
Best French Toilets (2026)
ToiletsRefined, softly curved one-piece and skirted silhouettes with a polished, Parisian-elegant profile, paired with verified MaP flush scores rather than a stylist's…
Read the guideThe trapway is the curved internal passage waste travels through after you flush. Its diameter, shape, and surface finish directly control whether your toilet clears the bowl completely or struggles with every flush.
Research updated June 2026.
A wider trapway (2.375 in. or larger) reduces resistance and clears bulk waste with less water pressure. Fully glazed passages cut friction further. Toilets with a 2.375 in. fully glazed trapway consistently earn MaP scores of 800 g or higher and rarely clog under normal household use.
The trapway is the S-shaped or P-shaped water channel built into the vitreous china body of the toilet, connecting the bowl outlet to the floor drain. Its internal diameter determines how large a solid object can pass without obstruction. A trapway that is too narrow creates a bottleneck where waste and paper accumulate, causing repeat clogs even when the flush pressure is adequate.
Manufacturers measure trapway size by the smallest circular opening the passage can accommodate. Standard toilets sold in the United States must meet a minimum passageway of 1.5 inches under ASME A112.19.2, but premium models voluntarily exceed this with 2-inch, 2.125-inch, or 2.375-inch passages. The difference between a 1.5-inch and a 2.375-inch trapway is not incremental; it is roughly 2.5 times the cross-sectional area, which translates directly into clog resistance.
Every toilet needs a trap (the curved water-seal section) to block sewer gases from entering the home. The trap is part of the same continuous vitreous china casting as the bowl and base, so its geometry is fixed at manufacture. That is why choosing the right toilet at the point of purchase matters more than any subsequent maintenance or cleaning product.
Trapway size affects flush performance in four measurable ways. First, a wider bore reduces the hydraulic resistance the water jet must overcome to push waste through. Second, a larger diameter accommodates bulkier solids before they can wedge against the walls. Third, a fully glazed passage (where the interior is coated with the same smooth ceramic glaze as the bowl) reduces friction so waste slides through rather than dragging. Fourth, a wider trapway paired with a well-designed rim or tower flush system creates a laminar flow that sweeps the entire bowl wall rather than just washing the center.
Plumbing engineers note that the trapway inlet -- the transition from the bowl sump to the curved passage -- is the most common clog point in residential toilets. A straight, wide entry with a fully glazed surface at this transition does more to prevent blockages than any additive or flush-booster product on the market. When comparing models, always ask for the fully-glazed trapway diameter, not the advertised "glazed trapway" claim, which may apply only to the exterior of the trap.
MaP (Maximum Performance) testing is the North American industry standard for measuring flush performance. Toilets are loaded with a standardized soybean paste media in increments of 100 grams and flushed until the bowl fails to clear completely. The highest weight cleared in a single flush with no residue is the MaP score, reported in grams (g). A score of 500 g is considered acceptable, 800 g is good, and 1,000 g is the maximum test level and the benchmark for high-performance models.
Toilets with trapway diameters of 2.125 inches or smaller tend to cluster between 500 g and 700 g in MaP testing. Models with a fully glazed 2.375-inch trapway consistently score 800 g to 1,000 g. The correlation is strong but not absolute, because the flush valve size, tank water volume, and rim or tower wash design interact with the trapway to produce the final score.
The MaP database, maintained at map-testing.com, lists thousands of tested toilets with their scores publicly available. When you see a manufacturer claim a "powerful flush," the most credible verification is an independent MaP score rather than a self-described test. EPA WaterSense, the certification program administered by the U.S. Environmental Protection Agency, requires that any toilet bearing the WaterSense label must pass MaP testing at a minimum of 350 g while using 1.28 GPF or less. That floor is relatively easy to clear; the real performance differentiation happens between 600 g and 1,000 g.
| Trapway Diameter | Typical MaP Score | Clog Risk | Common Models | GPF |
|---|---|---|---|---|
| 1.5 in. (unglazed) | 300 to 500 g | High | Builder-grade basics | 1.6 |
| 1.75 in. (partially glazed) | 400 to 600 g | Moderate | Mid-range gravity flush | 1.28 to 1.6 |
| 2.0 in. (fully glazed) | 600 to 800 g | Low | Kohler Highline, Cadet 3 | 1.28 to 1.6 |
| 2.125 in. (fully glazed) | 700 to 900 g | Very Low | TOTO Drake II, Gerber Viper | 1.28 |
| 2.375 in. (fully glazed) | 900 to 1,000 g | Extremely Low | American Standard Champion 4, TOTO UltraMax II | 1.28 to 1.6 |
A standard trapway has smooth vitreous china on its outer visible surfaces but may have a rough, unglazed interior through the curved channel. A fully glazed trapway receives the same ceramic glaze treatment on the inside of the passage itself, producing a glass-smooth surface with a very low coefficient of friction. Waste and paper move through with minimal drag, reducing the water pressure needed to complete a flush.
