Irrigation Calculator
Calculate irrigation water volume, run time and crop water requirement.
Please note: Planning figures based on published averages. Soil type, climate, variety, season and local practice all shift these results substantially. Base real decisions on a soil test, local extension guidance and the product label — particularly for fertiliser, lime and any crop protection product, where the label is a legal document.
What the Irrigation Calculator does
Irrigation demand starts from a depth of water in millimetres, either chosen directly or derived from crop evapotranspiration. Because one millimetre over a hectare is exactly ten cubic metres, converting depth to volume is straightforward — the real variable is how much of what you pump actually reaches the roots.
Formula
ETc = ET₀ × KcNet depth = ETc − Effective rainfallGross depth = Net depth ÷ System efficiencyVolume (m³) = Depth (mm) × 10 × Area (ha)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Calculate from | one of 2 options | Yes | — |
| Area | number | Yes | Accepts more than 0. |
| Area unit | one of 4 options | Yes | — |
| Depth to apply | mm | In some modes | Accepts more than 0. Shown Calculate from is A depth of water to apply. |
| Reference evapotranspiration | mm/day | In some modes | Accepts more than 0. Shown Calculate from is Crop water requirement (ET). |
| Crop | one of 9 options | In some modes | Shown Calculate from is Crop water requirement (ET). |
| Effective rainfall | mm/day | Optional | Accepts 0 or more. Shown Calculate from is Crop water requirement (ET). |
| Irrigation system | one of 6 options | Yes | — |
| Available flow rate | m³/h | Optional | Accepts 0 or more. |
How to use it
- Choose Calculate from and Area unit.
- Enter Area.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Available flow rate.
- Select Calculate.
Worked example
Applying 25 mm over one hectare with a centre pivot at 80% efficiency and a 20 m³/h supply.
- Area
- 1 ha
- Depth
- 25 mm
- System
- Centre pivot
- Flow
- 20 m³/h
25 mm net needs 31.25 mm gross, which is 312.5 m³. At 20 m³/h that is 15.6 hours of running.
Reading the result
- "Water lost to inefficiency" quantifies the entire cost of system choice for a fixed net depth requirement — it is exactly how much extra water and pumping time a lower-efficiency system consumes for the same crop benefit.
- "Water per hectare" isolates the rate from the total volume, making it possible to compare systems or fields of different sizes on a common basis without recomputing.
Assumptions and limitations
- System efficiency is treated as a single fixed percentage for the chosen system type — real efficiency varies with maintenance, wind exposure and terrain, so an aging or poorly maintained system loses more than the stated figure.
- In crop mode, ET₀ and effective rainfall are both single daily figures — the "over a 120 day season" projection assumes that one day's conditions hold for the whole season.
Common mistakes
- Using a region's average annual rainfall as "effective rainfall" instead of the actual recent figure — a seasonal average applied to any given day can wildly overstate irrigation savings in a dry spell or understate them right after a storm.
- Increasing net depth to "make up for" a less efficient system — the net depth target should stay the same regardless of system; changing it compounds the very inefficiency it is meant to work around.
Frequently asked questions
How much water is one millimetre of irrigation?
Ten cubic metres per hectare — 10,000 litres. It is the same as one litre per square metre.
What is a crop coefficient?
Kc scales reference evapotranspiration to a specific crop and growth stage. A mature maize crop transpires about 1.2 times the reference, a bare field far less.
Why does system efficiency matter so much?
Because you pay to pump the gross volume, not the net. Moving from flood at 50% to drip at 90% nearly halves the water bill for the same crop benefit.