Single Flush vs Dual Flush Toilet: What to Buy (2026)

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Quick answer from a licensed plumber: Single flush is one volume; dual flush gives a reduced and a full flush. On our four picks the full flush is 1.28 gpf either way, so dual flush saves water only on the reduced button, and it adds button rods you must set right. Want the simple one? Choose single flush. Will you use the reduced button? Choose dual flush and match the exact model. Federal law allows 1.6 gpf, but California and Washington set 1.28, and MaP lists Georgia, Colorado, Texas and others as barring non-high-efficiency toilets. We did not test any toilet.

Our take: want dual flush? TOTO Aquia IV CST446CEMFGN (MaP 800 g) or American Standard H2Option 2887.518 (MaP 600 g). Want one lever? Kohler Highline K-3889 (10-inch rough-in) or Kohler Cimarron K-31621 (12-inch), both 1,000 g.

The choice is smaller than the marketing makes it look. On the four toilets below the full flush is 1.28 gallons per flush (gpf) either way, and every one of them is listed by its maker as WaterSense. This page sets the rules, the water math, the MaP flush scores and the leak points side by side, from maker sheets and code.

Single Flush vs Dual Flush at a Glance

The table shows our four picks, not every toilet.

What the Rules Say

• Federal. The Department of Energy sets 1.6 gpf as the maximum for gravity flush tank water closets (10 CFR 430.32(q)). For a dual flush model, DOE’s certification rule has the maker report the flush volume observed in full-flush mode (10 CFR 429.30). So a dual flush with a 1.6 gpf full flush is legal federally.
• WaterSense, version 1.2. The label needs an effective flush volume of 1.28 gpf or less. For a dual flush toilet, EPA defines that as the average of two reduced flushes and one full flush. The toilet must also clear all test media in a single flush in at least four of five attempts. EPA’s residential toilets page says version 1.2 is in effect.
• WaterSense, version 2.0. EPA released it in May 2024. It drops the averaging and caps both the full and reduced flush at 1.28 gpf. EPA has paused its effective date, and version 1.2 stays in effect until further notice.
• California. The California code of regulations, title 20, section 1605.3, sets 1.28 gpf, or 1.28 as the dual-flush effective flush volume, for water closets sold or offered for sale on or after January 1, 2016. Those toilets must also pass the waste extraction test.
• Washington. The state plumbing code, WAC 51-56-0400 section 411.2, sets an effective flush volume of 1.28 gpf and calls for WaterSense performance criteria.
• Other states. MaP lists California, Georgia, Colorado, Texas, Massachusetts, Oregon, Washington D.C. and New York City as places that bar non-high-efficiency toilets, with some exceptions. We did not check each law. Ask your local building department.

How Much Water Does Dual Flush Save?

• The full flush saves nothing. On our picks it is 1.28 gpf, the same as the single flush models. Water is saved only on the reduced button.
• WaterSense’s count. TOTO: (1.28 + 2 × 0.9) ÷ 3 = 1.03 gpf. American Standard: (1.28 + 2 × 0.92) ÷ 3 = 1.04 gpf. That is about 19 to 20 percent below 1.28 (our arithmetic).
• MaP’s count. MaP says the best evidence points to fewer than two reduced flushes per full flush, and for MaP PREMIUM it counts them 1 to 1. At 1 to 1: TOTO 1.09 gpf, American Standard 1.10 gpf, about 14 to 15 percent below 1.28 (our arithmetic).
• Your house. EPA says a WaterSense dual flush toilet can save more water if you use the reduced flush. If your household seldom uses that button, the saving shrinks toward zero.

