How Do Toilets Work? The Flush Explained

Every toilet in every home works on the same fundamental principle that has not changed since Thomas Crapper's era: store a volume of water above the bowl, release it quickly, and let gravity and hydraulics do the rest. What has changed dramatically over the past thirty years is how efficiently each of those steps is engineered. The difference between a 1990s 3.5-gallon toilet and a modern 1.28-gallon model that clears 1,000 grams in one flush is not magic; it is the precise engineering of the flush valve, the bowl geometry, the trapway width, and the siphon dynamics that follow from all three. Understanding how a toilet works at that level of detail tells you why some models clog every other week while others go years without a plunger, why pressure-assisted units are louder, why dual-flush toilets save water without sacrificing clearing power, and how to read a spec sheet to distinguish a genuinely strong flush from marketing language. This guide walks through every stage of the flush cycle, explains each component in plain language, and names the real models from TOTO, Kohler, American Standard, Woodbridge, Swiss Madison and Gerber that demonstrate each principle best.

For buyers who are still deciding which toilet to purchase, this page pairs naturally with the pillar guide to the best flushing toilets and with the toilet buying guide for 2026, which translates the mechanics explained here into purchase decisions. For decisions about bowl shape and configuration, see one piece vs two piece toilets and round vs elongated toilets.

At a glance

Flush system comparison

A side-by-side snapshot of the three main flush families using a strong representative model of each. MaP grams measure actual waste cleared per flush. The tinted row marks the gravity flush as the best all-around choice for most homes.

Recommended toilets in this guide

Gravity siphonic

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Pressure-assisted

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Dual-flush gravity

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Washdown gravity

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What are the main parts of a toilet?

Before explaining how a toilet flushes, it helps to know what you are working with. The tank is the upper ceramic box bolted to the bowl. Inside it live the fill valve (also called a ballcock on older designs), which connects to your home's water supply line; the flapper or canister flush valve, which seals the opening at the bottom of the tank that leads into the bowl; the float, attached to the fill valve to sense the water level; and the overflow tube, a vertical standpipe that routes excess water into the bowl rather than letting the tank spill over. The handle or button connects by a chain or lift rod to the flapper or flush valve.

Below the water line, the bowl has two zones: the standing water pool (the water surface you see), held in place by the shape of the trapway, and the rim, which is the underside of the bowl's inner edge. On most siphonic toilets, small rim holes channel water from the tank into the bowl in a circular rinse pattern during the flush. TOTO's Tornado Flush system replaces rim holes with two or three directed nozzles that swirl water at higher velocity, scrubbing more surface area while using the same or less water. The trapway is the S-shaped internal passage that descends from the standing water, curves up over an internal crown, and then drops to the floor drain connection. That crown is what makes a siphon possible, and its height and width are two of the most important specs a toilet has.

How does a gravity flush toilet work step by step?

The gravity flush cycle has six distinct stages, and understanding each one shows you why some toilets are so much stronger than others at the same water volume.

Stage 1: Handle or button press. You press the flush handle. A lift chain (or lift rod on button-flush designs) pulls the flapper or canister valve upward, unsealing the large opening at the bottom of the tank. On older 2-inch valve toilets this opening is small, which throttles the water release. Modern designs, including TOTO's 3-inch G-Max and Double Cyclone valves, Kohler's 3-inch AquaPiston canister and American Standard's 4-inch Champion valve, use larger openings to dump the tank faster.

Stage 2: Tank dump. Water falls from the tank into the bowl through the flush valve opening. On a siphonic toilet, water enters the bowl both through the large flush valve opening and through the rim holes (or nozzles on TOTO's Tornado and Double Cyclone systems). The speed of tank emptying is governed by valve diameter, tank volume and the static head of water above the valve. A 3-inch valve on a full 1.28-gallon tank dumps its water in roughly two to three seconds. That speed matters because the siphon needs the bowl water to rise fast.

