About this calculator
How much rainwater a roof can collect from a given amount of rain, and which JoJo tank size will hold it.
How it’s calculated
Each millimetre of rain on one square metre of roof is one litre of water. JoJo suggests allowing about 15% for wastage (splashing, evaporation and the first flush), so litres collected = roof area (m², on plan) × rainfall (mm) × 0.85.
The calculator then picks the smallest vertical JoJo tank that holds that volume: 260, 500, 1,000, 1,500, 1,800, 2,000, 2,400, 2,700, 4,750, 5,250, 5,500, 10,000, 15,000 or 20,000 litres, or several 20,000 L tanks for very large volumes. Enter how much you use a day to see how long the water lasts.
Worked example
With these inputs:
- Roof area (plan, draining to the tank)
- 100 m²
- Rain to catch (mm)
- 25
- Water used from the tank per day (litres)
- 0
the result is:
- Rainwater collected
- 2,125 litres
- Rain on the roof
- 2,500 L
- After 15% wastage
- 2,125 L
Tips from the trade
- Use the plan area of the roof, not the sloping length; a steep roof doesn’t catch more rain.
- Size the tank for a heavy storm or a wet month, not the yearly total, unless you will use the water steadily.
- Keep gutters clean and screened so leaves don’t wash into the tank.
Common questions
How much water can I collect from my roof?
About 0.85 litres per m² of roof for every mm of rain, using JoJo’s 15% wastage allowance. A 100 m² roof in a 25 mm storm collects about 2,125 litres.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.