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Flush Performance: What the MaP Scores Say

• What MaP measures. It is a test to failure with soybean paste and toilet paper, up to 1,000 grams. MaP says WaterSense and the ASME standard pass or fail a model at 350 grams. MaP calls 600 grams or more excellent and says 500 or more is exceptional.
• Our four models. Kohler Highline K-3889 and Cimarron K-31621, 1,000 g. TOTO CST446CEMFGN, 800 g. American Standard 2887.518, 600 g. All four carry the WaterSense flag on the list.
• No simple rule. TOTO’s Drake CST776CSFG, a 1.6 gpf single flush, also scores 1,000 g on the list. TOTO’s 1.0 gpf dual flush sibling scores 500. Four models do not make a trend, so read the score for the exact model number.
• Which button? The MaP list does not say which button was tested. American Standard labels its light flush for liquid waste, and MaP says dual flush is meant to use the reduced flush for liquids.

Dual Flush Buttons, Valves and Slow Leaks

• Button styles. Our TOTO and American Standard picks have two lid buttons. American Standard also lists ADA dual flush models with paddles, where you push the paddle full stroke and release quickly. Fluidmaster’s Duo Flush kit is a lever: up for a half flush, down for a full one.
• Rod length. Both makers tell you to adjust the button rods. TOTO says each full turn is 1 mm and warns that rods too long may let water flow to the bowl. American Standard says to turn the white rod one turn counterclockwise at a time if the toilet does not flush properly, and to turn it back 1/2 turn clockwise if the flush valve then stays open.
• The seal. TOTO’s manual has a Servicing the Flush Valve section: remove the rubber seal gasket, rinse it and reinstall. TOTO’s parts manual lists the flush valve seal gasket as 9BU112E-A, and THU744 as a 10-piece pack. See our TOTO flapper and seal page. Korky sells dual flush seals, but its fit list names American Standard tanks 4339, 4348, 4349 and 4000 and TOTO ST453U, not the tanks in our picks. Buy the seal for your tank, not for “dual flush.”
• Dye test. The ghost-flush pattern is a slow leak. Our phantom flushing guide has the dye test. My advice: on a dual flush toilet, inspect the flush valve seal where that guide says flapper.
• Water pressure. TOTO lists a minimum flowing pressure of 8 psi. American Standard’s troubleshooting calls for at least 20 psi.

Retrofit Dual Flush Kits

• Fluidmaster Duo Flush (550DFR). The installation guide (rev. 04/15) says it is not intended as a retrofit for 1.28 or 1.0 gpf toilets, and lists as not compatible 3-inch flush valves, Kohler side-mount valves, Mansfield flush valves, ballcock fill valves and some low-boy one-piece tanks. Some of these can be swapped for a part Fluidmaster names first. You set the half and full dials by flushing with toilet paper.
• Availability. A search for “dual flush” on Fluidmaster’s US site returned no results on September 29, 2026, so we link no kit.
• Korky. Its dual flush page is for replacement seals, not a conversion kit.

Before You Buy

• Measure the rough-in. The Highline is a 10-inch model. The other three are 12-inch at minimum or standard. See toilet rough-in size.
• Pick the bowl and height. Round vs elongated, comfort height vs standard, and compact toilets for small bathrooms cover fit. All four picks are two-piece and elongated; for one-piece, see one-piece vs two-piece.
• Check your state. California and Washington need 1.28 gpf or less, and MaP lists several more states, Georgia among them, that bar non-high-efficiency toilets. Use the volume on the maker sheet for your exact model.
• Compare the exact model number. Family names hide different MaP scores and full-flush volumes.
• Keep bleach out of the tank. TOTO and Fluidmaster both warn that in-tank chlorine cleaners damage fittings.

FAQ

Is a dual flush toilet better than a single flush toilet?

Not on every point, and we did not test either. On the models here the full flush is 1.28 gpf either way, so a dual flush saves water only when you press the reduced button. It also adds two buttons whose rods must be set right. MaP scores for the four models here run from 600 to 1,000 grams, so check the score for the exact model.

Can I still buy a 1.6 gpf toilet?

