Watering Needs Calculator
How many gallons your garden bed needs each week.
Use ToolFree calculator
Crop Water Requirement Calculator
Weekly and seasonal irrigation gallons a planting needs, from area, reference evapotranspiration minus rainfall.
Crop Water Requirement Calculator
Chlorova · updated for 2026
Calculator
Quick samples
Click “Calculate” to see your results.
The climate figures are rounded reference evapotranspiration values, not your station data, and real weekly demand moves with temperature, wind, humidity, crop stage and canopy shade, so a heat wave can double a normal week and a cool cloudy one can erase it. Effective rainfall means what actually reaches the root zone, which is less than the gauge says on short storms and more on long soaks, and this tool subtracts the entered number in full. Use it to budget water and compare seasons, then tune against a rain gauge and your own soil dryness rather than treating the deficit as a schedule.
Plants do not drink gallons, they replace what evaporates from soil and leaves, and the gap between what the atmosphere takes and what the sky returns is the entire irrigation question. This crop water requirement calculator works that gap from a climate class, subtracting your effective weekly rainfall from a reference evapotranspiration figure, then translates the shortfall into the gallons your planting needs each week and across the whole season. Enter area and rainfall, and the water budget writes itself before you set a schedule.
Budgeting Crop Water in Steps
- Pick the summer climate class closest to your region and weather.
- Enter the planting area in square feet that needs water.
- Type the inches of rain that typically falls in an average week.
- Set how many weeks the crop will need supplemental water.
- Read the weekly deficit first, the shortfall driving everything.
- Check weekly gallons, then multiply out to the season total.
- Let the climate note adjust expectations for how thirsty your region runs.
- Treat a zero-deficit week as a skip, not a duty to water anyway.
How the Water Deficit Is Found
Evapotranspiration is the inches the atmosphere pulls from a planted surface each week, and the climate picker supplies a rounded regional value near a tenth of an inch in cool marine air up to thirty hundredths in desert heat. Subtracting effective weekly rainfall from that figure leaves the deficit, and a max function floors it at zero so a wet week simply needs no irrigation rather than returning negative gallons.
That deficit becomes volume the familiar way, area times inches times 0.623, since one inch over one square foot is roughly 0.623 gallons, giving the irrigation each week. Multiplying by the number of growing weeks produces the season demand, which is the figure worth knowing when you price water or size a rain system. Every result traces back to one subtraction, so the rainfall entry deserves the closest attention.
Worked Example: A Temperate Kitchen Garden
- Your 300 square foot garden sits in a continental climate.
- Reference evapotranspiration there runs near 0.15 in weekly.
- Summer rain averages 0.10 of an inch each week, partly.
- The deficit is 0.15 minus 0.10, leaving 0.05 inches.
- Weekly need is 300 times 0.05 times 0.623, about 9 gal.
- Over 14 growing weeks the season asks for roughly 131 gal.
- In a dry fortnight with zero rain that same bed wants 28.
- Small numbers, which is why mild summers in temperate gardens rarely irrigate much at all.
- Rain already covered two-thirds of the ET, leaving only a sliver to replace.
- Size a rain barrel or well from that roughly 131-gallon season figure.
Weekly Deficits by Climate and Rain
Irrigation demand in gallons per week for a 500 square foot planting, combining the reference evapotranspiration each climate class carries with the effective rainfall that offsets it. Scale the row up or down in proportion to your own area.
| Climate | ET in/wk | 500 sq ft at 0.1 rain |
|---|---|---|
| Hot arid | 0.30 | 62 gal/wk |
| Semi-arid | 0.25 | 47 gal/wk |
| Temperate | 0.15 | 16 gal/wk |
| Humid | 0.12 | 6 gal/wk |
| Marine coastal | 0.10 | 0 gal/wk |
Key Terms
- Evapotranspiration
- Water lost jointly from soil evaporation and plant transpiration, in inches.
- Reference ET
- Loss from a standardized grass surface, adjusted for crop and conditions in real schedules.
