Which projects does the state water budget apply to?

The state Model Water Efficient Landscape Ordinance (MWELO) applies to new construction with a landscape area of 500 square feet or more that needs a building or landscape permit, plan check or design review, and to rehabilitated landscapes of 2,500 square feet or more that need the same. Projects of 2,500 square feet or less can instead follow the prescriptive checklist in section 492. Cities can adopt stricter local versions, so the permit desk has the final word.

What formula does this calculator use?

The one in the ordinance: each zone contributes its plant rating divided by irrigation efficiency, times area, times annual reference evapotranspiration, times 0.62, in gallons per year. The budget cap multiplies reference evapotranspiration by 0.62 and by 0.55 of the regular landscape area for homes (0.45 for non-residential), plus areas permanently and solely dedicated to edible plants counted in full (recycled-water areas and park-type recreational areas count the same way, but private single-family residential landscapes are never recreational areas under the definition). The comparison is estimated use against the cap.

Where do the plant factor numbers come from?

From WUCOLS, the state plant water use classification: very low is under 0.10, low is 0.10 to 0.30, moderate is 0.40 to 0.60 and high is 0.70 to 0.90. This calculator uses the midpoint of each range. For a permit submission, look up your exact plant list in WUCOLS and use zone-specific factors.

Why are irrigation efficiency values fixed at 0.75 and 0.81?

Because the ordinance sets them, not the equipment vendor: 0.75 for spray heads and rotors, 0.81 for drip. Using drip raises the efficiency divisor, which lowers the computed water use for the same plants and area. That is why switching zones to drip often brings a project under the cap.

Is this enough for a permit application?

It gives the same arithmetic reviewers check first, so it is the right place to test a design before drawing anything. A full submission also needs the worksheet in the required format, plant lists, an irrigation design and other documents your city lists. Treat this page as the fast check, not the paperwork.

Why does my total change so much when I change the plant selection?

Plant choice is the strongest lever in the formula. Moving a zone from high water (0.8) to low water (0.2) cuts that zone to a quarter of its use before any irrigation change. Area is the second lever, irrigation type the third.

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Landscape water budget calculator

Estimate a California landscape project’s yearly water use and compare it against the state budget cap before anything gets drawn. Pick your city, describe the watering zones, and every number prints with the formula that produced it. Verified August 26, 2026 against the adopted 2025 state ordinance text.

City (sets reference evapotranspiration)

Project type

Edible garden area, sq ftCounted at 1.0, the full rate the state allows for food crops. Keep this area out of the zone list below: it is counted once, here.

Watering zones (group plantings that share a valve and water level)

Zone 1 area, sq ft

PlantsIrrigation

Zone 2 area, sq ft

PlantsIrrigation

Over the budget cap by 9,996 gallons per year. Shifting area to lower water plants or drip brings the total down.

Estimated total water use: 32,584 gallons per year
Budget cap for this project: 22,588 gallons per year

How that was worked out

Zone 1: 0.8 / 0.75 × 800 sq ft × 55.2 in × 0.62 = 29,204 gal/yr

Zone 2: 0.2 / 0.81 × 400 sq ft × 55.2 in × 0.62 = 3,380 gal/yr

The formula, spelled out

zone use = (plant rating ÷ irrigation efficiency) × area × ETo × 0.62 · §493.3

total use = sum of all zones (dedicated edible areas counted at 1.0)

budget cap = ETo × 0.62 × (0.55 × regular area + 1.0 × edible area) · residential

check: total use at or under the budget cap

ETo is annual reference evapotranspiration in inches for your city, from the adopted Appendix C table (676 California cities, the same dataset behind our plant guides). Irrigation efficiency is fixed by the ordinance: 0.75 for sprinklers, 0.81 for drip · §490.2(a)(38). The 0.62 converts inches over square feet into gallons. For non-residential projects the 0.55 becomes 0.45.

Worked example

A Sacramento home (ETo 55.2) with an 800 sq ft lawn on sprinklers (high water, 0.8) and 400 sq ft of shrubs on drip (low water, 0.2): the lawn needs (0.8 ÷ 0.75) × 800 × 55.2 × 0.62 = 29,204 gallons per year, the shrubs (0.2 ÷ 0.81) × 400 × 55.2 × 0.62 = 3,380 gallons, so the total is 32,584 gallons. The budget cap is 55.2 × 0.62 × 0.55 × 1,200 = 22,588 gallons, and the design is over the cap by about 10,000 gallons. Converting the lawn zone to low water plants on drip drops that zone to 6,760 gallons and brings the project comfortably under.

Questions people ask

Which projects does the state water budget apply to?: The state Model Water Efficient Landscape Ordinance (MWELO) applies to new construction with a landscape area of 500 square feet or more that needs a building or landscape permit, plan check or design review, and to rehabilitated landscapes of 2,500 square feet or more that need the same. Projects of 2,500 square feet or less can instead follow the prescriptive checklist in section 492. Cities can adopt stricter local versions, so the permit desk has the final word.
What formula does this calculator use?: The one in the ordinance: each zone contributes its plant rating divided by irrigation efficiency, times area, times annual reference evapotranspiration, times 0.62, in gallons per year. The budget cap multiplies reference evapotranspiration by 0.62 and by 0.55 of the regular landscape area for homes (0.45 for non-residential), plus areas permanently and solely dedicated to edible plants counted in full (recycled-water areas and park-type recreational areas count the same way, but private single-family residential landscapes are never recreational areas under the definition). The comparison is estimated use against the cap.
Where do the plant factor numbers come from?: From WUCOLS, the state plant water use classification: very low is under 0.10, low is 0.10 to 0.30, moderate is 0.40 to 0.60 and high is 0.70 to 0.90. This calculator uses the midpoint of each range. For a permit submission, look up your exact plant list in WUCOLS and use zone-specific factors.
Why are irrigation efficiency values fixed at 0.75 and 0.81?: Because the ordinance sets them, not the equipment vendor: 0.75 for spray heads and rotors, 0.81 for drip. Using drip raises the efficiency divisor, which lowers the computed water use for the same plants and area. That is why switching zones to drip often brings a project under the cap.
Is this enough for a permit application?: It gives the same arithmetic reviewers check first, so it is the right place to test a design before drawing anything. A full submission also needs the worksheet in the required format, plant lists, an irrigation design and other documents your city lists. Treat this page as the fast check, not the paperwork.
Why does my total change so much when I change the plant selection?: Plant choice is the strongest lever in the formula. Moving a zone from high water (0.8) to low water (0.2) cuts that zone to a quarter of its use before any irrigation change. Area is the second lever, irrigation type the third.

Related on WateringTimes

Sources

Model Water Efficient Landscape Ordinance, adopted text and appendices, effective 2025-01-02 · formulas §493.3, applicability §491, prescriptive option §492, efficiency §490.2(a)(38), ETo Appendix C · accessed 2026-08-23
WUCOLS plant water use classifications · plant factor ranges behind the plant selector · accessed 2026-08-23

WateringTimes is an informational gardening and landscape reference for California. Watering numbers are estimates computed from public state data. Nothing on this site is regulatory or professional advice of any kind. Check your water supplier’s current rules before relying on any schedule.