There are a number of different sources of climatic data online, NOAA (National Oceanographic and Atmospheric Administration and NCDC (National Climatic Data Center) both offer precipitation patterns specific locations

Identify and add up all of your potential uses, make an approximation of periodic rainfall in your area, measure your intended collection area (square feet of roof area) and calculate the potential volume that can be collected during a particular period of time. There are tabular charts available that can help simplify this process. Based on a 1" rainfall event, you can expect to collect between ½ and ¾ gallon of rainwater per square foot of roof area.

Rainfall frequency and potential collection volume are the primary factors in calculating required storage volume. An additional factor to consider is if you will depend entirely on rainwater or plan to supplement this volume during dry periods from an additional source.

No matter the capacity of the storage facility, it is critical to include an overflow. Overflow conditions can occur from extraordinary rainfall events to simple underutilization of the stored water. The overflow should be part of the system design to insure that it does not result in nuisance flooding or erosion issues. An actual benefit of an occasional overflow is to flush some of the floating debris that may accumulate in the storage vessel.

There are two areas to consider here:
a) Inlet Filtering of the collected water before it enters the storage vessel.
Regardless of the final intended use of the water, it is important to filter the water before it enters the storage facility to remove particulate, organic material and sediments that will increase the frequency of tank cleaning and therefore the cost of system ownership. Inlet filters available for smaller systems can be a simple screened baffle that installs in the downspout to large dual cascade filter packages mounted above or adjacent to the storage tank for larger systems.

b) Outlet Filtering the water as it is being supplied to point of final use.
Many systems designed to provide water for irrigation, plant watering and car washing may use only a screen in the pump suction or to filter large particulate.
If the system is used for non-potable indoor use i.e. toilet flushing or utility use should include sediment filters designed for periodic cleaning/replaceable filtration elements.

While collected rainwater can be used for drinking, it is essential to remember that unlike well water which has the benefit of natural purification in the subterranean aquifer, rainwater is considered surface water. All rainwater collection systems intended for potable (drinking water) use should be designed by a professional who takes into consideration the roofing material, collection system/piping materials, inlet and outlet filtration as well as the disinfection. Roth MultiTanks are manufactured with a wetted inner layer of HDPE that is NSF listed for drinking water contact which makes our tanks suitable for these applications.

Always check with local municipal authorities to be aware of codes and regulations that may impact your plans to install a rainwater harvesting system. This is particularly important in the Rocky Mountain and Southwestern US areas where extensive and longstanding water shed and water right policies are in force, you may find that you have no right to retain the rainwater that drips from the eves of your own roof!

Certainly the collection and use of rainwater will reduce annual household cost of water utility. There are drought affected areas of the US where use of alternate sources of water for irrigation/landscape management is becoming a building code requirement and a rainwater collection and storage system may allow the homeowner to avoid water use penalties while protecting property investment. LEED building credits are available for new construction depending on building class for including a rainwater harvesting system in project plans.