What this calculator does
Irrigation demand is evapotranspiration scaled by the crop and divided by the efficiency of the delivery system. This calculator produces the daily volume, the weekly volume and a rough emitter count, which is what you need before designing zones.
How to use it
- Enter the irrigated area and the reference evapotranspiration for the warmest month.
- Set the crop factor: turf grass ≈ 0.8, vegetables 1.0–1.2, drought-tolerant planting 0.3.
- Set the delivery efficiency — 75 % for good drip, 55–65 % for spray or a hose.
- Set the irrigation days per week so the weekly volume can be spread across them.
Governing formula
V = A · (ET₀ · Kc) / (1000 · η) Where:
-
ET₀= Reference evapotranspiration [mm/day] -
Kc= Crop coefficient [–] -
η= Irrigation efficiency [–]
Derived Equations:
V_week = V_day × 7 / n_days Worked example — 250 m² of turf in a 5 mm/day climate
The figures below were produced by running this page's own engine with the example inputs (Irrigated area (m²), Reference evapotranspiration (mm/day), Crop factor (Kc) (–), Irrigation efficiency (%), Irrigation days per week (days), Emitter flow rate (L/h), Hours per day (h)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Daily water requirement | 1333 L/day |
|---|---|
| Weekly requirement | 3.11 m³ |
| Crop water requirement | 4 mm/day |
| Indicative emitter count | 167 |
| Assessment | Crop requirement 4.0 mm/day → 1333 L/day over 250 m² (≈ 167 emitters running 2 h/day. Rotate zones against the 3-day/week schedule to hold storage. |
- Turf at Kc = 0.8 needs 4 mm a day.
- Over 250 m² at 75 % efficiency that is about 1,330 L a day.
- Irrigated three days a week, the weekly requirement is around 3.1 m³.
Assumptions & limitations
Engineering calculation. Applies a crop coefficient to reference evapotranspiration. No soil moisture deficit, rainfall contribution, irrigation uniformity or scheduling optimisation.
- Peak-month evapotranspiration over-estimates the annual average by 30–50 %.
- No rainfall offset, which in Mediterranean climates can remove half the irrigation need.
- Emitter count assumes a uniform runtime and does not account for pressure-compensating variation across a zone.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
Schedule mode — planting zones
A landscape plot is a set of planting zones with different crop factors, not one blended area. Enter each zone once and the totals give the peak demand the supply has to meet.
The schedule totals are the sum of every item's own engine result, so they match running this calculator once per item. An item the engine rejects is reported in place and contributes nothing to the totals.
- Zones are summed at peak reference evapotranspiration; the supply must also satisfy the simultaneous-use of the zones you actually run together.
- The emitter count sums each zone independently, so zones on a common manifold will need a single controller and valve schedule that this worksheet does not produce.
References & standards
- FAO Irrigation and Drainage Paper 56 — crop evapotranspiration guidelines.
- CIBSE Guide G — irrigation and landscape water management.