Cistern Sizing Calculator

About the Cistern Sizing Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter your daily water demand in gallons (irrigation, toilet flushing, etc.).
2. Enter the longest expected dry spell in days.
3. Optionally add a safety factor percentage.
4. View the recommended cistern size in gallons.
5. Shop for tanks that meet or exceed this volume.

Example Calculation

Tips & Best Practices

• Consult local weather data for reliable dry-spell duration — use the 90th percentile, not the average.
• Above-ground tanks are easier to install but exposed to temperature swings.
• Buried cisterns stay cool and are out of sight but cost more to install.
• Include a first-flush diverter in your system to improve water quality.
• Install a float valve or automatic fill from municipal supply as a backup.
• Opaque materials prevent algae growth; translucent tanks need UV treatment.

Design Dry Spell: The Key Variable

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 vs. Buried

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.

Backup Supply Planning

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.

Frequently Asked Questions

• What daily demand should I use?

  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).

• How do I determine my longest dry spell?

  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.

• What safety factor should I add?

  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.

• What tank materials are available?

  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.

• Can I combine a cistern with rain barrels?

  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.

• How much does a cistern cost?

  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.