How much water does your lawn actually need? FAO-56 vs the 25 mm-a-week rule, city by city

I grow Sir Walter on heavy clay in Ipswich, and that makes me suspicious of watering advice that arrives as a single number for the whole country. The lawn doesn't read the calendar. A wet Brisbane week has almost nothing in common with a dry Perth one.

So I put the familiar 25 mm a week rule up against a monthly water-budget calculation for each capital. The honest answer is that there isn't one weekly amount for every Australian lawn. Weather demand, useful rain, grass growth and your own irrigation setup all move it around.

There are dollar figures further down, and they need context before the numbers: these are FAO-56 scenario differences priced at FY2026–27 water tariffs; the gap for your household depends on how you water now. Change the baseline and the result changes. So does the way you average the weather.

What the 25 mm advice actually says

The rule is real enough. LawnPride says about 25 mm weekly including rain, during the growing season. Townsville City Council's guide gives 25 mm when dry and carves out the wet season. Isaac Regional Council calls 25 mm a rule of thumb.

These three guides were checked on 2 October 2026. None of that makes "add 25 mm from the tap every week regardless of rain" a fair reading of the advice. I'm using that literal setting as a benchmark, then showing what happens when you shorten the season or credit the rain.

One millimetre over one square metre is one litre. On the 150 m² example lawn used throughout this article:

• 25 mm × 52 weeks = 1300 mm, or 195 kL a year supplied by irrigation.
• 25 mm × 26 weeks = 650 mm, or 97.5 kL a year.

The second is just a half-year benchmark. October–March is the illustrative southern warm-season window here; it is not a growing-season definition for all Australia, and certainly not for Darwin.

How I calculated the FAO-56 estimate

The climate source is Queensland Government Long Paddock, FAO-56 reference ET0 on BoM-derived data. I downloaded SILO Patched Point data for the stations below, taking rainfall and reference evapotranspiration from the same daily series. Missing observations can be patched with interpolated estimates. ET0 is calculated with the FAO-56 method from the same station's daily weather.

SILO also assumes a fixed wind speed of 2 m/s, rather than using each station's observed wind. Its documentation and technical report explain that local wind matters, and that demand on a small irrigated patch surrounded by dry ground can be underestimated.

I used 1991–2020, summed each year's daily values into calendar months, then averaged the thirty January totals, thirty February totals, and so on. These are our calculations from SILO, not published BoM monthly normals. Airports are reference locations for the comparison, not a claim that every suburb shares the airport's weather.

Brisbane and Canberra include substantial patched rainfall. Canberra uses the historical Airport Comparison location throughout, including patches after its observations end.

The monthly arithmetic is:

Net irrigation = max(0, Kc × ET0 − 0.75 × rainfall)
Gross sprinkler supply = net irrigation ÷ 0.75
Annual supply = sum of the twelve monthly gross amounts
kL for this lawn = annual gross mm × 150 ÷ 1000

"Net" is water needed after useful rain; "gross" adds assumed irrigation losses back on top. Both 0.75 factors are scenario assumptions: one for effective rain, one for sprinkler efficiency.

For Kc, the grass coefficient, I used the lawn row in FAO Irrigation and Drainage Paper 56 Rev.1 (2025), Table 6.4: initial 0.50, mid 0.70, end 0.50. FAO gives growth stages, not an Australian calendar. My illustrative warm-season mapping is 0.70 in December–February, 0.50 in June–August, and steps of 0.05 through the intervening months. Darwin stays at 0.70 throughout, with no assumed southern winter slowdown. This mapping is for a warm-season lawn; cool-season lawns run a different calendar.

The coefficient choice matters. The original 1998 FAO-56 table, Table 12, lists warm-season turf at 0.80 initially and 0.85 at mid/end season, and cool-season turf at 0.90 initially and 0.95 at mid/end season. Those 0.80–0.95 values are higher than this illustrative Rev.1 mapping. A higher Kc raises the modelled irrigation demand and lowers the signed difference (fixed supply minus modelled supply). A positive difference can shrink or become negative; an already negative difference can become more negative. The research tables retain a constant Kc 0.70 sensitivity; that is not a calculation of the original table's higher coefficients.

Now the averaging catch, and it matters. Brisbane comes out at 467 mm gross when I calculate each year's monthly balances first and then average the thirty annual results: that is the headline method here. Balancing the monthly climate means instead gives 254 mm, retained only as a sensitivity. Wet years can't cancel dry-year irrigation once each monthly shortfall is floored at zero. These annual figures compare climates; for a timer setting, LawnSuite's watering engine (Pro) runs the FAO-56 balance daily against your zone's root-zone storage once smart watering is set up for a zone with a saved location. Forecast rain can adjust the advice; automatic rain credit into the stored balance is currently off, so check recent rain before setting a timer.