The practical difference shows up most clearly with toilet paper. Soft, multi-ply paper compresses poorly and can bridge across a narrow or rough trapway. A fully glazed 2.375-inch bore allows compressed paper to pass without catching on surface irregularities. Most premium toilets from TOTO, American Standard, and Kohler now include full glazing as a standard feature on their performance lines.
Glazing is not solely a friction issue. A smooth interior surface is also far easier to keep clean. In unglazed passages, mineral deposits and organic matter accumulate over years, gradually narrowing the effective bore and creating a rough surface that catches debris. Fully glazed trapways resist mineral buildup longer and respond better to periodic cleaning with a toilet brush and mild cleaner.
When shopping, the term "skirted trapway" sometimes appears. This refers to a design where the exterior of the toilet base is enclosed in a smooth ceramic skirt that hides the curved trap outline, giving a seamless look. A skirted trapway is an aesthetic choice and says nothing about the interior bore size or whether the interior is glazed. A skirted toilet can have a narrow, unglazed bore; a traditional exposed-trapway toilet can have a wide, fully glazed bore. Always check the specification sheet for the interior diameter.
Plumbers who service high-rise apartments consistently report that toilets with fully glazed trapways generate fewer service calls over a 10-year period than equivalent toilets with unglazed passages. The difference is especially noticeable in households that use thick or quilted bathroom tissue, where a rough interior surface acts like sandpaper that grabs the paper before it exits.
Flush technology and trapway diameter work together rather than independently. A large-diameter trapway cannot overcome a weak or poorly directed flush, and a powerful flush valve loses efficiency pushing waste through a narrow bore. The optimal toilet pairs a high-flow flush system with a wide, fully glazed trapway so both elements reinforce each other.
Gravity-flush toilets rely on the weight and momentum of water dropping from the tank. Tower flush valves (like those in the TOTO Drake or Kohler Cimarron) release 100 percent of the tank water in one burst rather than the restricted flow from a standard flapper, creating a stronger initial push. Pressure-assisted models use compressed air to blast water into the bowl, delivering extremely high instantaneous flow regardless of water pressure at the supply line.
Here is how the major flush types interact with trapway size:
Gravity flush with flapper valve: The oldest and most common design. Water releases as the flapper opens, then slows as the tank empties. A 2.375-inch trapway makes the most of this design because the wide bore requires less pressure to initiate movement. The American Standard Champion 4, which uses a standard flapper and a 2.375-inch fully glazed trapway, achieves a MaP score of 1,000 g at 1.6 GPF precisely because the wide bore compensates for any drop in flush energy as the tank empties.
Gravity flush with tower valve: Tower or canister valves (used in the TOTO Drake II and Drake) release water in a single, unrestricted column. This higher instantaneous flow rate amplifies the effect of a wide trapway because the water arrives with full momentum before slowing. The Drake II at 1.28 GPF earns a MaP score of 1,000 g with a 2.125-inch fully glazed trapway.
Dual-flush gravity: Dual-flush models offer a reduced liquid flush (typically 0.8 GPF to 1.0 GPF) and a full flush (1.28 GPF to 1.6 GPF). The liquid flush relies on a narrower water column and depends heavily on trapway smoothness to carry paper without clogging. TOTO's Aquia IV uses a 2.125-inch fully glazed trapway and reaches the 1,000 g MaP ceiling on its 1.28 GPF full flush. The Woodbridge T-0001 dual-flush model pairs a 2.375-inch trapway with its dual-flush tower valve for a strong, though not quite as high, full-flush score.
Pressure-assisted: Compressed-air models like the American Standard Cadet Pressure-Assisted or Kohler's Pressure Lite series generate extremely high instantaneous flow. A wide trapway is still beneficial but the pressure system can compensate for moderate bore restrictions. These toilets are louder and require municipal water pressure of at least 25 PSI to function; they are rarely used in residential settings but are common in commercial restrooms.
Tornado flush / Cyclone flush: TOTO's Tornado Flush and Kohler's AquaPiston channel water through angled rim jets that create a centrifugal wash rather than a top-down gravity fall. This rotational motion clears the bowl wall more completely and directs waste efficiently into the trapway inlet. These systems perform best with a fully glazed 2.125-inch or larger bore, as the rotational energy dissipates if the outlet is constricted.
