A cistern is a tank designed to collect and store water for later use. Unlike a well, which taps into groundwater, a cistern holds water that’s been delivered by truck, piped from a municipal source, or captured from rainfall running off a roof. People rely on cisterns in areas where groundwater is scarce, well yields are low, or water quality from natural sources is poor. They range from small plastic tanks holding a few hundred gallons to massive underground chambers storing tens of thousands.
Why Cisterns Exist
The core purpose of a cistern is bridging the gap between when water is available and when you need it. Rain falls unpredictably. Delivery trucks come on a schedule. Municipal water may not reach rural properties. A cistern gives you a reserve so that dry spells, seasonal shortages, or supply interruptions don’t leave you without water. This makes them especially common in rural homes, island communities, and arid regions where consistent water access isn’t guaranteed.
Beyond primary household supply, cisterns serve several other roles. They supplement low-yielding wells that can’t keep up with daily demand. They provide emergency backup water. They store rainwater for irrigating gardens and lawns, flushing toilets, filling cooling towers, or topping off ornamental ponds and fountains. In commercial buildings, collected rainwater is often used for wash applications and landscape maintenance, reducing the draw on treated municipal water.
A Technology Older Than Written History
Cisterns are among the oldest engineered water systems on Earth. The earliest known examples date to the Neolithic Age, when people in the Levant (modern-day Syria and Lebanon) built waterproof lime-plaster cisterns directly into the floors of their homes. By around 2,500 BC, the site believed to be the biblical city of Ai contained a massive cistern carved from solid rock, lined with large stones, and sealed with clay. It held nearly 450,000 gallons.
The Minoans on Crete built cylindrical underground cisterns ranging from about 5 to 23 feet in diameter and 8 to 16 feet deep, sealed with hydraulic plaster and fed by networks of terracotta pipes that channeled rainwater from surrounding surfaces. Greek islands like Santorini and Delos had smaller pear-shaped cisterns carved into rock for individual households. The Maya constructed bottle-shaped underground cisterns called chultuns, dug into limestone bedrock beneath buildings and ceremonial plazas, then plastered with cement.
In all these civilizations, the logic was the same: collect rain during the wet season so the community survives the dry one. Cisterns were also critical during sieges, when an enemy could cut off external water sources. And they weren’t always used for water. During biblical times, cisterns doubled as underground hiding places, prison cells, and even burial chambers.
Above Ground vs. Underground
Modern cisterns come in two basic configurations, each with trade-offs. Above-ground tanks are cheaper to install and easier to inspect, but they’re exposed to sunlight (which promotes algae growth) and temperature swings that can cause freezing in cold climates. Underground tanks maintain a consistent water temperature year-round, stay out of sight, and free up yard space. The trade-off is higher installation cost, since excavation is required.
Common Materials
The material you choose affects cost, lifespan, and maintenance. Polyethylene (plastic) tanks are the most affordable option, typically around $1,700 for a 2,600-gallon tank. They won’t rust, corrode, or leach chemicals into the water. Steel tanks cost more, ranging from about $2,200 to $3,700 for a similar size depending on the grade of steel. Galvanized steel resists rust through a zinc coating, while stainless steel is more resilient but pricier. Fiberglass tanks can last over 40 years with proper maintenance and handle temperature swings well, though they have seams that can be vulnerable over time. Concrete cisterns are the most durable and heaviest option, resistant to rust and physical damage during installation, making them a common choice for large underground systems.
Sizing a Cistern for Your Property
If you’re collecting rainwater, cistern size depends on your roof area and local rainfall. The basic formula: for every inch of rain, you can harvest about 0.62 gallons per square foot of roof. So a 1,800-square-foot roof in a location that gets 10 inches of usable rainfall annually would yield roughly 11,160 gallons per year.
A practical guideline from the New Mexico Office of the State Engineer recommends sizing your cistern at about one-third of your normal annual harvest. That accounts for the fact that rain comes in bursts and you’ll be using water between storms. For that 1,800-square-foot roof example, a cistern in the 2,000 to 3,000 gallon range would be appropriate. If your budget and space allow, going up to one-half of your annual harvest reduces overflow from heavy storms. To visualize capacity: a 1,000-gallon cistern is roughly the size of a compact car, a 2,000-gallon tank is about the size of a minivan, and a 10,000-gallon cistern would fill a large flatbed trailer.
Making Cistern Water Safe to Drink
Rainwater collected in a cistern is not automatically safe to drink. Rooftop runoff picks up bird droppings, dust, pollen, and whatever else has settled on the surface. If you plan to use cistern water for drinking, cooking, or bathing, it needs filtration and disinfection. Cisterns used for domestic purposes should only be filled with treated, potable water, never directly from an untreated lake, river, or rainfall without proper treatment in place.
After flooding or heavy contamination, the CDC recommends a thorough cleaning and disinfection process. You scrub the interior with a stiff brush using a solution of one cup of unscented household bleach per 10 gallons of water. Then you add a stronger bleach solution (3 cups per 100 gallons), run it through every faucet until you can smell chlorine, and let it sit for at least 12 hours before draining, refilling with safe water, and flushing until the chlorine smell is gone. After that, adding one tablespoon of bleach per 100 gallons helps prevent microbial growth in storage.
Ongoing Maintenance
A cistern isn’t a set-and-forget system. The City of Philadelphia’s maintenance guidelines offer a useful baseline. During your first year, inspect inlet structures, outlet structures, and storage areas monthly to get a sense of how quickly sediment accumulates. After that, quarterly inspections of the cistern and its control structures are typical. Gutters, gutter screens, and any first-flush diverters or catch basins should be cleaned as needed to keep debris from washing into the tank. Once a year, remove accumulated sediment and debris from the cistern floor, brush the inside surfaces, and disinfect thoroughly.
Legal Considerations for Rainwater Collection
If you’re planning to install a cistern for rainwater harvesting, check your state and local laws first. Most states allow it, but regulations vary significantly. Colorado, for example, restricted rainwater collection for over a century under its water rights laws, since capturing rain on your property could reduce flow to downstream claimants. The state loosened those rules in 2009 to allow limited individual collection. At least 18 other states have set specific limits on how much you can collect and what you can use it for. Some states have no statewide law at all, leaving regulation to local ordinances. A handful still actively restrict collection and use.