Eight capitals, one explicitly defined comparison

The first table compares the FAO-56 estimate with 25 mm of irrigation every week of the year regardless of rain — 1300 mm over a year. Treat it as the extreme end of the range. The rain-aware warm-season comparison further down is closer to the growing-season, rain-aware advice quoted above, and for Brisbane it comes out at about −6.2 kL, meaning the 25 mm target needed slightly less water than the modelled estimate. Neither is a measured saving.

Here is the mean of thirty separately balanced years against that benchmark. Water costs have been rerun using FY2026–27 residential water-usage prices, excluding fixed charges and sewerage, checked on 2 October 2026. Tier 1 → top means pricing every modelled difference at those two marginal prices; a real bill can cross tiers.

The FAO scenario uses approximately 70.1, 63.9, 75.5, 134.0, 117.2, 79.0, 61.3 and 136.6 kL/year respectively, against the fixed benchmark's 195 kL. Calculations use unrounded inputs; displayed figures are rounded.

An annual difference also hides when water is needed. Perth's January estimate is 184.2 gross mm, against 110.3 mm allocated to January from the fixed annual benchmark. So the benchmark can oversupply the annual total while undersupplying a peak month.

The tariff inputs, in AUD/kL, are:

Sources checked 2 October 2026. Brisbane's bulk charge is confirmed in the utility's worked residential example as well as the retail price table. Sydney's drought-triggered price is not a higher consumption tier and is excluded. Canberra and Hobart prices use the linked regulators' published figures; SA Water's are from its published 2026-27 price PDF. Perth prices can depend on the billing cycle and household tier; these are marginal-price scenarios, not a forecast of your bill.

The result changes when the baseline changes

Against the 26-week fixed benchmark, the annual water differences shrink, and some of them turn negative:

A minus sign means the FAO scenario needs more water than that fixed half-year allowance. It is not a recommendation to cut the lawn off at the smaller number. Darwin in particular shows why a southern warm-month calendar makes a poor national rule.

I also tested the rule with rain credited. Here, 25 mm remains a supplied/catch-depth target, as in Townsville's container test. I subtract the same effective rainfall month by month and stop at zero; I do not increase the remainder by dividing it by sprinkler efficiency. The FAO side still converts net demand to gross sprinkler supply. For a 26-week target spread over October–March, comparing only those same months gives:

The rain-credit factor remains my comparison assumption, not a figure supplied by those watering guides. Counting rain and choosing a realistic season change the comparison substantially. The full research tables also show the annual rain-aware version, a constant Kc 0.70 sensitivity and the lower climate-means balance.

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If you already water to the rain, the gap for you will be smaller; the tables show the difference between a fixed weekly habit and an FAO-56 balance. If you use rainwater or don't irrigate, these tables don't apply to your water bill.

On the seasonal care side, my Sir Walter Buffalo care guide and Kikuyu grass care guide each walk through turning a weather-based estimate into checks on an actual lawn.

What I'd do on an actual lawn

Start by measuring the area. The free lawn-size calculator gives you a place to start, and it matters because water volume scales directly with area.

Then measure the water. A bucket and a stopwatch tell you the flow from a hose; they don't tell you whether a sprinkler waters the whole lawn evenly. For that, put matching straight-sided containers across its coverage, run it for a measured time and compare the collected depths. Isaac Council's guide above describes the container approach. Dry corners and overspray need fixing before a longer run time does.

Watch what happens on your own soil. If water starts running off the clay, continuing the same run just moves it downhill. Stop, let it soak in, and reassess. A rain gauge and a check of moisture below the surface will teach you more than dividing an annual table by the weeks in a year.

Finally, follow local restrictions. Perth's watering-day roster and winter sprinkler switch-off, from 1 June to 31 August each year (checked 2 October 2026), take priority over anything in this calculation. The all-year benchmark is not a lawful Perth sprinkler schedule. Check your own supplier's current rules wherever you live.

I hand-water my lawn with a hose. I built LawnSuite to keep track of my applications, treatments and improvements. The useful question is how much water this lawn needs after the rain it actually received.

For the app side of watering, my feature update covers the watering features in detail.

About Macca: I built LawnSuite, whose watering feature uses an FAO-56-based water balance, and I use it on my own Ipswich lawn. The city figures here are a long-term research calculation; once smart watering is set up for a zone with a saved location, the app runs the same FAO-56 method daily for your own lawn. The daily app balance uses different weather inputs and turf coefficients from the city scenario. I've spent three years learning Sir Walter on heavy clay, including the mistakes, and read APVMA labels to gate the app's product database. For any lawn product you use, the label is the authority.

Data and method: the SILO Patched Point service supplies the daily rainfall and FAO-56 reference ET0 used here. Station IDs, the averaging period and calculation assumptions are given above.