Stage 3: Siphon initiation. As the rushing water fills the bowl faster than the trapway can drain it, the water level rises past the top (crown) of the internal trapway bend. The moment water flows continuously over that crown, it forms a seal between the air behind it and the drain ahead of it, and atmospheric pressure on the bowl's water surface pushes water through the sealed passage at high velocity. This is the siphon, and once it starts it sustains itself until air enters the system. The siphon is what creates the characteristic gurgle at the end of a flush and is also what pulls the bowl clean with more force than the water pressure alone would provide.

Stage 4: Bowl clearance. During the active siphon, water and waste travel through the trapway and into the drain line at high velocity. The siphon effectively amplifies the gravitational pressure of the water column, which is why a 1.28-gallon gravity toilet can clear 1,000 grams of solid waste in one flush even though the water volume is modest. The MaP (Maximum Performance) test, administered independently by map-testing.com, measures exactly this: how many grams of synthetic waste a specific toilet clears in a single flush without clogging. A 1,000-gram score means the toilet cleared the full test load every time.

Stage 5: Siphon break. As the tank empties, the flow of water into the bowl slows and drops below the rate needed to maintain the water seal at the trapway crown. Air enters the trapway, breaking the siphon. You hear the gurgle that signals flush completion. The bowl refills to its standing water level, which is set by the height of the trapway crown, and the process stops.

Stage 6: Tank refill. With the flapper now resting closed on the flush valve seat, the fill valve opens (triggered by the falling float) and begins refilling the tank from the supply line. A separate refill tube trickles water into the overflow tube during this time to restore the bowl water level. When the float reaches the set height, it shuts the fill valve off. Total cycle time from flush to full tank is typically 60 to 90 seconds on modern gravity toilets.

How does a pressure-assisted toilet work?

The key difference from a gravity toilet is where the energy for the flush comes from. In a gravity system, the energy is potential: the height of the water column above the bowl. In a pressure-assisted system, the energy is pneumatic: compressed air stored in a sealed inner vessel as your water line fills it. As water enters the sealed cartridge (most commonly a Flushmate vessel), it compresses the air pocket at the top of the vessel to line pressure, typically 25 to 80 PSI. When you press the flush actuator, an internal valve releases that compressed air, which pushes all the water in the vessel into the bowl in a high-velocity surge that is noticeably faster and louder than a gravity dump.

The result is a flush that does not depend on bowl geometry to build a siphon, because the force is so high that waste clears by raw hydraulic momentum rather than by sustained siphon action. This makes pressure-assisted toilets very clog-resistant, consistent under hard use and relatively independent of variations in your home's water pressure (they store energy as air, so lower supply pressure only slightly extends refill time). The consistent complaint is noise: the compressed-air release is a sharp, loud whoosh that can startle in a quiet home and is audible through walls. The Flushmate cartridge also costs significantly more to replace than a gravity flapper, and the external ceramic tank looks identical to a gravity tank but holds a sealed inner vessel instead of open water.

How does a dual-flush toilet work?

Dual-flush is not a separate flush mechanism in the way pressure-assist is; it is a smarter valve arrangement sitting on top of a gravity or washdown bowl. The tank holds a two-stage tower valve instead of a simple flapper. The small-flush side lifts only a partial valve section, releasing a short pulse of water (enough to rinse liquid waste through the trapway) and then reseating before the tank fully drains. The full-flush side lifts the complete valve section and empties the tank as a conventional gravity flush would, generating a strong enough siphon to clear solid waste. The TOTO Aquia IV's full 1.28-gallon flush clears at or near 800 grams on MaP testing, which is above the level needed for reliable solid clearance in most households.

The water savings come from frequency. Independent studies cited by the EPA WaterSense program suggest that in a typical household about 60 percent of daily flushes are liquid only. Replacing those with a 0.8-gallon half-flush instead of a 1.28-gallon full flush can save over 5,000 gallons per year in a four-person home. To earn EPA WaterSense certification, a dual-flush toilet must average no more than 1.28 gallons across both flush modes, weighted by the expected usage split. The honest caveat: some dual-flush designs, especially on washdown-style bowls, leave more residue and may need an occasional second flush for solids, which erases the savings. Choose a model with a high published MaP score on its full-flush mode to avoid this.

