1.28 GPF vs 1.6 GPF Toilet: Water Use and Flush Power

When you stand in a plumbing showroom comparing toilet spec sheets, two numbers appear constantly: 1.28 gallons per flush (GPF) and 1.6 GPF. Those 0.32 gallons sound trivial. Across a family of four over a year, they translate to roughly 4,000 to 7,000 gallons -- enough to fill a small backyard pool. More importantly, the gap in flush performance between the two standards has nearly closed thanks to a decade of engineering improvements in bowl geometry, trapway diameter, and jet design.

This guide cuts through the marketing language to explain what each standard actually means, how third-party MaP flush testing measures real performance, which specific toilet models perform best at each GPF rating, and whether spending more on a high-efficiency model pays off in water bill savings. Internal links throughout connect to deeper dives on related topics covered in our best flushing toilets roundup.

What Is the Difference Between 1.28 GPF and 1.6 GPF Toilets?

The practical difference comes down to water volume and the engineering required to move waste reliably with less water. A 1.6 GPF toilet relies on volume -- more water rushing through the bowl creates momentum to clear the trapway. A 1.28 GPF toilet must compensate with bowl shape, trapway design, and jet or siphon mechanics engineered to generate equivalent or superior hydraulic force using less water.

Early high-efficiency toilets (HETs) from the late 2000s struggled with this trade-off. First-generation 1.28 GPF models sometimes required two flushes to clear solid waste, effectively defeating the water savings. Today, the engineering has caught up. Independent MaP testing routinely records 1.28 GPF toilets scoring 800 to 1,000 grams -- the maximum rating -- which is identical to or better than many 1.6 GPF models on the market.

Does a 1.28 GPF Toilet Flush as Powerfully as a 1.6 GPF Model?

MaP testing is the standard buyers and plumbers rely on because it measures actual performance rather than manufacturer claims. The protocol was developed jointly by the Alliance for Water Efficiency, the Canadian Water and Wastewater Association, and various water utility partners. Toilets are tested in a certified lab using soybean paste mixed to simulate real-world waste density. A score of 350 grams is the minimum for EPA WaterSense certification; premium toilets routinely reach 800 to 1,000 grams regardless of GPF rating.

The key engineering factors that allow 1.28 GPF toilets to flush powerfully with less water include:

• Larger trapway diameter. A fully glazed 2-1/8 inch or 2-3/8 inch trapway allows solids to pass cleanly even with reduced water pressure. TOTO's CEFIONTECT glaze, for example, is applied inside the trapway to reduce friction and buildup.
• Optimized jet placement. The water jet at the bottom of the bowl directs water into the trapway at a precisely calculated angle to maximize siphon action with minimal water volume.
• Bowl shape and water surface area. Elongated bowls with a well-designed water spot (the pool of water visible in the bowl) keep waste submerged before flushing, reducing required flush force.
• Tower flush valves vs. flapper designs. Tower or canister flush valves -- used in Kohler's Class Five system and TOTO's G-Max -- release the full tank volume quickly and uniformly, creating stronger siphon action compared to older flapper-style valves that can slow water release mid-flush.

How Much Water and Money Does a 1.28 GPF Toilet Save Over a 1.6 GPF?

The EPA estimates that toilets account for about 30 percent of indoor household water use, making them the single largest indoor water consumer. Nationally, WaterSense-labeled toilets have collectively saved more than 580 billion gallons of water since the program launched in 2006, according to EPA program data.

Water cost savings vary significantly by geography. In drought-prone states like California, Arizona, and Colorado, municipal water rates are often two to four times the national average, and utility rebates for WaterSense toilets can reach $100 to $250 per unit. In these markets, the payback period for choosing a 1.28 GPF over a 1.6 GPF model can be as short as one to three years. In lower-cost water markets, the financial case is more modest, though the environmental benefit remains the same.

For households still using pre-1994 toilets using 3.5 to 5 GPF, the savings from upgrading to a 1.28 GPF model are dramatic. The EPA's WaterSense program estimates that replacing one pre-1994 toilet with a WaterSense-certified 1.28 GPF model saves more than 13,000 gallons per year -- and that many utility rebate programs prioritize these replacement scenarios with the highest rebate amounts.

Which Toilet Models Are Available at 1.28 GPF and 1.6 GPF?

Top 1.28 GPF Models with Strong MaP Scores

TOTO Drake II (1.28 GPF, Two-Piece) -- The Drake II uses TOTO's double cyclone flush system with two nozzles rather than a traditional rim wash to generate a powerful 1,000-gram MaP flush. The fully glazed 2-3/8 inch trapway and CEFIONTECT ion-barrier glaze reduce buildup. It is WaterSense certified and widely available. See our detailed TOTO Drake vs Drake II comparison for specifications across both generations.

TOTO UltraMax II (1.28 GPF, One-Piece) -- TOTO's one-piece flagship also scores 1,000 grams on MaP testing. The skirted design simplifies cleaning while the Universal Height (17.25 inches) suits most adults. CEFIONTECT glaze is standard on most configurations.

