Cistern Size for Family of Four: How Much Water Is Needed?

Water Demand and Cistern Sizing

To determine the ideal cistern size, we must consider the family’s water needs, which include drinking, cooking, hygiene, and other domestic uses. A general rule of thumb is to allocate 80-100 gallons per person per day (gpd). For a family of four, this translates to 320-400 gpd. Additionally, we need to account for the cistern’s reserve capacity, which should be at least 1,000 gallons to ensure a buffer during droughts or low rainfall seasons. This reserve capacity, combined with the daily water demand, will help us determine the minimum cistern size.

Calculating Cistern Size with Rainwater Harvesting

When incorporating rainwater harvesting, we can reduce the cistern size by accounting for the amount of water collected from the roof. A general rule of thumb is to allocate 0.7-0.8 inches of rainfall per square foot of roof area per year. For a typical roof with a 1,500 square foot area, this translates to 1,050-1,200 gallons of rainwater collected per year. By combining this harvested water with the allocated daily water demand, we can reduce the required cistern size. For example, if we allocate 350 gpd (average of 320-400 gpd) and collect 1,150 gallons of rainwater per year, we can size the cistern for approximately 5,000 gallons to meet the family’s water needs.

Sizing Considerations and Recommendations

When sizing a cistern, it’s essential to consider the water quality, storage tank material, and potential for water losses due to evaporation, seepage, or overflow. We recommend using a minimum tank material thickness of 1/8 inch for steel tanks and 1/4 inch for plastic tanks. Additionally, consider installing a first flush device to ensure clean water is collected and stored in the cistern. A well-designed and properly sized cistern will provide a reliable water supply for the family, reducing their reliance on external sources and promoting water conservation.