.

Take the worked example again. Total flow 96, supply 30, division 3.2. Three zones can carry 90 L per minute in total, which leaves 6 L per minute with no zone to run in. That is part of a head that needs 8 L per minute, and it cannot be split across zones. A result of 1.1 needs 2 zones, and a result of 5.01 needs 6 zones. Do not round down, because heads at the far end will throw less far, and head to head coverage will fail.

Common Mistakes With Zones

1. Rounding down instead of up. A result of 3.2 is 4 zones, not 3.
2. Using total flow from the wrong head count. Head count comes from the grid, not from a guess.
3. Using one flow per head for every head when part circle heads use less. Quarter heads and half heads use less flow than full heads. Check matched precipitation nozzles.
4. Mixing rotors and spray heads on one zone. They apply water at very different rates. Keep one head type per zone.
5. Using an unmeasured supply flow. Time a bucket fill at the outside tap with no other water running, and use that measured flow.
6. Treating an irregular lawn as a rectangle. Irregular lawns need hand layout, and a different head count means a different zone count.
7. Forgetting that zones run one at a time. More zones mean a longer total run time. If run time matters, check the irrigation run time calculator and the irrigation scheduling calculator after you have your zone count.

Frequently Asked Questions

Q: How many zones does my supply need for a small lawn?

A: Use the same formula. Work out total flow, divide by supply flow, and round up. A small lawn may need 1 or 2 zones, but the method does not change.

Q: How many heads can one zone run?

A: Divide available flow by flow per head, and use the whole number result. A 30 L per minute supply runs about 3 to 4 typical spray heads. At 8 L per minute per head, 3 whole heads fit in one zone.

Q: Why does my zone count go up when I add only a few heads?

A: Because zones rise in steps. Zones only rise when total flow crosses a whole number multiple of supply flow, so a small rise can make the count jump by one.

Q: Can I run all my heads in one zone if my supply is strong enough?

A: Yes, if total flow divided by supply flow is 1 or less, you need 1 zone. At 20 L per minute total flow, all three example supplies in this blog need 1 zone.

Q: How do I measure my supply flow before I work out zones?

A: Time how long a bucket takes to fill at the outside tap with no other water running, as the calculator page suggests.