Irrigation Calculator

How the Irrigation Calculator works

The formula. Litres per week = area in m² × (millimetres needed − rain in millimetres), because 1 mm of water over 1 m² is 1 litre. For example, 10 m² needing 25 mm with no rain takes 250 litres a week, or 83 litres a session watering three times. A second example: 50 m² needing 25 mm after 10 mm of rain takes 50 × 15 = 750 litres. Divide by the flow rate to see how many minutes to run the hose.

Plants need water by depth, not by minutes of hose. A lawn or vegetable bed in dry summer weather uses about 25 mm a week; that is 25 litres for every square metre, because one millimetre over one square metre is exactly one litre. Once you know the area, the litres follow, and once you know the flow rate of the hose, so does the time. Most gardens are watered too often and too little; this calculator helps you water less often and more deeply.

Millimetres to litres. Water need is quoted in millimetres because it is independent of area: 25 mm of rain and 25 mm of irrigation are the same thing. Multiply by the area in square metres for litres. A 50 m² bed at 25 mm is 1,250 litres a week, which surprises most people the first time they see it and explains why a quick evening sprinkle does little.

Rainfall. Subtract rain that has fallen in the week from the need; a rain gauge or a straight-sided jar tells you how much. Light showers of a few millimetres barely wet the surface, so only count rain that adds up to a proper amount. If rain exceeds the need, the result shows zero and you can skip watering.

Sessions. Deep watering two or three times a week is better than a little every day: it pushes roots down and lets the surface dry between times, which discourages slugs, moss and fungal disease. Newly sown seed and containers are the exception and need small daily amounts. The per-session figure divides the weekly total by the number of sessions you choose.

Run time. Time a hose or sprinkler into a bucket: a 10-litre bucket filled in one minute is 10 L/min. Typical garden hoses run at 8 to 15 L/min; a lawn sprinkler covers a set area at a rate you can measure by putting jars on the lawn and timing how long they take to reach 10 mm. The result divides the session volume by the flow rate to give minutes or hours. Drip lines are rated per emitter, so multiply the emitter rate by the number of emitters for the flow.

Costs. On a water meter you pay per cubic metre (1,000 litres) for supply and usually sewerage too, so a 50 m² lawn watered at 25 mm a week costs the price of about 5 m³ a month. A water butt of 200 litres holds enough for one session on 8 m² at 25 mm; the numbers make clear why butts help borders but cannot keep a lawn green.

Priorities in a dry spell. Established lawns can be left to go brown and recover with autumn rain. Concentrate water on vegetables in flower or fruit, anything planted this year, and containers. Water in the evening or early morning, at the base of plants rather than over the leaves, and mulch to slow evaporation.

Irrigation calculator: water needed by garden size

25 mm a week with no rain, watered three times a week through a 10 L/min hose.

Weekly water and run time per session by garden area

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