See our full breakdown on best flushing toilets for head-to-head MaP comparisons across flush technologies.
The American Standard Champion 4 is the benchmark for clog resistance, pairing a 2.375-inch fully glazed trapway with a large 4-inch flush valve to achieve a MaP score of 1,000 g at 1.6 GPF. The TOTO UltraMax II combines a 2.125-inch fully glazed trapway with the Tornado Flush system to reach 1,000 g at 1.28 GPF, making it the performance leader among WaterSense-certified gravity-flush one-piece models.
The Kohler Cimarron with AquaPiston and a 2-inch glazed trapway also reaches the 1,000 g MaP ceiling and is one of the strongest values at that performance tier. The American Standard Cadet 3, Gerber Viper, and Swiss Madison Concorde all offer fully glazed trapways of 2 inches or larger with MaP scores between 600 g and 800 g, suitable for most households.
The following models are widely cited in aggregated owner reviews and published MaP data for their trapway performance:
TOTO Drake (CST744SL): Two-piece, 1.6 GPF, 2.125-inch fully glazed trapway, E-Max flush, MaP 1,000 g. One of the best-selling high-performance toilets in North America. Recommended for households with heavy use or older drain lines.
TOTO Drake II (CST454CEFG): Two-piece, 1.28 GPF WaterSense, 2.125-inch fully glazed trapway, Double Cyclone flush, MaP 1,000 g. The water-efficient evolution of the Drake with identical performance.
TOTO UltraMax II (MS604114CEFG): One-piece, 1.28 GPF WaterSense, 2.125-inch fully glazed trapway, Tornado Flush, MaP 1,000 g. Quieter, lower profile, and easier to clean than two-piece alternatives.
American Standard Champion 4 (2034.014): Two-piece, 1.6 GPF, 2.375-inch fully glazed trapway, 4-inch wide flush valve, MaP 1,000 g. The widest standard trapway available in a consumer gravity-flush toilet. Particularly effective in homes with low water pressure.
American Standard Cadet 3 (2477.216): Two-piece, 1.28 GPF WaterSense, 3-inch flush valve, 2.375-inch fully glazed trapway, MaP 1,000 g. Combines water efficiency with the wide bore of the Champion 4 family.
Kohler Highline (K-3999): Two-piece, 1.28 GPF WaterSense, 2-inch fully glazed trapway, AquaPiston flush, MaP 600 g. A reliable everyday performer at a lower price point.
Kohler Cimarron (K-3609): Two-piece, 1.28 GPF WaterSense, 2-inch fully glazed trapway, AquaPiston flush, MaP 1,000 g. Kohler's performance flagship in the consumer gravity segment.
Woodbridge T-0001: One-piece dual-flush, 1.0/1.6 GPF, 2.375-inch fully glazed trapway, MaP 1,000 g on full flush. A high-value option combining a wide bore with a sleek skirted profile.
Swiss Madison Concorde (SM-1T803): One-piece dual-flush, 1.0/1.6 GPF, 2.375-inch fully glazed trapway, MaP 800 to 1,000 g. Gaining traction in the value-premium segment with verified independent scores.
Gerber Viper (21-302): Two-piece, 1.28 GPF WaterSense, 2.125-inch fully glazed trapway, MaP 1,000 g. Frequently recommended by plumbers for its durable flush mechanism and consistent long-term performance.
For deeper guidance on selecting between these models, see our TOTO vs American Standard comparison and our guide to best no-clog toilets.
Plumbing contractors frequently note that a 2.375-inch trapway is particularly valuable in older homes with cast-iron drain lines, where the interior pipe surface has roughened from decades of mineral buildup. The wide bore gives waste more room to pass even when the drain itself has reduced capacity, buying homeowners time before more extensive plumbing work is required.
GPF (gallons per flush) and trapway size have an inverse relationship in terms of clog risk. At 1.6 GPF, the larger volume of water can push waste through a narrower bore because raw water pressure compensates for bore restriction. At 1.28 GPF (the WaterSense maximum) and especially at 0.8 to 1.0 GPF liquid-flush settings, there is less water volume to do the mechanical work, making a wide, smooth trapway more important for reliable clearing.
EPA WaterSense requires a minimum MaP score of 350 g for 1.28 GPF toilets to earn certification, but the specification acknowledges that bore size and flush system design are left to manufacturers. This is why two WaterSense toilets at the same GPF can produce dramatically different results in daily use, with one scoring 400 g and another scoring 1,000 g on MaP.
