How much water does your lawn actually need this week?
“An inch a week” is an average. Your lawn never gets an average week. Enter a ZIP and see how this week’s drying compares with what is normal there for the same seven days of the year.
Why a fixed schedule drifts
Water leaves a lawn through evaporation from the soil and transpiration from the blades. The combined rate is measured as reference evapotranspiration, or ET0, and it is not just a temperature reading. It combines heat, wind, humidity, and sunlight.
That is why two 95 degree days can be completely different days for your grass. A still, muggy 95 pulls far less water than a 95 with 25 mph wind and dry air. A timer set once in spring cannot see any of it.
This tool compares the last seven completed days against the same seven calendar days averaged over the past five years at your location. That reference does not drift, so a long heat wave stays visible as a heat wave instead of quietly becoming the new baseline.
Why every source gives you a different number
Search how much to water a lawn and you will get one inch a week, an inch and a half, half an inch twice a week, and a rule about wetting the soil six inches deep. It reads like the experts disagree. They mostly do not. They are publishing three different quantities, and a fixed inches-per-week figure is only ever correct for the climate it was measured in.
The three quantities
Step 1
Water lost from a reference lawn (ET0)
How fast water leaves a well watered patch of grass, from heat, wind, humidity, and sunlight combined.
Never a controller setting on its own. It is the raw weather measurement everything else is built from.
Step 2
What your grass uses
ET0 multiplied by a crop coefficient for your grass type. Warm-season turf uses about 60% of ET0, cool-season about 80%.
Still not a controller setting. This is what the plant transpires, assuming every drop reaches it.
Step 3
What you apply
What the grass uses, divided by how efficient your sprinklers are. Always the largest of the three.
This is the controller setting. It is larger than plant need because no sprinkler is perfectly even.
Mixing these up is the single most common way a lawn gets underwatered on a schedule that looks correct. Setting a controller to the plant-need figure runs a deficit of roughly a third on a good system, and more than half on a poor one.
The same advice, read properly
| Source | What it says | What it is actually measuring |
|---|---|---|
| Colorado State Extension | 1.5 inches a week in July and August | Applied water including rainfall, for Colorado's climate and its predominantly cool-season lawns. |
| Clemson Extension | About 1 inch on heavy soil, half an inch on sandy soil | Depth per application, chosen to wet the soil 4 to 6 inches deep. Not a weekly total. |
| UC ANR | 60% of ET0 for warm-season turf, 80% for cool-season | What the plant uses, as a fraction of the local weather measurement. Scales to any climate. |
None of them is wrong. They answer different questions. This page starts from the weather at your ZIP instead of a national average, which is why its answer moves week to week when theirs cannot.
Change minutes, not days
When a week runs hot, make each watering longer rather than adding another watering day. Clemson Extension: “Watering every day with a small amount will encourage a shallow root system, making the grass less drought tolerant.” Deep and infrequent is what drives roots down, and roots are what carry a lawn through the next hot stretch.
If water runs off before it soaks in, split a single watering into shorter bursts with soak time between them rather than watering on more days.
Move in 10% steps
If you already have a weekly number that has been working, do not throw it out for this one. Move it 10% at a time and watch the lawn. Colorado State describes exactly this control, the Percent Key found on most timers, adjusted “in 10% increments.” Stepping means one odd week or one bad reading costs you 10%, not everything.
Where these numbers come from
- Reference evapotranspiration: Open-Meteo, computed with the FAO-56 Penman-Monteith equations for a well watered grass surface.
- Crop coefficients: UC ANR Center for Landscape & Urban Horticulture turfgrass Kc tables, annual averages for good quality turf.
- Sprinkler efficiency and the 10% stepping guidance: Colorado State Extension.
- Watering frequency and rooting depth: Clemson Extension Home & Garden Information Center.
Estimates from tables get you close. A catch cup test measures what your own heads actually deliver, and beats every assumption on this page.
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