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Irrigation Timer Calculator — Watering Duration & Frequency
Work out watering duration and frequency for your irrigation system.
Total area being irrigated — 1 mm of water = 1 litre per m².
Precipitation rate sets run time; drip/soaker apply more slowly but use 30-50% less water than sprinklers (RHS).
Sandy soils drain fast — water little-and-often; clay holds water, so fewer, deeper sessions (RHS).
Summer ~25 mm/week, spring/autumn 10-15 mm, winter usually none (RHS).
Adjust for wind, sun exposure, etc. (default 1.0)
How We Calculate This
Weekly water need = Area × mm needed per week (1 mm on 1 m² = 1 litre). Summer ~25 mm/week, spring/autumn 10-15 mm, winter usually none (RHS).
Session duration = (mm needed per session ÷ system precipitation rate in mm/hr) × 60 minutes. This is independent of area — a sprinkler applies the same depth per hour over any size of bed (Rain Bird / Hunter).
Precipitation rates: oscillating sprinkler ~12.5 mm/hr, soaker hose ~8 mm/hr, drip ~5 mm/hr (depth-equivalent). Soil type sets frequency — sandy soils little-and-often, clay deeper and less often (RHS).
Frequently Asked Questions
Run time depends on how fast your system applies water (its precipitation rate), not on the area watered — a sprinkler lays down the same depth per hour whether the lawn is small or large. A typical garden oscillating sprinkler applies about 12.5 mm per hour (10-15 mm). For a 25 mm weekly need spread over 3 sessions, that is roughly 8 mm per session, so about 40 minutes per session (around 2 hours per week in total).
Early morning (6-8 AM) is ideal. Less evaporation, leaves dry before evening (reducing disease), and water pressure is usually higher.
In summer, most gardens need 20-25mm per week. In spring/autumn, 10-15mm. Reduce by rainfall received.
Yes. Drip irrigation delivers water directly to roots with minimal evaporation. It uses 30-50% less water than sprinklers.
Morning is best. Plants take up water during the day and leaves dry quickly, reducing disease risk. Evening watering is the second choice but can promote slug activity and fungal problems.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.