WaterCalcLabGarden Water Requirement Calculator
Apply an ET?/Kc water balance for a garden hydrozone. ET? is a daily input, not calculated weather data.
Your result
Bars compare only results in the same unit group. They are normalized, not a time-series forecast.
Planning result based on your inputs.
How this tool works
Apply an ET₀/Kc water balance for a garden hydrozone. ET₀ is a daily input, not calculated weather data.
Dnet = max(0, ET₀ × Kc × days − effective rain); Vgross = A × Dnet/ηExample calculation
The prefilled form is a worked example. Its base-unit inputs and expected outputs are shown below; use the reset button to restore it.
| Input | Example |
|---|---|
| Area | 100 |
| Reference evapotranspiration ET₀ per day | 4 |
| Crop coefficient Kc | 0.8 |
| Days | 21 |
| Effective rain for the period | 5 |
| System efficiency (separate losses) | 0.9 |
Planning notes for this calculator
Build a simple water budget for one garden zone with similar plants and conditions. Use separate calculations for beds with different exposure or crop needs.
Choose meaningful inputs
- Area
- Use the planted area of the zone rather than the whole property including paths.
- Reference evapotranspiration ET₀ per day
- Use local daily reference evapotranspiration, ET₀, from an appropriate weather service. This calculator does not retrieve weather.
- Crop coefficient Kc
- Choose a coefficient suited to the crop and growth stage. The example is an assumption, not a universal garden value.
- Days
- Use days covered by the ET₀ assumption and effective-rain total.
- Effective rain for the period
- Enter the part of rainfall useful to the root zone during the same period; measured rainfall and effective rainfall can differ.
- System efficiency (separate losses)
- Enter the share of delivered water retained for the intended irrigation use. A lower fraction requires more supplied water.
Read the result correctly
ET₀ × Kc × days estimates crop evapotranspiration for the period. Effective rain reduces the calculated net depth, never below zero. Net volume describes that depth across the zone; gross volume includes your assumed delivery losses. This simplified balance does not measure current soil moisture, so compare it with observations before watering.
Common mistakes to avoid
- Do not subtract a weekly rain total from a single-day evaporation estimate.
- Do not apply one crop coefficient to every plant in a mixed garden without justification.
A useful next step
Check root-zone moisture, then use the irrigation or runtime tool to translate an appropriate net depth into delivery.
Follow a worked scenario
A separate illustrative scenario; these values do not change when you edit the form above.
- 4 mm/day × 0.8 × 7 days − 5 mm effective rain = 17.4 mm net depth.
- 100 m² × 17.4 mm = 1,740 L net; ÷ 0.90 gives about 1,933 L gross.
Frequently asked questions
Can the calculator tell me today’s weather?
No. You supply ET₀, crop coefficient and effective rainfall. A result using the presets is an illustrative scenario for those assumptions, not a local weather forecast.
What does a zero result mean?
Rain covers the estimated evapotranspiration in this simplified period balance. It does not prove every root zone is adequately moist or that no irrigation will be needed later.
Continue your plan
Irrigation Water Calculator
Convert a chosen net application depth into gross water volume with an explicit efficiency.
Open calculator ↗Garden irrigationDrip Irrigation Calculator
Calculate nominal zone flow and delivered volume from emitter count, measured emitter flow and runtime.
Open calculator ↗Garden irrigationSprinkler Runtime Calculator
Estimate runtime using the measured total flow of one zone and a chosen net dose.
Open calculator ↗Related reading
Garden irrigationHow to Reduce Garden Water Consumption
Record watering events, measured flow and runtime by zone. Note rainfall, weather and plant condition. A bill for the whole house mixes garden use with other consumption, so a submeter or event log can be useful. Establish which zones consume most before buying hardware; a small low-use bed offers less volume-saving opportunity than a poorly controlled large zone.
Read guide ↗
Garden irrigationWatering Different Soil Types
Texture helps describe water retention and movement, but rooting depth, compaction, organic matter, slope and current wetness also matter. Identify the soil in each relevant bed rather than assuming the whole property is uniform. Use observation or appropriate testing. A soil label alone cannot justify an exact interval for every plant and weather condition.
Read guide ↗
Garden irrigationHow Much Water Does a Garden Need?
Measure the beds actually being irrigated and choose a period such as one week. Exclude paved paths from the watered area unless overspray is being measured as a loss. Separate plants with different needs rather than applying one garden-wide depth without explanation. A container, vegetable bed and mature shrub zone may need different schedules even on the same property.
Read guide ↗Sources & method references
- FAO-56, Chapter 8: Soil water balance
Root-zone storage, allowable depletion and water-balance assumptions; our model is a simplified planning calculation.
- US EPA WaterSense: Watering tips
Practical observation and watering context, not a universal dose or a local schedule.
Method version 1.1 · Explanations and examples reviewed: 2026-10-11