Federal rules still allow 1.6 gpf for gravity flush tank toilets (10 CFR 430.32(q)). California sets 1.28 gpf for water closets sold since January 1, 2016, and Washington’s plumbing code sets an effective flush volume of 1.28 gpf. A 1.6 gpf single flush toilet does not meet either rule. MaP also lists Georgia, Colorado, Texas, Massachusetts, Oregon, Washington D.C. and New York City as barring non-high-efficiency toilets, with some exceptions. Check your local code.

Why does my dual flush toilet keep refilling or running?

The makers point to a few causes. TOTO says button rods that are too long may keep the flush valve activated and let water flow to the bowl, and that chlorine in-tank cleaners can damage tank fittings. American Standard lists a leaking or deformed flapper seal or debris on the seal surface, and says a button rod turned out too far can hold the flush valve open. Run a dye test, then start with the button rods and the seal.

Can I add a dual flush kit to my toilet?

Fluidmaster’s Duo Flush installation guide says the kit is not intended for 1.28 or 1.0 gpf toilets and is not compatible with 3-inch flush valves or Kohler side-mount valves. We did not find a Fluidmaster dual flush product on its current site, so we do not link a kit.

How does WaterSense count a dual flush toilet?

Under specification version 1.2, the effective flush volume is the average of two reduced flushes and one full flush, and it must not exceed 1.28 gpf. Version 2.0 would cap both modes at 1.28 gpf, but EPA has paused its effective date, so version 1.2 stays in effect until further notice.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia journeyman plumber. Every size, rating, dimension and spec on this page comes from the manufacturer pages and documents listed below, which we opened and read, or from the Amazon listing we link; nothing here comes from hands-on testing. Each linked product was shipped or sold by Amazon, new and in stock, when we checked on September 29, 2026. Where a link goes to Amazon we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.

Sources

• US EPA WaterSense: Residential Toilets (status of specification versions 1.2 and 2.0)
• US EPA: WaterSense Specification for Tank-Type Toilets, Version 1.2 (June 2, 2014)
• US EPA: WaterSense Specification for Tank-Type Toilets, Version 2.0 (May 2024, effective date paused)
• eCFR: 10 CFR 430.32, water closets (paragraph q)
• eCFR: 10 CFR 429.30, water closet certification reports
• Cal. Code Regs. tit. 20, § 1605.3 (Cornell LII copy), plumbing fixtures Table I
• Washington WAC 51-56-0400, Chapter 4 plumbing fixtures (section 411.2)
• MaP Testing: toilet types (single-flush, dual-flush and effective flush volume)
• MaP Testing: MaP PREMIUM requirements
• MaP Testing: FAQ
• MaP Testing: background and test method
• MaP Testing: all tank-type toilets list with MaP scores (dated September 24, 2026)
• TOTO: CST446CEMFGN Aquia IV spec sheet SS-02061
• TOTO: Aquia IV CST446 installation and owner’s manual
• TOTO: Aquia IV parts manual PD-01451
• American Standard: H2Option 2887 dual flush spec sheet (Rev. B 10/15)
• American Standard: H2Option 2886/2887/2889 installation guide with troubleshooting
• American Standard: H2Option 606CA002 product page
• Kohler: Highline K-3889 spec sheet (6-8-2024)
• Kohler: Cimarron K-31621 spec sheet
• Fluidmaster: Duo Flush 550DFR dual flush valve installation guide (Part 25-0183, rev. 04/15)
• Korky: Dual Flush Toilet Flush Valve Seal Kit 440 (fit list)
• Amazon listings for the four products linked above, including the model numbers they carry

Bottom Line

Single flush and dual flush are close. The full flush is 1.28 gpf on all four picks, so dual flush only saves water if you press the reduced button. Choose the TOTO Aquia IV or American Standard H2Option if you will, and keep the button rods set as the maker says. Choose the Kohler Highline (10-inch rough-in) or Cimarron (12-inch) for one lever. Match the exact model number, and check your state’s 1.28 gpf rule.