One-Piece vs Two-Piece Toilet: Pros, Cons and Differences
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1.28 GPF vs 1.6 GPF Toilet: Which Is Better?
Straight answers backed by MaP flush data, plumbing specs, and owner feedback.
Quick Answer
For most homes, a 1.28 GPF toilet is better due to water savings, EPA WaterSense certification, and modern engineering that achieves strong flush performance. A 1.6 GPF toilet may still be necessary for older plumbing systems with low slope or long drain runs, but 1.28 GPF models now match or exceed older flush power.
What Does GPF Mean and Why Does It Matter?
GPF stands for gallons per flush, the standard measure of how much water a toilet uses per flush. A 1.28 GPF toilet uses 20% less water per flush than a 1.6 GPF model, which can save over 2,000 gallons per year for a family of four.
GPF is the single most important specification for toilet water consumption. The federal maximum for new residential toilets in the United States since 1994 is 1.6 GPF, but many older homes still have toilets using 3.5 GPF or more. A 1.28 GPF toilet meets the EPA WaterSense criteria for high-efficiency toilets (HET), which require no more than 1.28 GPF and a minimum MaP score of 350 grams. WaterSense-certified toilets use at least 20% less water than the federal standard, and the EPA estimates that replacing an older 3.5 GPF toilet with a 1.28 GPF model saves a household about 11,000 gallons annually.
The difference between 1.28 and 1.6 GPF may seem small, but over millions of flushes nationwide, the water savings add up. A family of four flushing five times per day uses about 2,920 flushes per year. At 1.6 GPF, that is 4,672 gallons annually. At 1.28 GPF, it is 3,738 gallons, a savings of 934 gallons per year. Many municipalities offer rebates for installing WaterSense toilets, further incentivizing the switch.
| GPF Rating | Water Use per Flush | Yearly Water Use (4-person household) | WaterSense Certified |
|---|---|---|---|
| 1.28 GPF | 1.28 gallons | 3,738 gallons | Yes |
| 1.6 GPF | 1.6 gallons | 4,672 gallons | No (exceeds limit) |
| 1.0 GPF (dual flush) | 0.8-1.0 gallons (low) / 1.28-1.6 gallons (high) | 2,920-4,672 gallons | Yes (if high flush is ≤1.28) |
Which GPF Flushes Better: 1.28 or 1.6?
Modern 1.28 GPF toilets with advanced flush systems like the TOTO Tornado Flush or American Standard VorMax can achieve MaP scores of 800-1000 grams, equaling or exceeding many 1.6 GPF models. Older 1.6 GPF toilets may still outperform poorly designed 1.28 GPF units, but the best 1.28 GPF toilets flush better than most 1.6 GPF toilets.
Flush performance is measured by the MaP (Maximum Performance) score, which tests how many grams of solid waste a toilet can remove in a single flush. The minimum acceptable MaP score for WaterSense certification is 350 grams, but high-performing toilets often score 600-1000 grams. The MaP test uses a soybean paste mixture to simulate realistic waste loads and includes tissue paper to mimic real-world conditions. A MaP score of 1000 grams is the maximum tested and represents excellent flush power.
The key difference between 1.28 and 1.6 GPF toilets is not the water volume alone but the engineering of the flush system. Gravity-fed toilets with large trapways (2-1/8 inches or wider) and glazed passages move waste efficiently with less water. Pressure-assisted toilets use compressed air to boost flush power, achieving high MaP scores even at 1.28 GPF. Dual-flush toilets offer a 0.8-1.0 GPF option for liquid waste and a 1.28-1.6 GPF option for solid waste, balancing water savings with performance.
Expert Take
For homeowners concerned about clogs, look for a toilet with a MaP score of 800 grams or higher and a trapway diameter of at least 2-1/8 inches. Many 1.28 GPF models from TOTO, Kohler, and American Standard now achieve this threshold, making them reliable choices for most households. The TOTO Drake with Tornado Flush is a top performer at 1.28 GPF with a MaP score of 1000 grams.
