Measure the footprint
The Water Research Commission defines roof catchment area from the horizontal building and overhang footprint.
South African rainwater harvesting calculator
Rainwater calculator.
Match roof, rain and demand.
Estimate annual roof yield, first-flush volume, a practical reserve target, a starting tank size and basic gutter and downpipe checks.
Start calculating ↓LRoof yield · tank · first flush
01Roof-catchment yield
02Demand-based reserve
03First-flush volume
04Gutter and downpipe checks
01
Rainwater planning
Size the opportunity
Enter local annual rainfall and a realistic daily non-potable demand. A single annual average is useful for planning, but it cannot reproduce seasonal storage behaviour.
02
The method
How the estimate works
Each output shows its assumptions so you can replace defaults with the dimensions, pack sizes and rates available to you.
Apply rainfall losses
One millimetre over one square metre equals one litre; the runoff coefficient accounts for wetting, leakage, spillage and evaporation.
Allow unusable volume
The reserve target includes dead storage. WRC guidance says 10–20% may be unavailable when the exact value is unknown.
Protect water quality
WRC guidance covers first flush, gutter sizing, filtration, tank placement, overflow and treatment.
Important boundary
Annual yield is not guaranteed daily supply.
Rainfall timing, dry spells, roof cleanliness, tank overflow and actual demand determine reliability. Use local time-series data for a final tank design.
See all calculators →Real questions
Questions homeowners ask
How much rainwater can I collect from my roof?+
Multiply roof footprint in m² by rainfall in mm and the runoff coefficient. One millimetre falling on one square metre is one litre before losses.
What runoff coefficient should I use?+
A coefficient near 1 represents a smooth, efficient catchment. A conservative 0.8 starting value allows for losses, but roof material and system condition should guide the input.
How large should the first-flush diverter be?+
WRC guidance describes a typical diversion depth of 0.5–1.5 mm over the roof catchment. Dirtier roofs or longer dry periods can need more.
Can I drink harvested rainwater?+
Not without a suitable potable-water design, approved materials, filtration, disinfection, testing and maintenance. This tool is intended for planning, especially non-potable uses.