Enter your current water usage, local rate, and tank details to find out exactly how much you can save on your water bill — plus how fast the tank pays for itself.
Fill in your usage and costs — results update as you type
Current Water Usage & Cost
Daily Usage
Water Rate
Tank & Replacement Details
Tank Cost
$
% Replaced
%
Rate Increase
%/yr
Sewer charges & running costs optional
Sewer Usage Charge
$/L
Only if your bill charges sewer per unit of water used. Same unit as your water rate.
Tank Water Used For
Garden water never reaches the sewer, so it saves no sewer charge.
Running Costs
$/yr
Pump electricity, filters, cleaning. Subtracted from savings.
⚠ Please enter valid values for Daily Usage, Water Rate, and % Replaced (1–100%). Tank Cost and the other fields are optional.
Your Savings Results
Net Savings in Year 1
—
per year
Monthly Savings
—
Daily Savings
— /day
Water Replaced
— L/day
Payback Period
—
5-Year Savings
—
10-Year Savings
—
Water Charge Saved
— /yr
Sewer Charge Saved
— /yr
10-Year Net Gain
—
10-Year Cumulative Savings vs. Tank Cost
📖 How To Use
How to Use This Water Bill Savings Calculator
Getting an accurate savings estimate takes less than a minute:
Enter your daily water usage
Check your most recent water bill for monthly or quarterly usage, then divide to get a daily figure. US EPA WaterSense puts the average American family at more than 300 gallons (about 1,135 litres) a day at home; smaller households and homes outside the US often use much less. Your bill is the best source. You can switch between litres and gallons.
Enter your water rate
Find the volumetric (usage) rate on your bill — usually per kilolitre (kL) or cubic metre, per 100 cubic feet (CCF), or per 1,000 gallons. Select the matching unit. If your tariff is tiered, use the rate of the top tier you reach, because that is the water the tank saves. Leave out fixed service charges — a tank doesn't reduce them.
Enter your total tank cost
Include the purchase price plus installation, plumbing, first-flush diverter, pump and any rebate you will receive (as a negative adjustment to the price). This is used for the payback period. If you just want to see savings potential, you can leave it blank.
Set the percentage of water the tank will replace
This is the share of your total use the tank will actually supply over a year. EPA WaterSense data suggests toilets are about 30% of indoor use (around 20% of a typical US home's total), and outdoor use about 30% nationally. A tank can only supply what it collects, so in dry spells the real figure is lower — check with our Rainwater Harvesting Calculator.
Set your expected annual rate increase
Check your utility's published price path if it has one. Entering an increase shows how savings grow over time; leave it at 0 for a cautious estimate.
Tip: Then add the optional fields. If your bill charges sewer per unit of water, indoor uses like toilet flushing save that too. And subtract running costs — a pump's electricity and filter changes can eat a noticeable share of a small tank's savings.
📐 The Formula
How We Calculate Your Water Bill Savings
The savings model has four parts: water charges avoided, sewer charges avoided (where they apply), running costs, and price growth over time.
Daily water replaced (L) = Daily usage × (% replaced ÷ 100) Gross savings (Year 1) = Daily replaced × 365 × (water rate + sewer rate × indoor share) Year N gross savings = Year 1 gross × (1 + rate increase %)^(N−1) Year N net savings = Year N gross − running costs Payback period = years until cumulative net savings reach the tank cost
Cumulative savings over 5 and 10 years are the sum of each year's net savings. Running costs are held flat, which is slightly optimistic if electricity prices rise too.
Reference Water Rates by Region
How your bill shows it
Example rate
Per Litre
500 L/day for a year
Per kilolitre — Sydney Water, 2026–27
A$3.41/kL
A$0.00341
A$622
Per cubic metre (same as kL)
2.00/m³ (example)
0.0020
365
Per 1,000 US gallons
$6.00 (example)
$0.00159
$290
Per CCF (100 cu ft = 2,832 L)
$4.00 (example)
$0.00141
$258
Only the Sydney row is a real tariff (drinking water usage charge, Sydney Water, 1 July 2026 – 30 June 2027; Sydney has no separate per-kilolitre sewer charge for homes). The other rows show the unit conversion with example prices — use the rate on your own bill.
Typical Savings by Tank Application
Application
Share of a typical US home's use
At 500 L/day
Garden/irrigation only
about 30% nationally (more in dry areas)
up to ~150 L/day in season
Toilet flushing
about 20% (≈30% of indoor use)
~100 L/day all year
Toilets + garden
about 50%
~250 L/day
Whole house (treated)
up to 100%
limited by rainfall and tank size
Shares from US EPA WaterSense (70% of household use is indoors; toilets are nearly 30% of indoor use). These are upper limits — the tank only supplies what it has collected, so real replacement depends on your rainfall, roof area and tank size.
💡 When To Use
Who This Calculator Is For
Homeowners Considering a Rainwater Tank
If you're weighing the upfront cost of a rainwater tank against long-term water bill reductions, this calculator gives you the payback period and 10-year ROI to make an informed decision. Rising water prices improve the case over time, but be realistic: at modest water prices a tank may take a long time to pay back on bill savings alone.
