What this garden irrigation calculator does — and does not do
“Irrigation calculator” can mean anything from a watering-can estimate to professional hydraulic design. For a standalone garden tool, the most reliable common ground is volume and time. This page converts a chosen watering depth into litres, converts litres into runtime when you know the system flow, works backwards from flow and time to litres, and totals manufacturer-rated drip-emitter flows.
The calculator does not size pipe, calculate pressure loss, choose pumps, design sprinkler head spacing, balance zones or assess backflow protection. Those decisions depend on the actual equipment and installation.
Why millimetres are useful for watering
A depth of 1mm spread evenly over 1m² is exactly 1 litre. That makes rainfall-style depth a useful neutral input: 100m² at a selected 10mm equals 1,000 litres. The maths is exact, but the choice of watering depth is not universal advice. Plant type, soil, weather, season, establishment stage and root depth all change demand.
RHS guidance repeatedly stresses that water needs vary and that overwatering is possible. Its current tree-and-shrub guidance, for example, gives a particular dry-weather context of 30–50 litres per square metre per week for newly planted trees and shrubs, while general garden watering advice focuses on targeted, appropriate watering. That is why this calculator asks for your chosen depth instead of embedding one magic number.
Runtime depends on actual flow
If your system delivers 500 litres per hour and you want to apply 1,000 litres, the simple planning runtime is two hours. If your system is rated in litres per minute, the calculator converts that rate before dividing the target volume by flow. Real delivery can be lower or higher than nominal, so measuring actual flow can improve accuracy.
Drip emitters: add the rated flows, do not invent them
Current products illustrate why a generic emitter default is poor practice. GARDENA currently sells fixed 2L/h emitters, adjustable 1–8L/h emitters, adjustable 0–15L/h emitters and dripline with integrated 1.6L/h emitters. The calculator therefore asks for the rated flow printed for your emitter and multiplies it by the emitter count.
Worked examples
100m² × selected 10mm = 1,000L.
1,000L ÷ 500L/h = 2 hours = 120 minutes.
600L/h for 30 minutes = 300L.
20 emitters × 2L/h = 40L/h; 100L at that nominal flow takes 2.5 hours.
Link irrigation planning to stored rainwater carefully
The Water Butt Calculator estimates rainfall capture and storage. This irrigation page estimates demand and delivery. Comparing the two can show scale, but a water butt does not automatically have the pressure, outlet arrangement or pump required to operate a particular irrigation system.