# How do you calculate rainwater harvesting?

## How do you calculate rainwater harvesting?

For calculation we take the following formula: mean annual rainfall in mm x area in m² x runoff factor = collected rainwater in litres. In our example this means: 450 x 120 x 0.9 = 48 600 litres.

**How do I calculate how much water is in my roof?**

How Much Rain Water Runs Off Your Roof?

- Take the dimensions of the footprint of your roof and convert them to inches. (So, a 50′ x 20′ roof is 600″ x 240″.)
- Multiply the roof dimensions by the number of inches of rainfall. (In this example, 600″ x 240″ x 1″ = 144,000 cubic inches of water.)

**How do you calculate the roof area of rain runoff?**

The basic calculation is (a X r)/231=g where “a” is the area of your roof in square feet, “r” is the amount of rain in inches, and “g” is the amount of runoff in gallons.

### How do you calculate rainfall per square meter?

To estimate the amount of rainfall you can expect to harvest off your roof, there is a simple calculation you can do. One millimetre of rain falling on one square metre of roof will give you approximately one litre of water in your tank.

**How do you calculate the area of a roof?**

To calculate your roof area, simply multiply your roof length by your roof slope height, and multiply this by two. This should give you the total area of your roof, not accounting for a chimney or other sections of your roof not covered by tiles.

**How do you calculate water drainage?**

To calculate the volume of water that needs to be stored, multiply the amount of runoff from each drainage zone by 15. The runoff for each zone was in gallons per minute. Multiplying by 15 minutes leaves you with the amount of gallons to be stored.

#### How do you calculate rainfall in litres?

Calculating your catchment capacity Roughly speaking, 1 millimetre of rain over 1 square metre of roof equals 1 litre of water. This capacity can be calculated using the following formula: Annual rainfall (in millimetres) x Roof surface area (in square metres) = Roof catchment capacity.

**How do you calculate downspout capacity?**

- Measure the roof to get the watershed area, the area from which water will run off. Multiply length (peak to eave) by width to determine square footage.
- Find the rainfall intensity for your locale.
- Divide the inch per hour watershed of the roof by the 5-minute rainfall intensity.
- Size downspouts to fit the gutter.