Calculating How Much Rainwater Your Rooftop Can Capture

How Much Rain Can Your Roof Collect? A Simple Guide

An important factor when planning for rainwater collection is understanding how much water your roof can capture. This guide will help you figure this out and choose the right water tank size for your needs.

Step 1: Measure Your Roof Surface Area

To calculate how much rainwater you can collect, first measure your roof’s surface area:

• For Rectangular Roofs, simply multiply the Length by the Width.
• For L-shaped Roofs: Calculate the area of the main rectangle, calculate the area of the smaller section, then add them together. For example: If your main roof is 15m × 10m, with an extension of 5m × 2m: (15 × 10) + (5 × 2) = 150 + 10 = 160m².

Easy ways to measure your roof:

• Take a walk around your home – You can estimate your roof size by walking around your house outside. The average stride of a person is around 0.76 metres long. So, if it takes you 14 steps to walk one the side of your house, then 14 * 0.76m = 10.64m. Alternatively, you can get out a tape measure to be more precise, but if you don’t have one then a rough estimate should be fine.
• Google Maps’ Measure Tool – Another option is to use Google Maps, simply look up your home address in Satellite-View, zoom into your property, right-click on a corner of the roof and select “Measure distance,” then click around your roof perimeter for a precise measurement.

Make sure to factor in your downpipes:

Remember to only count the roof areas that drain to downpipes connected to your tank. Your roof may slope down to more than one downpipe –  half your roof might flow to one set of downpipes, while the other half flows to another set. So if you intend to place your water tank at one downpipe, it is important to know the surface area of the portion of your roof leading to that downpipe. If you’d like to harvest your whole rooftop, then you might need to redirect your gutters to your harvested downpipes or you may even consider using a second tank.

Step 2: Find Your Local Annual Rainfall

The next step is to look up your location’s average annual rainfall (in millimetres). You can find this information from:

• Bureau of Meteorology Annual, Seasonal and Monthly Rainfall
• Weather websites & apps that show historical data
• Your local water authority

For more information, view our guide on discovering the rainfall in your area.

Step 3: Calculate Your Rainwater Harvesting Potential

The basic formula to calculate rainwater harvesting is 1mm of rain falling on 1m² of roof = 1 litre of water. That means:

Roof Area (m²) × Annual Rainfall (mm) = Potential Rainwater Harvest per Year (litres)

For example, if you have a roof area of 150m² and your location gets 800mm of rain per year, your roof’s rainwater harvesting potential would be:

150m² × 800mm = 120,000 litres per year

Remember that you won’t capture 100% of this water due to factors like:

• Water loss from evaporation
• Water bounce / splashing off gutters, debris filters
• First flush systems diverting initial rainfall
• Water loss from overflow during heavy downpours if your tank is already full

Most systems capture about 80-90% of what falls so you might want to multiply your result by 0.8 to get a more realistic estimate.

Overall, this simple calculation gives you a good estimate of your roof’s rainwater harvesting potential, which helps you choose an appropriately sized rainwater storage tank for your needs.

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