How are low-flow toilet water savings calculated?

Subtract the replacement toilet's average volume per flush from the existing toilet's volume, then multiply by flushes per person, household size, days occupied, and the share of household flushes handled by the replaced toilets.

How much water does a WaterSense labeled toilet use?

EPA states that WaterSense labeled toilets use 1.28 US gallons per flush or less and are independently certified for efficiency and performance.

What flush rate should I use for an older toilet?

Use the rated volume marked on the fixture when possible. EPA planning assumptions are about 5 gallons per flush before 1980, 3.5 gallons for 1980 through 1994, and 1.6 gallons after 1994.

How does the calculator handle a dual-flush toilet?

It calculates a weighted average from the full-flush volume, reduced-flush volume, and the percentage of flushes expected to use the reduced mode.

Does the payback include all costs and bill charges?

No. Simple payback uses the purchase and installation costs, rebates, and variable water or sewer rate entered. It excludes fixed utility charges, financing, maintenance, taxes, and changes in usage.

Are my household and cost inputs tracked?

No. Calculations, copying, and text-file downloads happen locally in your browser, and the tool does not store or transmit the values entered.

Low-Flow Toilet Water Savings Calculator

Compare an existing toilet with a low-flow, WaterSense labeled, or dual-flush replacement. Estimate water savings, variable utility savings, project cost, and simple payback. Your inputs stay in your browser.

Toilet replacement details

1. Compare flush volumes

Volume units Existing toilet estimate Existing volume (gal/flush)The fixture's marked rating is better than an age estimate. Replacement type

Replacement estimate Replacement volume (gal/flush)

2. Household toilet use

People in householdEPA suggests 2.6 if household size is unknown. Flushes per person per dayEPA planning default; edit for your household. Occupied days per year Flushes served by replaced toilets (%)Use less than 100% when other toilets will remain in service.

3. Utility rate and project cost

Currency Variable water + sewer rateInclude only charges that change with use. Rate basis Toilets purchasedUsed for project cost, not to multiply household flushes. Purchase cost per toilet Installation cost per toilet Rebate per toilet Analysis period (years)

Using EPA planning defaults for a 1980–1994 toilet and household use.

Estimated water and cost savings

How the toilet savings calculation works

Water-use formulas

weighted dual-flush volume = full volume × (1 − reduced share) + reduced volume × reduced share

annual water saved = (old volume − new average volume) × people × flushes/person/day × occupied days × use share

The tool treats household flushes as a total demand. The number of toilets purchased affects project cost, while the “flushes served” input controls how much of household demand moves to the new fixtures.

Cost and payback

annual bill saving = annual volume saved ÷ billing volume × variable rate

simple payback = net project cost ÷ annual bill saving

Net project cost is purchase plus installation minus rebates. Payback is not shown when annual savings are zero or negative. A rate per cubic metre is converted using 1 m³ = 1,000 litres; one US liquid gallon is 3.785411784 litres.

Planning limit: rated flush volume does not capture leaks, double flushing, user behaviour, seasonal occupancy, tiered tariffs, or fixture-life changes. Check the fixture label and recent utility bill for a personal estimate. Confirm product compatibility and installation requirements with the manufacturer, local code, or a qualified plumber.

Assumptions, sources, and interpretation

Last reviewed: August 3, 2026

Existing-toilet presets: the EPA WaterSense calculator methodology uses about 5 gpf for toilets installed before 1980, 3.5 gpf for 1980–1994, and 1.6 gpf after 1994. It assumes 5.05 flushes per person per day and suggests 2.6 people when household size is unknown.
WaterSense replacement: EPA residential toilet guidance states that WaterSense labeled toilets use 1.28 gpf or less and are independently certified for efficiency and performance. The page also notes that actual household savings vary.
Other replacement presets: 1.0 and 0.8 gpf are comparison scenarios, not WaterSense claims. Enter the exact rated volume from the product specification.
Cost scope: only the variable rate entered is applied. Fixed charges, minimum bills, tiers, taxes, financing, maintenance, repair, and future rate changes are excluded.

Frequently asked questions

How much water does a low-flow toilet save?

It depends on the old and new flush volumes and how often the toilet is used. Replacing a 3.5 gpf toilet with a 1.28 gpf model saves 2.22 gallons each time the replacement avoids the old toilet's flush, before accounting for leaks or repeat flushes.

What does 1.28 gpf mean?

It means 1.28 US gallons per flush, equal to about 4.85 litres. EPA's WaterSense label requires qualifying toilets to meet water-efficiency and performance criteria; not every product described as “low-flow” is necessarily WaterSense labeled.

Where can I find my toilet's flush volume?

Look for a stamp or label behind the seat hinge, inside the tank, on the tank lid, or in the model's manual. If the tank and bowl were mixed, use the complete fixture's compatible rated volume rather than estimating from tank capacity.

How should I model a dual-flush toilet?

Enter the rated full and reduced flush volumes, then estimate what percentage of uses will use the reduced button. The result is a weighted planning average, not a guarantee of real-world behaviour.

Why doesn't the calculator multiply water savings by the number of toilets?

A household's total flushes are distributed among its toilets. Multiplying the same household demand by every fixture would overcount use. Instead, set the share of household flushes handled by the replacements; the fixture count is used only for purchase, installation, and rebate totals.

Should the utility rate include sewer charges?

Include the metered wastewater portion if it changes when water use falls. Exclude fixed or minimum charges that will remain on the bill. For tiered rates, use the marginal rate that applies to the water likely to be saved.

Does a rebate make payback immediate?

Only when total rebates equal or exceed purchase and installation costs. Rebate eligibility, funding, approved products, and application timing vary, so confirm the current program before buying.

Are my inputs sent anywhere?

No. Calculations, copy actions, and the text-file download run locally in your browser. The tool does not put values into the page URL or send them to Starlight Tools.