Why head-to-head spacing is not wasteful
The instinct is that two sprinklers should be placed so their circles just touch, because anything more looks like watering the same ground twice. That instinct produces a lawn with rings in it, and understanding why is the single most useful thing on this page.
A sprinkler does not lay water down evenly across its circle. Whether it is a fixed spray or a gear rotor, the distribution is heavily weighted towards the head. Close in, the ground is getting the full stream at short range. At the outer edge of the throw, the same volume of water is being spread across a much larger ring of ground, and droplets are landing at the end of their travel with whatever the wind has done to them along the way. Put catch cups in a line out from a single head and the readings fall off steadily from the head to the edge — often to a quarter or less of the near-head figure by the last cup.
So a single pattern is never even. The design fixes that by adding the patterns together. Place head B so its stream reaches head A, and the thin outer part of A's pattern lands on the same ground as the thick inner part of B's, and vice versa. The two triangles add up to something close to flat. That is the entire logic of head-to-head spacing: the overlap is not redundancy, it is the mechanism.
The calculator makes this visible with the "ground under one head only" figure. At head-to-head spacing that number is zero — every square foot is reached by two or more patterns. Stretch a square grid to 145 percent of radius and barely any ground is technically dry, yet half of it is now covered by a single head, which means half the lawn gets roughly half the water the rest gets. That half is where the browning appears, and no run time fixes it because running longer scales the wet and the dry parts equally.
Triangular against square
A square grid places heads on a plain rectangular lattice. A triangular grid offsets every other row by half a spacing, so each head sits between two heads in the row above. For the same throw radius, triangular spacing covers about 15 percent more ground per head, because the worst-covered point sits closer to the surrounding heads.
| Grid | Row spacing | Ground per head | Worst point at 100% spacing | Goes dry beyond |
|---|---|---|---|---|
| Square | Same as head spacing | spacing squared | 0.71 x radius from four heads | 141% of radius |
| Triangular | 0.866 x head spacing | 0.866 x spacing squared | 0.58 x radius from three heads | 173% of radius |
Those "goes dry beyond" figures are the point at which ground receives nothing at all, and they are much further out than anyone should ever go. They are worth knowing only because they explain why a badly stretched grid can look almost acceptable in spring and fail in July: the ground is technically getting water, just not much of it, and the deficit only shows when the plant is under stress.
In practice, triangular suits open lawn where the shape does not fight you. Square suits narrow strips, rectangular beds and anywhere the layout has to follow a hard edge, and it is far easier to set out with a tape. Most real systems end up as a triangular field in the middle with square rows chasing the boundaries.
Perimeter heads and arcs
Heads go on the boundary, not inside it. A head set back from the edge of a lawn cannot water the edge, and the strip beyond it is exactly the strip that dries first because it sits against paving or a fence that radiates heat. So the first and last row sit on the boundary line, throwing half circles inwards, and the corners throw quarter circles.
This is why the head count in the calculator is higher than dividing the area by the ground-per-head figure. A perimeter head serves half the ground an interior head serves, and a corner head a quarter, but it still costs a head, a fitting and its share of the zone flow. On a small or awkwardly shaped lawn the perimeter can be most of the system.
Matched precipitation nozzles matter here. A half-circle nozzle should flow half what a full-circle nozzle of the same radius flows, so that both put down the same depth per hour. If the half circle flows the same as the full circle, the perimeter gets double the depth and the middle of the lawn gets shorted, and the zone can never be scheduled correctly. Manufacturer nozzle sets are usually designed to match; mixed-brand or mixed-vintage nozzles on one zone very often are not.
Wind is a spacing problem, not a run time problem
Wind shortens the throw on the upwind side and stretches it downwind, and it strips fine droplets out of the pattern entirely. The common response is to tighten the grid: something like 95 percent of radius in light wind, 85 percent in moderate wind, and 75 percent or less where the site is genuinely exposed. Those are the figures the calculator offers, and they are trade practice rather than physics, so treat them as a starting point that a catch cup test can confirm or overturn.
The other lever is droplet size. Larger droplets carry through wind better than a fine mist, which is why rotors and rotating-stream nozzles hold up in conditions that shred a fixed spray. And the cheapest lever of all is timing: watering before dawn puts the system into the calmest, coolest part of the day, which cuts both drift and evaporation without spending anything.
Before the trenching starts
Every lateral on this grid is a trench. Have the underground utilities located before any digging begins — the national call-before-you-dig service exists for this, it is free, and it covers the buried services that will ruin your week. Locating is worth doing even on ground you believe you know, because service runs are not always where the paperwork says.
If the system will be fed from a drinking water supply, backflow prevention is a public health matter rather than a detail: an irrigation system sits in soil, fertiliser and whatever else is on the ground, and without protection a pressure drop in the main can pull that back into the drinking water. Which device is required, how it must be installed and how often it must be tested are set by your water authority, and this page gives no guidance on any of it. Ask them before you connect anything.
Once the grid is settled, the flow it demands decides how it splits into zones, which the irrigation zone calculator works out from your measured supply. The depth this spacing actually applies per hour is on the precipitation rate calculator, and the only honest way to check whether the grid worked is the catch cup audit.
Questions people ask
What does head-to-head spacing actually mean?
It means the distance between two adjacent heads equals the throw radius of the nozzle, so the water from each head lands on the next head. It does not mean the circles just touch — that would be double the radius, and it leaves the ground in between badly under-watered. The reason for the overlap is that a sprinkler applies far more water close to itself than at the end of its throw, so the patterns are designed to be stacked. Overlapping them is what turns two uneven cones into an even depth across the ground.
Is triangular spacing worth the trouble over square?
On open lawn, usually yes: for the same nozzle it covers about 15 percent more ground per head, which is 15 percent fewer heads, fittings and gallons per minute. The catch is that setting it out takes more care, every second row is offset by half a spacing, and it fits awkwardly against straight boundaries where you end up filling gaps with extra heads anyway. A sensible compromise on most domestic layouts is a triangular field through the open middle and square rows following the edges.
Can I just space heads further apart and run the zone longer?
No, and this is worth being blunt about. Running longer multiplies every part of the pattern by the same factor, so the ratio between the wettest and driest ground does not change. If a stretched grid gives one patch half the depth of another, doubling the run time gives that patch the depth it needed while giving the rest twice what it needed. You get the same dry spots plus a much larger water bill, and on anything but sand the extra water on the wet areas runs off or drains past the roots.
What throw radius should I put in?
The figure from the manufacturer nozzle chart for the exact nozzle you are installing, read at the pressure that will be present at that head. That last part is where most layouts go wrong. Radius depends strongly on pressure — a rotor that throws 34 feet at 55 psi may only manage 25 at 30 psi — and the pressure at the head is always lower than the pressure at the meter once pipe friction, fittings and any rise up the garden have taken their share. If you are unsure, install a head, run it, and measure where the water actually lands.
Why are my dry spots always in the same places?
Because they are geometry, not weather. Work through it in this order: spacing wider than the throw radius; a nozzle too small for the spacing it was given; pressure at the head below what the nozzle needs, which shortens the throw; a half-circle nozzle flowing the same as the full-circle nozzles around it, which starves the middle; arcs misadjusted so a spot is hit by one head instead of two; or a head sunk below grade and firing into the turf. A catch cup audit will separate a spacing fault from a pressure fault in about twenty minutes.