Rainwater Harvesting System Cost Calculator

Getting a system total takes eight quick entries. The tool works out your yearly harvest first, then prices the tank and the rest of the system.

1. Enter your roof area in square feet and your annual rainfall in inches, then the collection efficiency. 80% is typical.
2. Enter the tank size in gallons and the tank price per gallon, then the gutters and downspouts, the pump and filters, and the installation cost.
3. Press Calculate, or press Enter. The total appears first and large, with the yearly harvest, the tank cost and the cost per gallon of storage below. Press Reset to clear the form and start over.

The rainwater harvesting formula has two parts, the water and the money. Yearly harvest = roof area x annual rainfall x 0.623 x collection efficiency. The 0.623 is the gallons in one inch of rain over one square foot, from the Texas Water Development Board Texas Manual on Rainwater Harvesting. Collection efficiency is the share you actually keep after first flush, evaporation, splashing and leaks, usually about 80%.

Total cost = tank cost + gutters and downspouts + pump and filters + installation. Tank cost = tank size x price per gallon. The cost per gallon of storage is the total divided by the tank size, and it lets you compare a cheap barrel with a big cistern on equal terms. Our concrete calculator works out the pad a large tank sits on.

A home has a 1,000 sq ft roof, 36 in of rain a year, and 80% collection efficiency. The plan is a 2,500 gal tank at $1.50 per gallon, $600 of gutters and downspouts, $450 of pump and filters, and $800 of installation. Here is exactly what the calculator does.

1. Yearly harvest: 1,000 x 36 x 0.623 x 0.80 = 17,942 gallons.
2. Tank cost: 2,500 x $1.50 = $3,750.
3. Total: $3,750 + $600 + $450 + $800 = $5,600.
4. Cost per gallon of storage: $5,600 / 2,500 = $2.24.

As a second check, an 800 sq ft roof in 30 in of rain at 80% collects 800 x 30 x 0.623 x 0.80 = 11,962 gallons a year, and a 55 gal barrel at $1.50 a gallon with $100 of gutters costs $82.50 + $100 = $182.50.

A system can start under $100 for a single 55 gallon rain barrel, and a fully automated high capacity underground system runs several thousand dollars. Basic 55 gallon barrels run $50 to $120 each. Clean reconditioned 275 gallon IBC totes cost $100 to $250, while brand new food grade totes run $350 to $500. Large vertical plastic cisterns of 1,000 to 2,500 gallons cost $1,000 to $3,200 for the tank alone.

Above ground tanks are cheaper and easy to fit, and they suit garden watering and most homes. Underground tanks cost more to install but free up space and keep the water cool and dark, which suits larger whole house systems. Pad prep adds $150 to $800, first flush diverters and leaf strainers add $100 to $300, and a pump adds $250 to $700 when you need pressure.

Your harvest is the water your roof can supply, not the water you will use. Size the tank to hold about 18 days of water, which is the standard UK method in BS 8515. Work from whichever is smaller, the rain your roof collects or the water you use. A tank of about 5% of your yearly harvest is a good starting point.

Garden water needs only a screen and a first flush diverter. House water needs more, with cartridge filters and a UV unit for drinking use, and that treatment adds to the price. Many water districts offer rebates of $0.50 to $1.00 per gallon of installed storage, so check your local district before you buy.

Potable treatment is the biggest jump, because filters and a UV unit add hundreds to thousands of dollars. Long plumbing runs add pipe and labor, and an underground cistern adds excavation on top of the tank price. A pressure pump with a pressure tank and switches adds $250 to $700 when gravity feed is not enough.

Check local rules before you build, because some areas restrict rainwater use or require permits for large cisterns. For water treatment inside the house, our home water softener installation cost calculator prices a related water system the same way. Get itemized quotes for the tank, the pump and the install separately, because those three lines move the total the most.