Irrigation Water Budget Calculator
A water budget is the ceiling your irrigation schedule should never exceed. It starts from evapotranspiration (ET) - what weather and plants demand - corrected by your system's efficiency and local rainfall.
Inputs
Result
Planning estimate - not a substitute for a pressure-tested engineering design.
Formula
Weekly gallons = (ET × Kc − effective rainfall) × area (sq ft) × 0.6233 ÷ efficiency. Reference ET (ET0) comes from weather data; the plant coefficient Kc scales it to your planting.
Example
2,000 sq ft of turf (Kc 0.7) in a 1.5 in/week ET week, 75% efficiency, 0.25 in rain: demand = 1.05 in, net = 0.8 in → 0.8 × 2000 × 0.6233 ÷ 0.75 ≈ 1,330 gal/week (~5,760/month).
Assumptions
- Reference ET is a weekly average from a nearby source.
- Effective rainfall excludes light showers that evaporate before use (roughly < 0.25 in per event).
- The Irrigation Association identifies water budgeting - ET, plant type, area, efficiency, water quality and rainfall - as a core irrigation management practice.
Limitations
This tool is a planning aid for DIY design and pre-design work. It does not replace an on-site pressure and flow test, a catch-cup uniformity test, or local code and backflow requirements. For steep slopes, long pipe runs, large elevation changes or commercial-scale systems, have a qualified irrigation designer verify the plan before installation.
What This Means
The budget is the truth test for your controller schedule: run the numbers on your actual run times with the Water Use Calculator, and if usage exceeds budget, shorten sessions or fix efficiency.
Next Step
Convert this budget into a schedule with the Run Time Calculator, then verify real consumption with the Water Use Calculator.
Related Tools
Sources
Frequently Asked Questions
What is a water budget in irrigation?
It is the calculated amount of water a landscape needs over a period, based on ET, plant type, area, efficiency and rainfall - used as the ceiling for scheduling and conservation targets.
Where do I find local ET values?
State university extensions, CIMIS (California), regional weather networks and the Irrigation Association publish reference ET; many controllers can pull it automatically.
Why multiply ET by a plant coefficient?
ET0 describes a reference grass surface. Landscapes differ - shrubs use roughly half, turf most of ET0 - so Kc scales the weather demand to the planting.