Irrigation water calculator
Estimate how much water your garden needs each week and over the season, and what it may cost.
Your estimate
Water per week
1,573 L
- Water per season
- 31,467 L
- Water cost per season
- £79
- Net water need per week
- 11.8 mm
Estimates for planning only — not a quote, survey or financial advice.
How this is calculated
Formula
- Plant water use (ETc) = reference evapotranspiration (ET₀) × crop coefficient (Kc) × 7 days
- Net need = ETc − effective rainfall (never below zero)
- Water to apply = net need ÷ watering efficiency × area (1 mm on 1 m² = 1 litre)
- Season = weekly volume × weeks · cost = season volume × water price
Assumptions
- Method: FAO-56 crop-coefficient approach (FAO Irrigation and Drainage Paper 56).
- ET₀ used per climate: cool 3, temperate 4, warm 5, hot and dry 6.5 mm per day — rounded summer averages, not local measurements.
- Coefficients: lawn 0.8, vegetables 1.0, mixed borders 0.6, shrubs 0.5, drought-tolerant 0.3. Efficiency: drip 90%, sprinklers 75%, hose 60%.
- Check local hosepipe bans or watering restrictions.