What is a lawn water requirement?
A lawn loses water in two ways: it evaporates from the soil, and the grass releases it through its leaves. Together this is evapotranspiration. Measured as a depth over the lawn, like rainfall, it works for any lawn size. Extension guidance such as Rutgers FS555 also states lawn needs as a depth per week.
Instead of assuming a fixed weekly target, this calculator starts from reference evapotranspiration (ETo): the water use of a standard reference grass under local weather, defined in FAO Irrigation and Drainage Paper 56. Your actual turf uses a fraction of that, set by its crop coefficient (Kc). Multiply the two and you have turf water use. Subtract rain and you have the net amount irrigation must replace.
Which Kc should I use for turf?
The presets use the species factors for turf in the landscape coefficient method described in A Guide to Estimating Irrigation Water Needs of Landscape Plantings in California. A local value from your extension office or water utility is better than a generic one.
| Grass type | Kc preset | Examples |
|---|---|---|
| Cool-season | 0.80 | Kentucky bluegrass, tall fescue, ryegrass |
| Warm-season | 0.60 | Bermudagrass, zoysia, St. Augustinegrass |
| Your own value | You enter it | From a local source or a Kc table |
See the grass type water needs calculator, the lawn watering by grass type chart and the crop coefficient table for more on species differences.
Check local rules first. Many water utilities set watering days, hours or seasonal limits. Where they apply, they come before any calculated amount.
How does this lawn water requirement calculator work?
Four steps turn weather data into a volume. Everything except the Kc preset comes from your own inputs.
1. Put ETo on the same period as the result
Period conversion
Weekly ETo = Daily ETo × 7
Daily ETo = Weekly ETo ÷ 7
No conversion is made when your ETo already matches the result period.
2. Turf water use and net depth
Depth
Turf water use (ETc) = ETo × Kc
Net depth = ETc − Rainfall
If rain meets or exceeds ETc, the net depth is zero. It is never negative.
3. Convert depth to volume
US units
Gallons = Area (sq ft) × Depth (in) × 0.623
One inch over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches, so 144 ÷ 231 ≈ 0.623.
Metric units
Liters = Area (m²) × Depth (mm)
One millimeter over one square meter is exactly one liter.
Depths show 2 decimals in US units and 1 in metric, the same precision used in the math. Volumes are rounded to a sensible level, such as the nearest 10 gallons, because the inputs are estimates.
When this calculator isn't the right tool
- A simple weekly target with a run time: use the lawn watering calculator.
- Minutes per zone: use the how long to water lawn calculator, which needs your sprinkler rate.
- Crops, orchards and vineyards: use the crop evapotranspiration calculator.
- Finding ETo itself: see the reference ET calculator.
This page belongs to the lawn calculators. It is designed for established turf. New seed and sod are watered on a different pattern.
Lawn water requirement example
A 5,000 sq ft cool-season lawn (Kc 0.80). The local ETo is 0.25 in per day, and a rain gauge caught 0.25 in this week.
- ETo for a week: 0.25 × 7 = 1.75 in.
- Turf water use: 1.75 × 0.80 = 1.40 in.
- Net depth: 1.40 − 0.25 = 1.15 in.
- Volume: 5,000 × 1.15 × 0.623 ≈ 3,584 gal, shown as about 3,580 gallons.
The same calculation in metric
A 450 m² warm-season lawn (Kc 0.60) with a weekly ETo of 35 mm and 6 mm of rain:
- Turf water use: 35 × 0.60 = 21.0 mm.
- Net depth: 21.0 − 6 = 15.0 mm.
- Volume: 450 × 15.0 = 6,750 L, shown as about 6,750 liters.
How do I use the result?
Treat the net depth as the amount your lawn used and irrigation must replace, then plan the delivery separately.
- Match periods. The most common error is entering a daily ETo but treating the answer as weekly. Set the ETo selector and the result period before reading the number.
- Turn depth into minutes. Divide the depth you plan to apply per session by your sprinkler rate. Use the how long to water lawn calculator, or measure the rate with the catch-can test calculator.
- Allow for uneven coverage. Sprinklers do not apply water perfectly evenly. See the distribution uniformity calculator and scheduling coefficient calculator.
- Compare with what you applied. The lawn water deficit calculator works from the gap between use and supply.
- Read rain carefully. Use your own gauge. The effective rainfall calculator explains why measured rain overstates what the roots receive.
What changes the result?
| Factor | Effect on result | What to check |
|---|---|---|
| ETo | Result rises and falls in direct proportion to ETo. | Use a nearby station and confirm the value is per day or per week. |
| Kc (grass type) | Result rises and falls in direct proportion to Kc. | Prefer a local value. The presets are typical, not measured. |
| Rainfall | Subtracted in full from turf water use. | Use a yard gauge. Enter the total for the same period as the result. |
| Lawn area | Volume rises in direct proportion to area. | Measure only the grass. Leave out beds and paving. |
| Result period | A weekly result covers seven days of ETo. | Set the period and the ETo basis so they match your data. |
Soil, shade, mowing height and irrigation efficiency also affect real-world water use, but this calculator does not model them.
Frequently asked questions
What is the water requirement of a lawn?
A lawn's water requirement is the depth of water its grass uses through evapotranspiration, which is evaporation from the soil plus water released by the leaves. It is estimated by multiplying reference evapotranspiration (ETo) by a turf crop coefficient (Kc). Rain that reaches the lawn reduces how much irrigation has to supply.
How do I calculate turf water use from ETo?
Multiply the ETo for your period by the turf Kc. For example, 1.75 in of ETo in a week times a Kc of 0.80 gives 1.40 in of turf water use. Subtract the rain you measured to get the net depth, then multiply by the lawn area to get gallons or liters.
Where do I find ETo for my lawn?
Many weather networks, extension services and water utilities publish daily or weekly ETo for a nearby station. Use a station close to you and note whether the value is per day or per week. This calculator lets you choose either and converts it for you.
Which crop coefficient should I use for turf?
The presets use 0.80 for cool-season grass and 0.60 for warm-season grass, the species factors used in the landscape coefficient method. Local extension offices or your water utility may publish values tuned to your climate and grass. If they do, choose the custom option and enter that value instead.
Does this calculator tell me how long to run my sprinklers?
No. It gives the net depth and volume of water the turf uses. Turning that depth into minutes needs your sprinkler application rate, which is what the how long to water lawn calculator does. Sprinkler uniformity also means you must apply more than the net amount.
Should I add extra water for sprinkler inefficiency?
The result here is the water the turf uses after rain, not the amount to send through the sprinklers. Uneven coverage, wind and runoff mean the applied amount has to be larger. The distribution uniformity calculator and the scheduling coefficient calculator cover that adjustment.
Is all measured rain counted?
Yes. Every bit of rain you enter is subtracted in full. Light showers may be intercepted by the grass and evaporate, and heavy rain can run off, so real benefit can be smaller. The effective rainfall calculator looks at that in more detail.
Related tools: how long to water lawn calculator, distribution uniformity calculator, effective rainfall calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- ETo × Kc, the period conversion and both worked examples verified by hand.
- Method checked against FAO-56 and the landscape coefficient method.
- Results checked to match in US and metric units.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56.
- University of California Cooperative Extension and California Department of Water Resources. A Guide to Estimating Irrigation Water Needs of Landscape Plantings in California: The Landscape Coefficient Method and WUCOLS III. [VERIFY URL]
- Rutgers Cooperative Extension. Best Management Practices for Watering Lawns (FS555).