Size a rainwater cistern based on daily demand and the longest expected dry spell. Calculate the tank volume needed to bridge gaps between rain events.
A cistern bridges the gap between rainy periods and dry spells, storing enough water to meet your non-potable (or potable) demand even when it hasn't rained in weeks. Proper cistern sizing ensures you never run out while avoiding the cost and space of an oversized tank.
The fundamental sizing formula is simple: multiply your daily water demand by the number of days in the longest expected dry spell. This gives you the minimum storage volume. Adding a safety factor of 10–20% accounts for unexpected demand or longer-than-average droughts.
Cisterns range from simple 500-gallon above-ground tanks to buried 10,000+ gallon systems. Materials include polyethylene, fiberglass, concrete, and galvanized steel. The right choice depends on your volume needs, available space, climate, and budget. This calculator gives you the target volume — from there, you can shop for systems that match.
By calculating this metric accurately, energy analysts gain actionable insights that inform equipment selection, system design, and operational strategies for maximum efficiency and savings.
Under-sizing a cistern means running out of water during dry spells; over-sizing wastes money and space. This calculator gives you a data-driven target based on your actual demand and local climate. This quantitative approach replaces rough estimates with precise figures, enabling facility managers to identify the most cost-effective opportunities for reducing energy consumption.
Cistern Size (gal) = Daily Demand (gal) × Dry Spell (days) × (1 + Safety Factor)
Result: 1,725 gallons
50 gal/day × 30 days = 1,500 gallons base. With a 15% safety factor: 1,500 × 1.15 = 1,725 gallons. A 2,000-gallon tank would be the nearest standard size.
The dry spell duration drives cistern size more than any other factor. A 15-day dry spell requires half the storage of a 30-day dry spell. Invest time in researching your local climate data to get this number right — it directly affects system cost.
Above-ground cisterns are cheaper to install and easier to maintain but take up yard space and are subject to temperature extremes. Buried cisterns are invisible, thermally stable, and protected from UV, but cost 2–3× more to install due to excavation and structural requirements.
Even well-sized cisterns can run dry during exceptional droughts. Plan a backup supply — whether a municipal fill connection, well, or delivered water — so you're never without water when the cistern is empty.
If using harvested water for irrigation only, estimate garden demand (typically 50–200 gal/day depending on area and climate). For whole-house non-potable, add toilet flushing (15–25 gal/day) and laundry (20–30 gal/day).
Review historical weather data for your area. Look at the longest consecutive dry period over the past 10–20 years. Use this as your design dry spell rather than the average.
A 10–20% safety factor is standard. In areas prone to drought or with limited municipal backup, increase to 25%. If you have a municipal fill backup, 10% may be sufficient.
Polyethylene (most common, affordable, lightweight), fiberglass (durable, UV-resistant), concrete (long-lasting, heavy, expensive), and galvanized steel (durable, can be lined). Each has trade-offs in cost, lifespan, and installation complexity.
Yes. Some people use rain barrels for immediate garden use and a larger cistern for long-term storage. The systems can be connected so barrels overflow into the cistern.
Above-ground polyethylene tanks cost $0.50–$1.50 per gallon of capacity. A 2,500-gallon tank runs $1,500–$3,000 plus installation. Buried systems cost more due to excavation and site work.