Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Gallons to apply this week
639
Gross irrigation depth
1.02
Net irrigation need (after rain)
0.82
Crop evapotranspiration (ETc)
1.12
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How to use this
- 1Enter reference et (weekly) (in).
- 2Enter crop coefficient (kc).
- 3Enter effective rainfall this week (in).
- 4Enter irrigated area (sq ft).
- 5Enter irrigation system efficiency (%).
- 6Read your gallons to apply this week on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Evapotranspiration (ET) measures total water lost from soil evaporation and plant transpiration combined, and it's the standard basis for scientific irrigation scheduling used by farmers, golf courses, and landscape water managers. Reference ET (ETo) — published daily by agricultural extension weather networks like CIMIS in California — represents water use of a standardized grass reference surface; multiplying it by a crop coefficient (Kc) that varies by plant type and growth stage gives actual crop water use (ETc). Subtracting effective rainfall from ETc tells you exactly how much supplemental irrigation to apply, which avoids both under-watering (crop stress) and over-watering (wasted water, nutrient leaching, and root disease) that flat weekly watering schedules cause.
Worked example
Using the values the calculator loads with:
Inputs
- Reference ET (weekly): 1.4 in
- Crop coefficient (Kc): 0.8
- Effective rainfall this week: 0.3 in
- Irrigated area: 1000 sq ft
- Irrigation system efficiency: 80 %
Results
- Gallons to apply this week: 639
- Gross irrigation depth: 1.02
- Net irrigation need (after rain): 0.82
- Crop evapotranspiration (ETc): 1.12
What each field means
Inputs
- Reference ET (weekly) (in)
- The reference et (weekly) used in the calculation, measured in in. Starts at 1.4 in so you have a working example on load. Accepted range: 0.1–4 in.
- Crop coefficient (Kc)
- The crop coefficient (kc) used in the calculation. Starts at 0.8 so you have a working example on load. Accepted range: 0.1–1.3.
- Effective rainfall this week (in)
- The effective rainfall this week used in the calculation, measured in in. Starts at 0.3 in so you have a working example on load. Accepted range: 0–5 in.
- Irrigated area (sq ft)
- The irrigated area used in the calculation, measured in sq ft. Starts at 1000 sq ft so you have a working example on load.
- Irrigation system efficiency (%)
- The irrigation system efficiency used in the calculation, measured in %. Starts at 80 % so you have a working example on load. Accepted range: 40–100 %.
Results
- Gallons to apply this week
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Gross irrigation depth
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Net irrigation need (after rain)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Crop evapotranspiration (ETc)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where do I find reference ET (ETo) for my area?
State agricultural extension networks publish it daily or weekly — CIMIS in California, AZMET in Arizona, and similar mesonet systems in most farm states. If unavailable, a rough national summer average of 1.0-1.8 inches per week works as a starting estimate.
How do I pick a crop coefficient (Kc)?
Kc varies by plant type and growth stage: cool-season turfgrass runs about 0.6-0.8, warm-season turf 0.5-0.7, most vegetables 0.7-1.15 depending on stage, and mature trees or shrubs often 0.5-0.9. FAO-56 publishes detailed Kc tables for hundreds of crops by growth stage.
What counts as 'effective rainfall'?
Not all rain reaches the root zone usefully — heavy downpours run off, and light rain can evaporate before infiltrating. A common rule of thumb treats about 75-80% of rainfall under 1 inch as effective, with diminishing returns above that as runoff increases.
Why does system efficiency matter for the calculation?
Drip irrigation delivers 85-95% of applied water to the root zone, while overhead sprinklers lose 15-30% to evaporation, wind drift, and uneven coverage, so you need to apply proportionally more gross water through an inefficient system to deliver the same net amount the crop actually needs.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Evapotranspiration Irrigation Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation" target="_blank" rel="noopener">Evapotranspiration Irrigation Calculator — RevenueLab</a> (2026).</p>
Source: [Evapotranspiration Irrigation Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation) (2026).