The math is straightforward. A 1.6 GPF toilet delivers roughly 6 liters of water per flush. A 1.28 GPF toilet delivers about 4.8 liters. A 0.8 GPF liquid flush delivers only about 3 liters. As GPF drops, the system must compensate through better flush geometry (directed jets, Tornado Flush, AquaPiston) and reduced trapway resistance (wider bore, full glaze). Manufacturers that cut GPF without compensating on trapway design or flush technology produce toilets that clog frequently regardless of their WaterSense label.
This is one reason the MaP database is more useful than GPF ratings alone when selecting a toilet. A 1.28 GPF toilet with a MaP score of 1,000 g will outperform a 1.6 GPF toilet with a MaP score of 500 g in both water savings and clog frequency. The combination of a wide fully glazed trapway and a directed flush system is what allows modern high-efficiency toilets to achieve 1,000 g MaP scores at 1.28 GPF.
For context on water efficiency decisions, see our guide to 1.28 GPF vs 1.6 GPF toilets and our overview of WaterSense certified toilet models.
The ASME A112.19.2 plumbing standard requires a minimum 1.5-inch interior trapway diameter. Most builder-grade toilets meet only this minimum. Performance-grade toilets voluntarily exceed it with 2-inch, 2.125-inch, or 2.375-inch bores. Always check the manufacturer specification sheet rather than assuming any toilet exceeds the minimum.
A fully glazed trapway has the same smooth ceramic glaze coating on the inside of the curved waste passage that the bowl surface has. This reduces friction so waste slides through with minimal resistance. A partially glazed or unglazed trapway has a rougher interior that can catch toilet paper and create clogs over time.
A 2.375-inch fully glazed trapway provides greater cross-sectional area and lower clog risk, but a 2.125-inch fully glazed trapway paired with a strong flush system (Tornado Flush, Double Cyclone, or similar) can match or exceed 1,000 g MaP performance. The TOTO Drake II achieves 1,000 g with a 2.125-inch bore. Neither dimension is universally superior; the full flush system design must be evaluated alongside the bore.
Yes. The American Standard Champion 4 (model 2034.014) is one of the few widely available consumer toilets with a 2.375-inch fully glazed trapway, published in American Standard's own specification sheets. This, combined with the 4-inch wide flush valve, accounts for its 1,000 g MaP score and its reputation as one of the most clog-resistant toilets sold at retail price points.
The model number is printed on the inside of the tank lid or on a label inside the tank above the water line. Once you have the model number, search the manufacturer website or MaP's public database at map-testing.com. Manufacturer specification PDFs typically list the trapway diameter and whether it is fully glazed. If the documentation is unavailable, a plumber can measure the bore with a sizing ball.
No. The trapway is cast as part of the vitreous china body and cannot be altered or replaced separately. If your toilet clogs frequently due to a narrow trapway, the only solution is replacing the entire toilet. Temporary measures like using less toilet paper or adding a flush booster can reduce frequency but will not fix the underlying bore restriction.
For a family home with regular use, a MaP score of 600 g or higher is generally sufficient for clog-free performance with standard two-ply toilet paper. Heavy users, homes with older drain lines, or households using thick multi-ply paper should target 800 g or higher. A score of 1,000 g provides the maximum rated protection against clogs and is the benchmark used by high-performance models from TOTO, American Standard, and Gerber.
Yes and no. The skirted exterior hides the trapway curve but does not change the interior bore size. If a skirted toilet has a wide, fully glazed trapway, it will clog no more often than a traditional exposed-trapway model. However, if a clog does occur in a skirted toilet, using a toilet auger can be slightly more awkward because the curved ceramic exterior limits the angle at which the snake can enter the trap.
Not inherently. Dual-flush design is a flush valve choice, not a trapway specification. Several dual-flush models, including the Woodbridge T-0001 and Swiss Madison Concorde, pair dual-flush operation with a 2.375-inch fully glazed trapway. The risk with dual-flush toilets is the reduced-volume liquid flush (0.8 to 1.0 GPF), which provides very little water pressure. In these models, a wide fully glazed trapway is more important than in single-flush toilets to prevent the liquid flush from leaving residue.
Yes, to a meaningful degree. The Tornado Flush creates a centrifugal water motion that keeps solids suspended and moving into the trapway rather than settling. This directionality allows the TOTO UltraMax II and Drake II to achieve 1,000 g MaP scores with a 2.125-inch bore, which would be a mid-range result on a standard gravity flapper toilet. The combination of directed flush and fully glazed bore is what drives performance in TOTO's top-tier models.
