Calculate how much rainwater a roof or catchment area collects from the rainfall depth, or find the catchment area or rainfall needed for a target volume.

Uses the horizontal catchment footprint. Collection efficiency (optional) accounts for losses; blank means 100%.

Collection losses
Share of rain actually collected after first-flush, splash and overflow losses. Roof systems are often assumed to collect 75–90%.

Related Calculators

Catchment Area Formula

CA = HW / (RFD × 0.6233766234)

Variables:

  • CA is the horizontal catchment footprint (ft²), not the sloped roof surface
  • HW is the ideal water volume (US gal) before collection and storage losses
  • RFD is the rainfall depth (in)

Divide the ideal water volume in US gallons by rainfall depth in inches, then by 0.6233766234. The familiar 0.623 factor is rounded. One inch over one square foot is 1/12 cubic foot, giving about 0.6234 US gallons. Use rainfall and water volume from the same time interval; measured collected water with unknown losses cannot identify the true catchment area.

How to Calculate Catchment Area?

The following steps outline how to calculate the Catchment Area.

  1. First, determine the ideal water volume (US gal) before losses. 
  2. Next, determine the rainfall depth (in). 
  3. Next, use CA = HW / (RFD × 0.6233766234), with US gallons, inches and square feet.
  4. Finally, calculate the horizontal catchment area; rainfall must be greater than zero for a unique inverse result.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

harvested water (gal) = 45

rainfall depth (in) = 34. The ideal catchment area is 45 / (34 × 0.6233766234) ≈ 2.1232 ft².

FAQs

How do I find catchment area for drainage?

Choose the drainage outlet, then use a topographic map or elevation survey to trace the surrounding drainage divide: the boundary enclosing land that sends surface runoff to that outlet. Measure the enclosed horizontal area, using a map scale or GIS area tool. Check the actual drainage routes where pipes, roads or other works change natural flow. For a roof, measure the horizontal footprint that drains to the collection point. This calculator relates rainfall depth, area and collected volume; it does not map a watershed boundary or estimate peak drainage flow.

What is Rainwater Harvesting?

Rainwater harvesting is the process of collecting, storing, and using rainwater from surfaces like roofs for various water needs, including irrigation, flushing toilets, and laundry, reducing the demand on traditional water supplies.

Why is the Catchment Area important in Rainwater Harvesting?

The catchment area is crucial because it determines the potential volume of rainwater that can be collected. A larger horizontal catchment area receives more rainwater for the same rainfall depth. Collection efficiency, first-flush losses and storage capacity determine how much is actually available for use.

How can I increase the efficiency of my Rainwater Harvesting system?

Efficiency can be increased by ensuring the catchment area is clean and unobstructed, using high-quality components for water collection and storage, and regularly maintaining the system to prevent leaks and contamination.

Are there any legal restrictions on Rainwater Harvesting?

In some areas, there may be legal restrictions or guidelines on rainwater harvesting, including permits, usage, and treatment requirements. It's essential to check local regulations before installing a rainwater harvesting system.