Rainwater Harvesting Calculator

Size your cistern and estimate annual yield from roof collection. Use the precipitation data from your prework climate section.

Collection System

Roof area (sq ft)Typical house: 1200-2400 sq ft

Annual precipitation (mm)From your prework Climate section

Roof type

Household size

Monthly Collection vs Demand

BC west coast precipitation pattern. Red line = your monthly water demand. Bars above the line = surplus months.

Using your prework report

Your report's Climate section shows annual precipitation from NASA POWER 30-year averages. The monthly chart shows exactly when rain falls and when the dry season creates a gap. Key things to look for:

Annual precipitation (mm) goes directly into the calculator above
June-August gap is typically 60-80% less rain than winter months on the coast
East of the Coast Range (Okanagan, Cariboo) gets 300-500mm and needs a well as primary source
Cistern should bridge the 3-4 month dry season if rainwater is your primary supply

Rainwater collection for potable use in BC requires filtration and treatment. Check with your regional health authority for requirements. This calculator estimates collection volume only.