Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Annual cost savings
$1,847
Annual gallons saved
194,472
Total retrofit cost
$8,400
Payback period
4.55
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How to use this
- 1Enter number of toilets to retrofit.
- 2Enter current toilet flush volume (gal/flush).
- 3Enter low-flow toilet flush volume (gal/flush).
- 4Enter uses per fixture per day.
- 5Enter combined water + sewer rate ($/1,000 gal).
- 6Enter installed cost per toilet ($).
- 7Read your annual cost savings on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
Retrofitting older fixtures with WaterSense-certified low-flow toilets, faucets, and showerheads is one of the fastest-payback water conservation investments available, often paying back in under two years in commercial buildings with moderate to heavy restroom traffic. This calculator compares your current fixture flow rates against low-flow replacements, factoring in daily uses per fixture and combined water plus sewer cost (sewer rates are often based on metered water use even though the water doesn't return to the sewer system, which is a detail that inflates real-world savings beyond the water cost alone). It's the standard calculation behind LEED water-efficiency credit applications and behind most municipal water utility rebate program qualification worksheets, which typically require documented baseline and post-retrofit flow rates.
Worked example
Using the values the calculator loads with:
Inputs
- Number of toilets to retrofit: 30
- Current toilet flush volume: 3.5 gal/flush
- Low-flow toilet flush volume: 1.28 gal/flush
- Uses per fixture per day: 8
- Combined water + sewer rate: 9.5 $/1,000 gal
- Installed cost per toilet: 280 $
Results
- Annual cost savings: $1,847
- Annual gallons saved: 194,472
- Total retrofit cost: $8,400
- Payback period: 4.55
What each field means
Inputs
- Number of toilets to retrofit
- The number of toilets to retrofit used in the calculation. Starts at 30 so you have a working example on load.
- Current toilet flush volume (gal/flush)
- The current toilet flush volume used in the calculation, measured in gal/flush. Starts at 3.5 gal/flush so you have a working example on load.
- Low-flow toilet flush volume (gal/flush)
- The low-flow toilet flush volume used in the calculation, measured in gal/flush. Starts at 1.28 gal/flush so you have a working example on load.
- Uses per fixture per day
- The uses per fixture per day used in the calculation. Starts at 8 so you have a working example on load.
- Combined water + sewer rate ($/1,000 gal)
- The combined water + sewer rate used in the calculation, measured in $/1,000 gal. Starts at 9.5 $/1,000 gal so you have a working example on load.
- Installed cost per toilet ($)
- The installed cost per toilet used in the calculation, measured in $. Starts at 280 $ so you have a working example on load.
Results
- Annual cost savings
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Annual gallons saved
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total retrofit cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Payback period
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why does sewer cost matter here if the water doesn't go into the sewer differently?
Most utilities bill sewer charges based on metered water consumption as a proxy for wastewater volume, regardless of where the water actually ends up, so cutting water use cuts your sewer bill proportionally too. That combined rate is why fixture retrofits often show better paybacks than a water-only rate calculation would suggest.
Are there rebates for low-flow fixture retrofits?
Many water utilities, especially in drought-prone regions of the Southwest and California, offer per-fixture rebates for WaterSense-certified toilet and faucet replacements, sometimes $50-200 per toilet. Check your local water utility's conservation program before budgeting the full retrofit cost out of pocket.
Do low-flow toilets actually work as well as older high-flow models?
Early-generation low-flow toilets (mid-1990s to early 2000s) had a real reputation for clogging and requiring double-flushing, but current WaterSense-certified models (1.28 gpf or less) use improved bowl geometry and trapway design that perform comparably to older 3.5-gallon models in independent flush-performance testing (like the Maximum Performance Testing program).
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Water Use Intensity Calculator
Gallons of water per square foot or per employee, benchmarked.
LED Retrofit Payback Calculator
Simple and rebate-adjusted payback period for switching to LED.
Building Energy Use Intensity (EUI) Calculator
kBtu per square foot per year, benchmarked against ENERGY STAR medians.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Water Fixture Retrofit Savings Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/water-fixture-retrofit-savings
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/water-fixture-retrofit-savings" target="_blank" rel="noopener">Water Fixture Retrofit Savings Calculator — RevenueLab</a> (2026).</p>
Source: [Water Fixture Retrofit Savings Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/water-fixture-retrofit-savings) (2026).