How Much Rainwater Will My Roof Collect

Rainwater collection calculation formulas and equations.

How much rainwater will my roof collect. Keep in mind most rain falls over 2 3 months of a year. Annual rainfall in millimetres x roof surface area in square metres roof catchment capacity. Roughly speaking 1 millimetre of rain over 1 square metre of roof equals 1 litre of water. Roof area ft2 x precipitation amount in x 0 623 amount collected gallons easy formula to remember.

1 of rain x 1 sq. Concrete tile may leave you with alkalinity in your water but this isn t necessarily a problem. You can use the precipitation map below to find an approximate amount for your area. Most cisterns and full rainwater collection kits can collect several hundred gallons of water or more at a time.

1 of rainfall over 1 000 sf will yield 623 gallons to calculate the amount of rainwater you can collect you need to know your annual average precipitation for your area. To get an idea how much water the roof of your own house might yield use the rainfall harvest calculator above. That s 600 less gallons you have to pay for and use from your town water supply. With over 600 gallons of high quality water available from a single inch of rain falling on 1000 square foot surface even an 85 collection efficiency estimated for clay tile will give you a lot of high quality water.

It isn t uncommon for an area to collect up to 350mm of rain in just one month even in rural areas. Amount of water since los angeles is commonly plagued by drought you may be wondering if collecting rainwater is useful. An average suburban roof of 200m 2 with such a rain fall can potentially harvest 100 000l of rainwater 200m 2 500mm 100 000l. 0 623 gallons easy to remember formula.

Roof area m2 x precipitation amount mm amount collected liters. So 70 80 1400 means you have 1225 net sq ft of collection area. If your average rainfall was 25 inches year your annual collection potential is 1 120 x 25 28 000 gallons year. So each inch of rain on your roof will have 763 gallons of water going down your downspouts.

1 of rain on 1 000 sf roof will yield 623 gallons. Just a half inch of rain falling on a 1 000 square foot roof will yield 300 gallons of water. To find how much rain you can collect in an average rain year multiply this number by the average inches of rain. That is the theoretical best case outcome for this scenario.

When you look at the numbers it s surprising how much water you can collect every time it rains. One inch of rain falling on 1000 sq ft of roof will deposit 623 gallons of water. This capacity can be calculated using the following formula.