Rainwater Harvesting Calculator
Calculate annual rainwater yield from your roof. Based on South African rainfall data for 8 cities. Includes tank sizing, water bill savings, and 10-year ROI.
Quick Calculator Get a fast estimate
m²
%
Annual Yield
68 kL
Monthly Average
5 704 L
Recommended Tank Size
6 000 L
Garden Coverage
4 m²
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Rainwater Harvesting Formula
Annual yield (litres) = Roof area (m²) × Annual rainfall (m) × Collection efficiency. Typical efficiency is 75–85% — losses from evaporation, first-flush diversion, and overflow. A 120m² roof in Durban (1,008mm/yr) at 80% efficiency yields over 96,000 litres per year.
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Extended Calculator More options, charts, and scenario comparison
m²
%
Annual Rainwater Yield (Johannesburg)
67,3 kL/year
Average Annual Rainfall — SA Cities
| City | Annual Rainfall | Yield (120m² roof) |
|---|---|---|
| Durban | 1,008 mm | 96,768 L |
| Johannesburg | 713 mm | 68,448 L |
| East London | 695 mm | 66,720 L |
| Pretoria | 674 mm | 64,704 L |
| Gqeberha | 593 mm | 56,928 L |
| Cape Town | 515 mm | 49,440 L |
| Bloemfontein | 562 mm | 53,952 L |
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Professional Calculator Complete parameters, sensitivity analysis, and detailed breakdown
Full Rainwater ROI Analysis
m²
%
R/kL
%/yr
R
L
Intended Uses
System Analysis
68,4 kL/year — R 1 917/year saving
Demand covered
82%
Capital cost
R 11 500
Payback
5 years
10-Year Savings Projection
| Year | Water Tariff | Annual Saving | Cumulative |
|---|---|---|---|
| Year 1 | R28/kL | R 1 917 | R 1 917 |
| Year 2 | R30,8/kL | R 2 108 | R 4 025 |
| Year 3 | R33,88/kL | R 2 319 | R 6 344 |
| Year 5 | R40,99/kL | R 2 806 | R 11 701 |
| Year 7 | R49,6/kL | R 3 395 | R 18 183 |
| Year 10 | R66,02/kL | R 4 519 | R 30 545 |
10-year cumulative saving: R 30 545
Water Quality Note: Harvested rainwater is suitable for garden irrigation, toilet flushing, and car washing without treatment. For drinking and cooking, a first-flush diverter + activated carbon filter + UV sterilisation is required. Check with your local municipality regarding permits for rainwater harvesting systems.
Frequently Asked Questions
Rainwater can be made safe for drinking with proper treatment: a first-flush diverter (discards the first 10–25 litres contaminated with bird droppings and dust), followed by activated carbon filtration and UV sterilisation. Without treatment, use for garden, toilet flushing, and car washing only.
A first-flush diverter automatically discards the first 10–25 litres of roof runoff at the start of each rainfall event, which contains the highest concentration of bird droppings, dust, pollen, and pollutants. This significantly improves water quality. Budget R300–R800 for a quality diverter.