How the calculator works it out
A millimetre of water spread over a square metre is exactly one litre. So once you know how many millimetres an hour a system puts down, the run time and the volume follow directly:
catch-can rate (mm/h) = water in the cans (mm) ÷ test minutes × 60 · drip line rate (mm/h) = dripper L/h ÷ ( dripper spacing (m) × line spacing (m) )
drippers: run time (min) = litres per plant ÷ ( dripper L/h × drippers per plant ) × 60
The drip line formula treats each dripper as watering the rectangle between its neighbours and the next line. Rain Bird's published example checks it: a 2.3 L/h dripper every 46 cm, with lines 46 cm apart, applies 2.3 ÷ (0.46 × 0.46) = 10.9 mm an hour, the figure in its table.
The catch-can test, step by step
Water utilities recommend it because sprinkler rates vary so much with nozzle, spacing and pressure. The method below is the one Power and Water sets out for Darwin gardens, and Water Corporation uses the same idea in Perth.
- Place several straight-sided cans across the area in a grid. Empty tuna or pet-food cans work.
- Run the system for 10 minutes.
- Measure the depth in each can with a ruler, in millimetres, and take the average.
- Divide 100 by that average to get the minutes needed for 10 mm. Or enter the numbers above and the calculator does it.
If some cans hold far more than others, the watering is uneven. Fixing the nozzles or spacing does more good than running the system for longer, which only overwaters the wet spots.
Typical application rates
| System | Rate | Time for 10 mm |
|---|---|---|
| Pop-up spray heads | 25–64 mm/h (38 typical) | 9–24 min (16 typical) |
| Rotors | 6–19 mm/h | 32–100 min |
| Drip line, 2 L/h every 30 cm, lines 30 cm apart | 22 mm/h | 27 min |
| Drip line, 2 L/h every 30 cm, lines 40 cm apart | 17 mm/h | 36 min |
| Drip line, 3 L/h every 30 cm, lines 40 cm apart | 25 mm/h | 24 min |
Spray and rotor figures are Colorado State University Extension's ranges for lawn sprinklers. The drip lines are 13 mm tubes of the kind sold in Australian hardware stores: Pope's Drip Eze has 2 L/h drippers and Holman's Techline 3 L/h drippers every 300 mm. Because sprays and rotors put water down at such different rates, they should never share one valve or station: one part of the garden would flood while the other stayed dry.
How much water to give
Water Corporation's advice for Perth is that a garden only needs 10 mm on each watering day, and Power and Water gives the same figure in Darwin. Watering deeply and less often encourages deeper roots than a daily sprinkle. New turf is the exception: it needs frequent light watering until the roots take hold, which the turf calculator works out.
Watch for run-off. A pop-up spray at 38 mm an hour can put water down faster than clay, compacted soil or a slope can soak it up. If water pools or runs onto the path, split the run time into two or three shorter cycles with a break between them so each one has time to sink in.
For trees and shrubs on drippers, the calculator works in litres per plant instead, because drippers wet a patch around each plant rather than the whole bed.
When you can water
Permanent water-use rules set the hours sprinklers may run, and they differ between utilities:
- Sydney: Sydney Water's Water Wise Guidelines allow sprinklers and watering systems before 10 am and after 4 pm. Drip irrigation and smart watering systems may also run between those hours.
- Victoria: the permanent water-saving rules allow watering systems only between 6 pm and 10 am. A hand-held hose with a trigger nozzle, a watering can or a bucket may be used at any time.
- Elsewhere: Perth runs a sprinkler roster, and other utilities have their own rules. Check your water utility's website before setting a timer.
Watering questions
How long should I run my sprinklers?
Long enough to put down about 10 mm. For typical pop-up sprays at 38 mm an hour that is about 16 minutes; for rotors at 6–19 mm an hour it is 30 minutes to well over an hour. A catch-can test gives your own figure: 100 divided by the millimetres collected in 10 minutes.
How long should drip irrigation run?
Work out the rate first: dripper flow in L/h divided by the area each dripper serves in m². A 2 L/h drip line with drippers every 30 cm and lines 30 cm apart puts down 22 mm an hour, so 10 mm takes about 27 minutes.
How many litres is 10 mm of water?
10 litres for every square metre. A 50 m² lawn needs 500 L for one 10 mm watering, or 1,000 L a week if you water twice.
What is a catch-can test?
A way to measure how fast your sprinklers apply water. Set several straight-sided cans across the area, run the system for 10 minutes and measure the average depth in millimetres. Multiply by 6 for millimetres per hour, or divide 100 by it for the minutes needed to give 10 mm.
Sources and method
- Water Corporation, How efficient is my irrigation system? – 10 mm per watering day and the catch cup test.
- Power and Water, Living Water Smart: catch can test – 10-minute test, 100 ÷ mm collected, 10 mm per watering.
- Colorado State University Extension, Sprinkler types used for lawn irrigation – pop-up spray and rotor application rates.
- Rain Bird, Calculating application rates for XF dripline – worked example used to check the drip line formula.
- Pope, 13 mm Drip Eze tube (2 L/h); Holman, 13 mm Techline PC drip tube (3 L/h every 300 mm); Hills Irrigation, Antelco pressure-compensating button drippers (2, 4 and 8 L/h, colour-coded).
- Sydney Water, Water Wise Guidelines; Victorian Government, Permanent water saving rules.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error