- Effective rainfall
- The part of a rain fall that reaches roots, not the total the gauge records.
- Deficit
- ET minus effective rain, the irrigation requirement in inches of water.
- Crop coefficient
- A multiplier scaling reference ET to a specific crop and growth stage.
Irrigation Scheduling Notes from Extension
- Reference ET describes grass under full sun, and your actual crop differs by a coefficient, so dormant mulched beds need less than the number here while a flowering tomato canopy needs more.
- Rain under fifteen hundredths of an inch barely wets beyond the leaves, which is why schedulers count light showers as nearly zero effective rainfall rather than crediting the gauge in full.
- Weekly totals beat daily habits, because one deep soaking matching the deficit feeds roots downward, while the same gallons split into sips stay in the top inch where evaporation claims them.
- Shade, wind and humidity move demand more than temperature alone does, so a breezy dry site can outrun a hotter humid one on the same calendar week.
- The season figure is a budget rather than a prescription, and the honest check stays a hand in the soil at root depth two days after a good rain.
Common Mistakes & Fixes
- Crediting every recorded light drizzle as effective rainfall, then wondering why beds stay dry.
- Using a regional average through a heat wave instead of adjusting for the week you have.
- Watering to the daily habit rather than applying the weekly deficit in one deep soak.
- Ignoring crop stage, since tiny seedlings and senescing ends drink far less than the peak.
- Treating the zero-deficit answer as permission for clay slopes, where runoff still starves roots.
- Assuming a season figure is a fixed schedule rather than a budget the weather can rewrite.
When to Use This (and When Not To)
Use it for season planning and water budgeting, when you want gallons rather than minutes and need to compare a dry-year scenario against a normal one. It stops at the requirement, so convert the answer into run time with the sprinkler-run-time or soaker-hose calculators, price those gallons with the garden-watering-cost tool, and lean on the watering-needs calculator for plant-by-plant defaults.
Crop Water Requirement Calculator: Frequently Asked Questions
How much irrigation does a garden really need?
The difference between what evaporates and what rains, expressed in inches over your area, which for a temperate three-hundred square foot plot often works out under ten gallons in an average week. Arid sites on the same area can need six times that, and wet summers can need nothing at all.
What is evapotranspiration in plain terms?
It is the water the land loses upward, both evaporating from soil and transpiring through leaves, measured in inches the same way rainfall is. One inch of ET means one inch of water must come back from rain or irrigation to keep the surface balanced.
Why does my rainy week show zero need?
Because rainfall met or beat evapotranspiration, and the calculator floors the deficit at zero rather than reporting negative irrigation. That is genuinely correct, a soaking week refills the root zone and the next watering should wait until the soil dries to root depth.
Does 0.623 gallons per square foot matter here?
It is the conversion constant doing all the unit work, since one inch of water over one square foot weighs out to about 0.623 gallons. Every gallon figure on this page is simply your area times the deficit inches times that constant.
How do I turn this into a watering schedule?
Apply the weekly deficit as one deep soaking, then convert gallons to minutes using your actual delivery rate. A sprinkler or soaker sized with the run-time calculators closes the loop from this page's requirement to a timer setting you can trust.
Should fruit trees use this method too?
As a rough budget yes, entered at their wetted root-zone area rather than the canopy footprint, since young trees drink far less than bearing ones. Mature orchards respond more to soil moisture at depth than to any weekly inches figure, so check with a probe before irrigating.
Sources & Further Reading
- Understanding Evapotranspiration and Irrigation Scheduling — University of California Agriculture and Natural Resources. Basis for the ET-minus-rain deficit model used on this page.
- Watering Established Trees, Shrubs and Perennials — Penn State Extension. Underpins the deep-soak versus frequent-light-watering guidance here.
- Estimating Outdoor Water Use and Irrigation Cost — University of Florida IFAS Extension. Source of the depth-to-gallons conversion and climate demand ranges.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated