Sprinkler Coverage Calculator
Space heads at the radius, not the diameter.
Work out Sprinkler Coverage. Space heads at the radius, not the diameter. States the assumption instead of hiding it.
To cover 200 m²
8 heads
26 min per session, 2 times a week
Heads are spaced at about their throw radius rather than their diameter, so the patterns overlap. A sprinkler delivers far less water at the edge of its circle than at the centre, and spacing at the full diameter leaves dry rings that only become visible weeks later. The supply cannot run all 8 heads at once, so this needs 3 zones run in sequence — and the total watering time multiplies accordingly. Fewer, longer sessions encourage deeper rooting than daily light watering.
How the Sprinkler Coverage Calculator works
How many sprinkler heads a lawn needs, the precipitation rate they deliver, and how long to run them. Heads go at about their throw radius apart so the patterns overlap — spacing at the diameter leaves dry rings.
Also known as: how many sprinkler heads do i need · sprinkler spacing calculator · how long to run sprinklers · lawn irrigation run time
Head-to-head spacing, and the dry rings
A sprinkler does not apply water evenly across its circle — it delivers far less at the edge than near the centre. Spacing heads at their full throw diameter therefore leaves a ring of under-watered grass between them.
The convention is head-to-head spacing: heads set about one throw radius apart, so each pattern reaches the next head and the overlapping edges add up to an even application. It uses more heads and it is the only arrangement that waters uniformly.
The failure is slow to appear. Dry rings show up weeks later as patchy colour, by which point the cause is not obvious, and the usual response — running the system longer — over-waters everywhere else to compensate.
Precipitation rate and run time
Precipitation rate is how fast the system applies water, in millimetres per hour, calculated from flow per head over the area each head covers. It is what converts a watering target into a run time, and it varies enormously between head types.
Which is why rotors and fixed sprays must never share a zone. Rotary heads apply water several times more slowly than fixed sprays, so a mixed zone over-waters the spray area and under-waters the rotor area no matter how long it runs.
A catch-cup test measures the real rate and the real uniformity: identical containers on a grid, a normal cycle, then measure each. It reveals both the actual precipitation rate and the dry spots a spacing calculation assumes away.
When and how often
Deeper, less frequent watering drives roots down and produces a lawn that survives dry spells. Daily light watering keeps roots shallow, where the soil dries first, which makes the lawn more dependent on the irrigation rather than less.
Early morning is the best time. Evening watering leaves foliage wet overnight and encourages fungal disease; midday loses a large share to evaporation before it reaches the root zone.
On clay soils and slopes, the precipitation rate frequently exceeds what the soil can absorb, and the excess runs off. Cycle-and-soak — splitting a run into shorter cycles with gaps — delivers the same total while letting each portion infiltrate.
Where to go next
The Sprinkler Coverage question rarely arrives on its own. These are the ones that usually come with it:
- Plant Watering Calculator — Water thoroughly and less often, not lightly and daily.
- Rain Barrel Calculator — One millimetre on one square metre is one litre.
- Pump Head Calculator — Choosing on static lift alone is why pumps stall.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How far apart should sprinkler heads be?
About their throw radius, so adjacent patterns overlap. A sprinkler delivers far less water at the edge of its circle than at the centre, and head-to-head spacing is what evens it out.
What is precipitation rate?
How fast a sprinkler applies water, in millimetres per hour, calculated from flow rate over the area each head covers. It is what converts a watering target into a run time.
How long should I run sprinklers?
Long enough to deliver the weekly requirement across your chosen number of sessions. Fewer, longer sessions encourage deeper rooting than daily light watering.
How much water does a lawn need?
Commonly cited around 25 mm a week including rainfall, varying with grass type, soil and climate. A rain gauge tells you how much of it nature already supplied.
Why do I need multiple zones?
Because the supply cannot run every head at once. Total demand divided by available flow gives the number of zones, and each runs in sequence — which multiplies the total watering time.
When is the best time to water?
Early morning. Evening watering leaves foliage wet overnight and encourages fungal disease; midday loses a large share to evaporation before it reaches the roots.
What is distribution uniformity?
A measure of how evenly a system applies water across the area, found by catching water in cups on a grid. Poor uniformity means over-watering everywhere to satisfy the driest spot.
How do I do a catch-cup test?
Place identical containers across the area, run a normal cycle, and measure each. It reveals both the real precipitation rate and the dry spots a spacing calculation assumes away.
Should I water every day?
Rarely. Deeper, less frequent watering drives roots down and produces a more drought-resistant lawn. Daily light watering keeps roots shallow and vulnerable.
What is cycle and soak?
Splitting a run into shorter cycles with gaps, so water infiltrates rather than running off. It is essential on clay soils and slopes where the precipitation rate exceeds infiltration.
Do rotors and sprays mix on one zone?
They should not. Rotary heads apply water far more slowly than fixed sprays, so mixing them on one zone over-waters part of the area and under-waters the rest.
How much does a rain sensor save?
Meaningful amounts in a wet climate, because a timer with no sensor irrigates during rainfall. It is one of the cheapest efficiency additions to an existing system.
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