Looking to calculate your home rainwater collection volume from regular rainfall events? Calculating the amount of rainfall in gallons of water involves variables fully dependent on where you live and the entire size of the rainwater harvesting (RWH) system.

Needed variables include: (1) Expected rainfall depth in inches; (2) Catchment area size in square feet, and; (3) Expected RWH system efficiency percentage.

**Rainfall Collection Equation**

Total Harvested Rainwater = Rainfall Depth (in) x Catchment Area (ft^{2}) x 0.623 x System % Efficiency

Knowing how to calculate the gallon-amount of rainwater collected by a system is highly useful for rainwater harvesters across residential home owners, commercial/industrial businesses, as well as agriculture and livestock uses. The rainfall collection equation is valuable for approximating and balancing water supply and demand, in long-term water projections, and when tank maintenance may be needed as well.

To calculate the amount of water a rain barrel may collect from local rainfall: use the Rainfall Collection Equation. The individual parts of the equation are explored below.

**Equation | Rainfall Depth**

As may be obvious, but the more rainwater volume during rain events and/or more frequent rain events, directly relates to the total amount of water that will be available to be collected by your RWH system. The amount of this rainfall volume–depth in inches–varies extensively with your system location and geographic area. Knowing what annual average precipitation rates are for your specific location is important in determining your RWH system’s functionality in terms of the total water volume that will be available for supply and use.

**Equation | Catchment Area**

The rainfall catchment area is the rainfall’s initial point of contact with the RWH system. Catchment areas will have large, exposed surface areas intending to capture rainwater on impact and direct it toward system plumbing, pre-filters, diverters, and last to system tank(s).

Catchment areas are often the roofs of buildings, but they can be any effective rain-collecting device or structure. Roof catchment areas are measured in square feet based on the roof’s footprint / perimeter. Slope, pitch and height of the roof do not typically affect rainfall collection volume, except in instances of overflow and rapid water velocity. Catchment areas with greater total surface areas will be able to capture more water.

When considering catchment area size, measure and calculate using only those areas connected with the rainwater harvesting system. For example, rain gutters diverting water to rain tanks are installed only on 80% of the roof’s total footprint area, so only 80% of the catchment area’s square footage should be considered and incorporated into the rainfall collection equation.

**Equation | Conversion Factor**

The rainfall equation features a conversion factor: 0.623. This value, 0.623, is an equation constant derived from unit values used to convert inches of rainfall over a square foot area to total volume of gallons rainwater. The conversion factor remains constant while the equation’s other variables change.

**Equation | RWH System Efficiency**

The last component of the rainfall equation is system efficiency. In rainwater harvesting, system efficiency refers to the percentage of the catchment area / roof that is actually connected to system waterways and plumbing. System efficiency is also designed to account for water loss that may be associated with rapid rainfall that can cause overflow and water loss due to constraints experienced by the system components or steep catchment areas.

**Rainfall Equation Takeaways**

After understanding your local annual rainfall averages, rain catchment size, and system efficiency, the total potential volume of collected rain water can be calculated using:

Total Harvested Rainwater = Rainfall Depth (in) x Catchment Area (ft^{2}) x 0.623 x System % Efficiency

The equation is highly useful in approximating:

- Total rainfall volume in gallons regularly expected;
- The size of rainwater tank needed or that is appropriate, and;
- The amount of rainfall required to sustain desired use applications.

**RWH Scenarios with Example Calculations**

- Personal residence with roof footprint 1,100 ft
^{2}wants to know how many gallons of water can be captured from an average monthly rainfall amount of 2.97 inches, assuming 100% system efficiency:**Total collected rainfall (gallons)**= 2.97 inches x 1,100 ft^{2}x 0.623 x 100% =**4,142 gallons**of rainfall potentially collected from an average month.

- Poultry house agricultural application with 32,000 ft
^{2}metal roof RWH system wants to know how much rainfall will be required to fill the farm’s 100,000 gallon multi-rainwater tank system that operates 90% efficient:- 100,000 gallon rain tank =
**x inches**of rainfall x 32,000 ft^{2}x 0.623 x 90% =**57 inches**of rain.

- 100,000 gallon rain tank =

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The professionals here at NTO are always ready to assist every need concerning RWH systems, equipment, and questions; should you have any: contact us.