American Standard Cadet 3 (1.28 GPF) -- One of the most reviewed toilets in the mid-range segment, the Cadet 3 at 1.28 GPF scores consistently high on MaP testing (800 to 1,000 grams depending on configuration). The PowerWash rim scrubs the bowl with each flush and the fully-glazed trapway is 2-3/8 inches. Learn more in our American Standard Cadet 3 review.

Woodbridge T-0001 (1.28 GPF, One-Piece) -- The Woodbridge T-0001 is a popular value-oriented one-piece with a soft-close seat included and WaterSense certification. Owner reviews consistently note adequate flush performance for everyday use, though MaP scores are not publicly listed by the manufacturer.

Gerber Avalanche (1.28 GPF) -- Gerber's Avalanche line uses a 3-inch flush valve -- larger than the industry standard 2-inch -- to deliver a high-volume, fast flush at 1.28 GPF. MaP testing records 1,000-gram performance on select configurations.

Swiss Madison Sublime II (1.28 GPF) -- The Swiss Madison Sublime II is a rimless, wall-faced one-piece that uses a fully skirted design. WaterSense certified; owner reviews note strong flush performance for a value-priced one-piece.

Top 1.6 GPF Models Still Worth Considering

American Standard Champion 4 (1.6 GPF) -- The Champion 4 remains one of the most clog-resistant 1.6 GPF toilets on the market, with a 4-inch accelerator flush valve and a 2-3/8 inch fully glazed trapway that produces a 1,000-gram MaP score. American Standard markets it specifically as a large-trapway, high-clearance design. Our full American Standard Champion 4 review covers performance and long-term owner satisfaction.

Kohler Highline Classic (1.6 GPF) -- A stalwart in the Kohler lineup, the Highline Classic at 1.6 GPF is widely used in rental properties and new construction. It uses Kohler's Class Five flush technology with a 3-1/4 inch by 2-1/8 inch trapway. MaP scores range from 800 to 1,000 grams depending on configuration.

Kohler Cimarron (1.28 GPF and 1.6 GPF) -- Kohler offers the Cimarron in both ratings, making it a useful direct comparison point. Owner reviews and aggregated feedback suggest the 1.28 GPF Cimarron performs nearly identically to the 1.6 GPF version in everyday use, consistent with MaP test results.

Is a 1.28 GPF Toilet Better for the Environment and for Utility Rebates?

The EPA WaterSense program provides a searchable rebate finder on its website (epa.gov/watersense) where homeowners can locate utility rebates by ZIP code. In many California and Arizona municipalities, rebates stack with state-level water conservation incentives, further reducing net purchase cost. Some jurisdictions -- including parts of California -- mandate WaterSense-level efficiency for new construction and renovation, effectively making 1.28 GPF or lower the only compliant option.

For environmental impact, the water efficiency advantage of a 1.28 GPF toilet compounds over its 10 to 25 year service life. A household of four replacing two 1.6 GPF toilets with 1.28 GPF WaterSense models conserves approximately 75,000 gallons over 20 years -- per toilet -- which represents meaningful stress relief for municipal water treatment infrastructure, especially in water-scarce regions.

Are There Any Situations Where a 1.6 GPF Toilet Is the Better Choice?

However, these situations are less common than they once were. Most residential plumbing installed after 1990 has adequate drain slope for 1.28 GPF performance, and most modern 1.28 GPF toilets have trapways large enough to pass waste without relying on volume alone. If you are uncertain about your drain configuration, a licensed plumber can assess drain slope before a toilet purchase.

A second scenario where 1.6 GPF remains relevant is replacement parts availability. In commercial settings, rental units, or older buildings where standardization matters, a 1.6 GPF Kohler or American Standard tank may already be the resident model and replacing with an identical unit simplifies parts sourcing. This is a logistical argument, not a performance one.

Dual Flush: The Hybrid Option

Dual-flush toilets offer a third path, combining a reduced-volume flush (typically 0.8 to 1.0 GPF) for liquid waste with a full flush (1.28 or 1.6 GPF) for solid waste. The TOTO Aquia IV, for example, is a dual-flush model rated 0.8/1.28 GPF and earns WaterSense certification. When used as designed -- partial flush for liquids, full flush for solids -- a dual-flush toilet can reduce average water consumption below 1.0 GPF per flush for many households.

Owner reviews of dual-flush models note a learning curve in the first weeks of use, but long-term satisfaction is generally high. For households focused on maximum water conservation, a dual-flush model like the TOTO Aquia IV or the Woodbridge T-0001 in dual-flush configuration represents the lowest water consumption option while maintaining reliable solid waste clearance. Read more about this option in our guide on how dual flush toilets work.

Low Water Pressure Considerations

Homes with water pressure below 25 psi can sometimes experience reduced flush effectiveness in any gravity-feed toilet, including 1.28 GPF models. In low-pressure scenarios, pressure-assist toilets -- which use compressed air in the tank to boost flush force -- are available in both 1.28 GPF and 1.6 GPF configurations. Pressure-assist 1.28 GPF models from American Standard and Gerber score near the maximum on MaP testing even at lower inlet pressures, at the cost of louder flush noise.

• EPA WaterSense, epa.gov/watersense
• MaP flush testing, map-testing.com
• Manufacturer published specifications
• Alliance for Water Efficiency, allianceforwaterefficiency.org
• Energy Policy Act of 1992, U.S. Department of Energy