Rainwater Harvesting Tank Size Calculator

Rainwater Harvesting Guide

Collection Calculation

Annual harvest = roof area × annual rainfall × runoff coefficient. Runoff coefficient: tiles/slates ≈ 0.90 (small losses to wetting and splash). Flat felt roof ≈ 0.85. Green roof ≈ 0.50-0.65. Example: 60m² pitched tile roof in London (600mm annual rainfall): Harvest = 60 × 0.60 × 0.90 = 32.4m³ = 32,400 litres/year. This is a significant amount — equivalent to approximately 162 bathloads of water. However, distribution is uneven: summer rainfall often falls when demand for irrigation is highest.

Demand and Tank Sizing

Garden irrigation: approximately 15 litres/m² during dry summer months. An 80m² garden: 80 × 15 × 3 dry months = 3,600 litres peak demand. Toilet flushing: approximately 30 litres/person/day = 10,950 litres/year per person. Laundry: approximately 9,000-13,000 litres/year per household. Tank sizing rule: accommodate approximately 5-7% of annual demand as storage. For garden only use (3,600L peak): a 2,000-3,000L tank provides useful buffer. For toilet and laundry (40,000L demand): a 5,000-7,500L

Installation and Regulations

Simple water butt: £20-80. Connects to a downpipe. Suitable for garden use. 200-500L capacity. Very simple DIY installation. Medium system (1,000-3,000L tank, submersible pump, overflow to sewer): £500-1,500 installed. Suitable for heavy garden use and car washing. Large underground system with whole-house non-potable supply: £3,000-8,000 professionally installed. Includes tank, pump, filtration, mains backup valve, separate pipework. Building Regulations: rainwater used inside the building requ

Economics and Payback

UK average water cost (metered): approximately £2.20 per m³ (combined water and sewage). Annual saving from 30m³ harvested: 30 × £2.20 = £66/year for garden-only use. For a simple £150 water butt setup: payback approximately 2-3 years. Large underground system (£5,000): saving £200-350/year from toilet and garden combined: payback 14-25 years — financially marginal. Non-financial benefits: reduced mains water demand. Reduced sewage system load during heavy rain (stormwater management). Resilienc