Property Developers and Builders
Many municipalities now require water-efficient design or offer rebates for rainwater harvesting. Use this calculator to model the savings for prospective buyers and demonstrate return on investment as a selling point.
Farmers and Agricultural Operators
Irrigation water at agricultural rates can be expensive. If you're connected to a metered supply for any portion of your irrigation, a storage or harvesting tank can yield substantial savings — especially in drought-prone seasons when rates spike.
Businesses Reducing Operating Costs
Commercial water usage for cooling towers, landscaping, toilet facilities, and cleaning operations is a significant cost centre. Even a 20% reduction in mains usage can produce thousands of dollars in annual savings at commercial tariff rates.
Be realistic: Payback varies enormously. Where water is cheap, a household rainwater tank may not pay for itself on bill savings alone within its life; where water is expensive, rebates are available, or the tank also provides backup supply, the case is much stronger. Run your own numbers rather than relying on a rule of thumb.
🧮 Worked Example
Worked Example: A Sydney Household Tank
A household uses 500 L a day and plans a tank for toilets and garden that should supply about 40% of its use (200 L/day). The tank costs A$3,000 installed, the pump and filters cost about A$40 a year to run, and the household assumes prices rise 3% a year.
Step
Working
Result
Water saved per year
200 L × 365
73,000 L (73 kL)
Water charge avoided
73 kL × A$3.41
A$249
Sewer charge avoided
Sydney has no per-kL sewer charge
A$0
Net Year 1 savings
A$249 − A$40
A$209
Payback
Cumulative net savings reach A$3,000
about 11.8 years
10-year net savings
Sum of Years 1–10
A$2,454
On bills alone, this tank takes about 12 years to pay back — well within a typical 20-year life, but not quick. Rebates, a cheaper tank, a higher replacement share or a utility that charges sewer per kilolitre would all shorten it. A tank also has value bills don't capture, such as water for the garden during restrictions.
❓ FAQ
Frequently Asked Questions
How much can I save on my water bill with a rainwater tank?
Savings depend on your water rate, daily usage, and how much mains water the tank replaces. For example, a Sydney household using 500 L/day that replaces 30% of it (150 L/day) avoids about A$187 a year at Sydney Water's 2026–27 usage charge of A$3.41/kL. With a 3% yearly price rise, that grows to about A$210 by Year 5. Subtract any pump and filter running costs for the net figure. Use this calculator with your own numbers for a precise figure.
How do I find my water rate per litre?
Look at your most recent water bill. Find the volumetric charge — it may be listed as per cubic metre (m³ = 1,000 L), per kilolitre (kL = same as m³), per 100 cubic feet (CCF = 2,832 L), or per 1,000 US gallons (3,785 L). Divide the rate by the volume to get a per-litre figure, or use the unit selector in this calculator — it handles the conversion automatically. Use the usage (volumetric) charge only, not fixed service charges.
What percentage of water use can a rainwater tank replace?
It depends on what you connect the tank to and your local rainfall. According to US EPA WaterSense, toilets account for nearly 30% of indoor use — around 20% of a typical home's total — and outdoor use is about 30% nationally. So toilets plus garden can be around half of household use, if the tank can keep up. In practice the limit is often rainfall and tank size: a tank that runs dry in summer replaces much less than its connected uses. Our rainwater calculators show how much your roof and tank can actually supply.
How long does it take for a water tank to pay for itself?
It varies too much for a single answer. Payback = tank cost ÷ net yearly savings, adjusted for rising prices. In the Sydney example above it is about 12 years; with a cheaper tank, a rebate, higher water prices or a sewer charge per kilolitre it can be much shorter, and with cheap water it can be longer than the tank's life. Always subtract running costs, which many estimates leave out.
Are there government rebates for water tanks?
Yes — many governments offer rebates or subsidies for rainwater tank installation, particularly in water-stressed regions. Schemes come and go, and many have closed, so check your water utility's and local government's websites for what is currently on offer. Where a rebate is available, subtract it from the tank cost in this calculator — it can shorten the payback period considerably.
How do I calculate water bill savings for a business or commercial property?
Enter your business's total daily water consumption (from your bill), your commercial tariff rate, the total installation cost, and the percentage you expect to replace with stored water. Commercial rates are often higher than residential, so savings and ROI can be substantially better. For large sites, consider running multiple scenarios with different replacement percentages to find the optimal tank size.
Why does the rate increase percentage matter so much?
Because savings grow with the price of water. At a 5% yearly increase, a tank that saves $200 in Year 1 saves about $268 in Year 7, and total savings over 10 years are about 26% higher than with flat prices ($2,516 vs $2,000). That shortens the payback period compared with simply dividing tank cost by Year 1 savings. Price rises aren't guaranteed, so also run the calculator with 0% for a cautious view.
✅ Sources & Review
Sources and review
The usage shares and the example tariff on this page come from these references:
US EPA WaterSense: How We Use Water — average family uses more than 300 gallons a day at home; about 70% indoors and 30% outdoors nationally.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