How Much Water Can You Save with 1.28 GPF?
Switching from a 1.6 GPF to a 1.28 GPF toilet saves about 0.32 gallons per flush, which adds up to roughly 934 gallons per year for a family of four. Over the toilet's 20-year lifespan, that is nearly 19,000 gallons saved per toilet.
The EPA calculates that WaterSense toilets save the average family 13,000 gallons of water per year compared to older 3.5 GPF models. Even compared to standard 1.6 GPF toilets, the 20% reduction is significant. For a household with multiple toilets, the savings multiply. A home with three toilets switching from 1.6 GPF to 1.28 GPF saves about 2,800 gallons annually, enough to fill a small swimming pool.
Water savings also translate to lower utility bills. Depending on local water rates, a family can save $30-$100 per year per toilet. Many water utilities offer rebates of $50-$100 per WaterSense toilet, offsetting the purchase cost. Environmentally, reducing water consumption lowers the energy needed for water treatment and pumping, reducing your carbon footprint.
Do 1.28 GPF Toilets Work with Older Plumbing?
Most 1.28 GPF toilets work fine with standard residential plumbing, but homes with long horizontal drain runs (over 15 feet), low slope (less than 1/4 inch per foot), or cast iron pipes may require a 1.6 GPF toilet for adequate waste transport. Check local plumbing codes and consult a plumber if unsure.
The concern with low-flow toilets is that less water may not carry waste far enough through the drain line, leading to clogs or buildup. Modern 1.28 GPF toilets address this with larger trapways (2-1/8 inches vs. 1-3/4 inches on older models) and optimized bowl shapes that create a strong siphon. However, if your home has 3-inch drain pipes instead of the standard 4-inch, or if the toilet is far from the main stack, a 1.6 GPF model may provide extra margin.
Rough-in measurement is another compatibility factor. Most toilets are designed for a 12-inch rough-in (distance from the wall to the center of the floor flange), but 10-inch and 14-inch options exist. Both 1.28 and 1.6 GPF toilets are available in all standard rough-in sizes. The American Standard Champion 4 is a 1.6 GPF model known for handling difficult plumbing situations due to its 4-inch flush valve and large trapway.
| Plumbing Condition | Recommended GPF | Reason |
|---|---|---|
| Standard 4-inch drain, short run | 1.28 GPF | Sufficient water for waste transport |
| Long horizontal run (>15 ft) | 1.6 GPF | Extra water helps push waste |
| Low slope (less than 1/4 in/ft) | 1.6 GPF | Reduced gravity requires more water |
| Cast iron pipes with buildup | 1.6 GPF | Higher flow clears debris |
| 3-inch drain pipe | 1.6 GPF | Narrower pipe needs more water |
Is 1.28 GPF More Expensive Than 1.6 GPF?
Purchase prices for 1.28 and 1.6 GPF toilets are similar, with both ranging from budget to premium. Long-term savings from lower water bills make 1.28 GPF the more economical choice, especially with utility rebates that can reduce upfront cost.
There is no significant price premium for WaterSense-certified 1.28 GPF toilets. Brands like TOTO, Kohler, and American Standard offer models at various price points in both GPF ratings. The main cost difference comes from features like comfort height, elongated bowls, and flush technology, not GPF alone. Installation costs are identical since both use the same rough-in and plumbing connections.
Over a 20-year lifespan, the water savings from a 1.28 GPF toilet can total $600-$2,000 depending on local rates. Many municipalities offer rebates of $50-$100 per WaterSense toilet, which can cover the price difference if any. The Kohler Highline is a popular 1.28 GPF model that combines affordability with strong flush performance and comfort height design.
Which GPF Should You Choose for Your Home?
Choose a 1.28 GPF toilet for most residential applications due to water savings, EPA WaterSense certification, and modern flush performance. Choose a 1.6 GPF toilet only if your home has challenging plumbing conditions or if local codes require it for specific installations.