There is a weak correlation in that elongated-bowl models tend to be from premium lines that also feature wider trapways, while round-bowl economy models tend to use minimum-spec trapways. However, the bowl shape itself (round vs elongated) does not determine the trapway size. An elongated toilet with a 1.5-inch bore will clog far more often than a round-bowl model with a 2.375-inch bore.
Residential water supply pressure typically ranges from 30 to 80 PSI, but the toilet tank fills through a fill valve and stores water at atmospheric pressure. The flush event is driven by gravity (tank height) and the volume of water released, not by line pressure. Low supply pressure slows tank refill time but does not reduce flush power. This means trapway size and flush valve design matter equally in both high- and low-pressure homes, except for pressure-assisted models that require at least 25 PSI to compress the internal air chamber.
Yes. Kohler publishes specification sheets for all major product lines that include the fully-glazed trapway diameter. The Cimarron and Highline lines list a 2-inch fully glazed trapway. The Highline Arc and some Wellworth models also list 2-inch bores. Kohler does not currently offer a consumer gravity-flush toilet with a 2.375-inch bore, so American Standard remains the leader in maximum bore size at the retail level.
The flush valve diameter controls how quickly water exits the tank, not the trapway bore. A 4-inch valve (like the one in the American Standard Champion 4) releases water roughly 70 percent faster than a standard 2-inch flapper, creating a stronger initial push. This high-flow release works synergistically with the 2.375-inch trapway: the fast water column reaches the trapway with more momentum before slowing. A 3-inch valve on the American Standard Cadet 3 produces a similar effect, albeit slightly less aggressively than the 4-inch valve.
Possibly. A wide toilet trapway eliminates the toilet itself as a bottleneck but cannot clear buildup inside the drain pipe downstream. If your toilet clogs frequently even after upgrading to a large-bore model, the issue may be a partial blockage 6 to 12 inches into the floor flange or further along the drain line. A plumber's camera inspection can identify buildup in cast-iron drain pipes, which is common in homes built before 1980.
Not in the standard residential gravity-flush market. The 2.375-inch bore has been the maximum in widely available consumer toilets since the American Standard Champion 4 introduced it over a decade ago. Some commercial pressure-assisted units use larger passages, but these are designed for institutional use and require higher supply pressure than residential water systems typically deliver.
Yes. The TOTO Aquia IV dual-flush toilet features a 2.125-inch fully glazed trapway with TOTO's SanaGloss surface treatment on both the bowl and the trapway interior. The SanaGloss ionic barrier makes the surface even smoother than standard glaze, reducing adhesion of waste and mineral deposits. The Aquia IV scores 1,000 g on MaP at its 1.28 GPF full flush setting.
Fully glazed trapways require very little specific maintenance. Regular bowl cleaning with a toilet brush prevents mineral buildup at the bowl outlet, the entry point of the trapway. Avoid using solid in-tank cleaners with bleach tablets, as these can degrade the fill valve and flapper over time, leading to reduced flush strength that stresses any trapway. A monthly flush with a bucket of hot (not boiling) water can clear soap residue accumulation inside the passage.
Yes. The Gerber Viper is frequently cited in plumber recommendations for its 2.125-inch fully glazed trapway and 1,000 g MaP score at 1.28 GPF. Gerber is a professional-grade brand that distributes primarily through plumbing wholesalers rather than home improvement retailers, so it is less visible to consumers but has a strong track record in installed performance and parts availability.
The Woodbridge T-0001 matches American Standard's 2.375-inch fully glazed trapway, making it one of the widest bores available in a one-piece toilet. TOTO's consumer one-piece models (UltraMax II, Nexus) use a 2.125-inch bore and compensate with advanced flush technology. In real-world clog resistance, both approaches yield 1,000 g MaP scores. The Woodbridge T-0001 appeals to buyers who want a one-piece skirted design with a large bore at a lower price than premium Japanese brands.
Trapway size is the single most predictive specification for clog resistance in a gravity-flush toilet. A fully glazed bore of 2.125 inches or larger, paired with a strong flush valve design, consistently delivers MaP scores at or near 1,000 g regardless of GPF rating. If you flush frequently, have heavy household use, or live in a home with older drain lines, prioritizing trapway diameter and full glaze treatment in your next toilet purchase will deliver more noticeable improvement than any brand loyalty or price premium. The American Standard Champion 4 and Cadet 3 lead on raw bore size; the TOTO Drake II and UltraMax II lead on flush efficiency per gallon; the Gerber Viper and Woodbridge T-0001 offer strong performance at accessible price points. All four paths lead to the same outcome: reliable, clog-free flushing that holds up over years of daily use.
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.
Researched by Marcus Bell · Last updated July 1, 2026 · Our review method

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