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Cistern Catchment Information
About RainBoxes | Cistern Catchment Info | Diagrams


How Much Rain Can You Catch?
A square foot of horizontal surface receives more than a ½ gallon of water with each inch of rainfall. To get the effective square footage of your roof for catchment purposes, measure the sides of your house from eave to eave. Multiply the length times the width to get the square footage of the catchment surface. Multiply that amount times 0.625 to find the total gallons the roof can catch per inch of rain. If only part of the roof is used for catchment, calculate only for that area. 

For example:
Let's say the building you have to work with is 27 x 20 feet which is an average 2 car garage. The eaves add 1½ feet on all sides, so the roof covers an area of 30 x 23 feet.

30' x 23' = 690 square feet of catchment area
690 sqft. x 0.625 gallons of water = 431.25 gallons caught per inch of rain.

This calculation provides theoretical value, a general estimate of the amount your RainBox, rainwater catchment system can capture. Some rain will evaporate, splash off, or overflow the gutters; some water will be lost if you have a first-flush diverter; and if it rains a lot, the cistern may overflow. 

Once you know the effective roof area, take a look at a rainfall chart for your area. Let's say you live in area where 8 inches of rain falls on average every year. How much water would that provide for your irrigation? The roof area calculated above will catch 431 gallons of water per inch of rain. If 8 inches of rain falls, you have 431 gallons x 8 inches of rain = 3,448 gallons. An irrigation system using 100 emitters @ 2 gallons of water every hour running 10 minutes every other day for 30 days would need 500 gallons per month. So, the rainfall that would be enough to sustain the irrigation system for June through October would need to capture 2,500 gallons.

The next step is to consider the cistern's storage capacity. We've seen that there would be enough rain on average to run your irrigation system. So we need to calculate the size of the cistern to accommodate the rain. Say you wanted to just have enough to run your system you would need a RainBox which would hold a minimum of 2500 gallons of water.

Each RainBox holds 32 gallons of water. 

An area 11 ft. X 9 ft. X 4.5 ft deep would hold 96 RainBoxes which would be 3,072 gallons of rain water. If you wanted to capture the average amount of rain produced in a year you can see you would need a cistern that would hold approximately 3,500 gal. To catch all the average amount of rain you would need an area 13 ft. X 12 ft. X 4.5 ft. deep would hold 135 RainBoxes which would hold 4,320 gallons of rain water.