For a typical home with standard 4-inch drain pipes and reasonable slope, a 1.28 GPF toilet from a reputable brand will provide reliable performance and significant water savings. Look for models with MaP scores of 800 grams or higher, elongated bowls for comfort, and ADA-compliant heights if needed. The TOTO Drake and Kohler Highline are excellent choices that meet these criteria.
If your home has known plumbing issues such as frequent clogs, long drain runs, or old cast iron pipes, a 1.6 GPF toilet like the American Standard Champion 4 may provide extra peace of mind. However, many modern 1.28 GPF toilets now include features like pressure assist or dual flush that can overcome these challenges. Always check the MaP score and trapway size when comparing models.
Expert Take
For most homeowners, the choice is clear: a 1.28 GPF toilet with a MaP score of 800 grams or higher offers the best balance of water savings and flush performance. Only opt for 1.6 GPF if you have verified plumbing limitations or prefer the simplicity of a standard gravity flush without advanced engineering.
Frequently Asked Questions
Is 1.28 GPF enough to flush solid waste?
Yes, modern 1.28 GPF toilets with MaP scores of 800-1000 grams easily handle solid waste. Look for models with large trapways and glazed passages for best performance.
What is the difference between 1.28 GPF and 1.6 GPF?
1.28 GPF uses 0.32 gallons less per flush, meeting WaterSense criteria. 1.6 GPF is the federal maximum for new toilets since 1994. Both can perform well, but 1.28 GPF saves more water.
Are 1.28 GPF toilets more prone to clogging?
Not necessarily. High-quality 1.28 GPF toilets with MaP scores of 800+ grams and 2-1/8 inch trapways clog less than poorly designed 1.6 GPF models. Proper installation and maintenance matter more than GPF alone.
Can I replace a 1.6 GPF toilet with a 1.28 GPF?
Yes, as long as the rough-in measurement matches (usually 12 inches). No plumbing modifications are needed. Most replacements are straightforward and improve water efficiency.
Do 1.28 GPF toilets qualify for rebates?
Yes, if they are WaterSense certified. Many water utilities offer rebates of $50-$100 per toilet. Check your local provider for specific programs.
What is a MaP score and why does it matter?
MaP (Maximum Performance) score measures how many grams of solid waste a toilet can flush. Minimum is 350g, good is 600g, excellent is 800g, and maximum tested is 1000g. Higher scores mean better flush power.
Is 1.6 GPF better for old plumbing?
Sometimes. Homes with long drain runs, low slope, or 3-inch pipes may benefit from the extra water volume. Consult a plumber to assess your specific situation.
What is WaterSense certification?
WaterSense is an EPA program that certifies toilets using no more than 1.28 GPF and meeting minimum MaP score of 350 grams. It ensures water efficiency and performance.
Do dual-flush toilets use more or less water than 1.28 GPF?
Dual-flush toilets typically use 0.8-1.0 GPF for liquid waste and 1.28-1.6 GPF for solid waste. Overall water use can be lower than a single 1.28 GPF toilet, but performance varies by model.
Which GPF is best for a basement toilet?
For basement toilets with long horizontal runs or pump systems, a 1.6 GPF model may be recommended to ensure waste reaches the main line. Check local plumbing codes for specific requirements.
Sources
- EPA WaterSense, epa.gov/watersense
- MaP Flush Testing Program, map-testing.com
- Manufacturer published specifications
How we rank & our data sources
We do not run physical lab tests. Rankings are built from published, verifiable data and real owner feedback, never paid placement.
- MaP (Maximum Performance) flush scores
- EPA WaterSense certification & GPF ratings
- Manufacturer specifications (trapway size, flush-valve, rough-in)
- Verified owner reviews from Amazon, Home Depot & Lowe’s
Researched by BFT Editorial Team · Last updated February 2, 2026 · Our review method
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