Irrigation Flow Calculator
Irrigation is measured by precipitation rate — the depth of water applied per hour, in mm/hr, exactly like rainfall.
Enter Values
How to use this calculator
- Enter the zone area in square metres and the target precipitation (application) rate in mm/hr — the tool returns the required system flow in L/min, L/h and L/s.
- Optionally add a run time per cycle to see total litres used and the water depth applied, and a per-head flow to estimate how many sprinklers the flow supports.
- Use the flow figure to size your supply pipe and check it against the flow your mains or pump can actually deliver.
How it works
Irrigation is measured by precipitation rate — the depth of water applied per hour, in mm/hr, exactly like rainfall. The key identity is that 1 millimetre of water spread over 1 square metre equals 1 litre. So the volume flow a zone must receive is simply flow (L/hr) = precipitation rate (mm/hr) × zone area (m²), and dividing by 60 converts it to L/min, or by 3,600 to L/s.
If you supply a run time, the water used per cycle is flow (L/min) × run time (min), and the depth applied is rate (mm/hr) × time (hours). If you supply a per-head flow, dividing the system flow by the per-head flow gives the number of sprinkler heads the flow can drive. The tool assumes uniform coverage with no overlap or distribution-uniformity losses, so real systems apply the water somewhat unevenly and may need more flow at the edges.
Worked example
Flow for a 150 m² lawn zone at 12 mm/hr. A pop-up sprinkler zone covers a 150 m² lawn and is designed to apply water at 12 mm/hr. Enter Zone area = 150 and Precipitation rate = 12. Because 1 mm over 1 m² equals 1 litre, the zone needs 12 × 150 = 1,800 L/hr, or 1,800 ÷ 60 = 30 L/min (0.5 L/s). Add a Run time of 20 minutes and the tool shows 600 L used per cycle, applying a 4 mm depth. If each sprinkler head flows 8 L/min, the zone can drive 3.75 heads worth of flow.
Common mistakes
- Confusing precipitation rate (mm/hr, a depth) with flow rate (L/min, a volume). This tool converts one to the other using the zone area — you must enter both the area and the rate.
- Using the plan area of the whole garden instead of the wetted area actually covered by that one zone. Flow scales directly with area, so an over-stated area over-sizes the pipe and pump.
- Assuming the calculated flow is what your tap or mains can deliver. This is the demand of the zone; you still have to confirm the supply can provide that flow at adequate pressure, or split the garden into more zones.
Frequently asked questions
What is precipitation rate in irrigation?
Precipitation rate (also called application rate) is the depth of water an irrigation zone applies per hour, expressed in mm/hr — the same unit as rainfall. Sprays are typically 25–50 mm/hr, rotors 8–20 mm/hr, and drip lines much lower. It lets you compare irrigation directly against a plant's water needs and against your soil's ability to absorb water.
How do you convert mm/hr to litres per minute?
Multiply the precipitation rate (mm/hr) by the zone area (m²) to get litres per hour, because 1 mm over 1 m² is 1 litre, then divide by 60 for litres per minute. For example, 12 mm/hr over 150 m² is 1,800 L/hr, which is 30 L/min.
Does this size my pipe or pump for me?
No. It gives the flow demand of one zone. Use that figure as an input to pipe-sizing and pump-selection, and always confirm your mains or pump can deliver that flow at the pressure your sprinklers need. Check local plumbing and water-authority rules for backflow prevention and connection requirements.
Why might my system need more flow than the calculator shows?
The calculation assumes perfectly uniform coverage. Real sprinklers overlap and vary, so distribution uniformity is below 100%. Designers commonly add a margin so the driest spots still get enough water, which raises the effective flow the zone draws.
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