What is a trapway and why does its size matter?

The trapway is the bottleneck of the entire flush system. No matter how much force the flush generates, waste has to fit through the trapway's narrowest point to exit. A wider trapway is not automatically better for siphon strength (the siphon depends on the trapway filling and sealing, which a too-wide trapway resists), but it is better for passing chunky or fibrous waste without jamming. The engineering balance is a trapway wide enough to be clog-resistant but narrow enough to form a reliable siphon at the water volumes available from a 1.28-gallon flush.

Most quality siphonic toilets today use a fully glazed trapway of 2 to 2.125 inches internal diameter. TOTO's CeFiONtect ceramic glaze, applied to the trapway walls on models like the Drake II, UltraMax II, Vespin II and Aquia IV, is a nano-glaze engineered to be smoother than conventional glaze, which reduces the surface grip on waste and means less cleaning chemical is needed to maintain it over time. American Standard solved the clog problem differently on the Champion 4: instead of a refined siphon, it used a massive 4-inch flush valve and a 2.375-inch trapway to pass waste by sheer size before a siphon even needs to form. Both strategies work; they just produce different flush characters, with TOTO being quieter and American Standard being more brute-force.

Which toilet has the strongest flush?

The 1,000-gram MaP score is the highest rating the independent MaP (Maximum Performance) testing program awards, and it is achievable with a gravity toilet. TOTO, Kohler and American Standard all have multiple residential models at that ceiling. The TOTO Drake II achieves it through a combination of a 3-inch G-Max and Double Cyclone flush valve, a fully glazed 2.125-inch CeFiONtect trapway, and a bowl geometry engineered to start the siphon at low water volumes. The Kohler Highline uses a 3-inch Class Five canister valve and the same 1.28-gallon volume. American Standard's Champion 4 uses a 4-inch valve and 2.375-inch trapway to punch through with volume and passage width instead of refined siphon geometry.

The TOTO Vespin II and Kohler Cimarron also hit 1,000 grams and deserve mention, as does the Woodbridge T-0001, which reaches strong MaP scores in most dual-flush full-flush configurations and is the strongest value in the modern skirted segment. The Gerber Viper, a straightforward two-piece gravity siphonic toilet, consistently achieves 1,000 grams and is among the best-value options for buyers who want proven performance without premium pricing. The Gerber Avalanche, Gerber's pressure-assisted line, matches that score with compressed-air force rather than siphon geometry, making it the best choice for very heavy-use commercial-grade situations.

What is a good MaP score?

MaP testing was developed jointly by Canadian and American water utilities in the early 2000s to give consumers and procurement officers an objective measure of flush performance that did not rely on manufacturer claims. The test uses soybean paste formed into cylinders of set weight, placed in the bowl, and flushed. Each toilet is tested multiple times and the highest gram load it clears every time without failure becomes its MaP score. A 1,000-gram score means the toilet cleared a kilogram of simulated waste without failure on every repetition, which is more than any single residential flush cycle typically encounters.

The test database at map-testing.com is publicly searchable by brand, model and flush volume. The EPA's WaterSense program cross-references MaP scores when evaluating toilets for certification: to earn the WaterSense label at 1.28 GPF, a toilet must achieve a minimum MaP score of 350 grams, but the top-rated WaterSense models far exceed that minimum. Models from TOTO (Drake II, UltraMax II, Vespin II, Entrada, Aquia IV), Kohler (Highline, Cimarron, Santa Rosa, Memoirs), American Standard (Champion 4, Cadet 3), and Gerber (Viper) hold both WaterSense certification and MaP scores of 800 to 1,000 grams simultaneously. A toilet that reaches 1,000 grams while using only 1.28 gallons is doing something genuinely well-engineered, not just spending more water to score higher.

What is EPA WaterSense certification and why does it matter?

EPA WaterSense is a voluntary certification program run by the United States Environmental Protection Agency that labels toilets (and other plumbing fixtures) meeting strict water-efficiency criteria without sacrificing performance. For toilets, the WaterSense standard requires a maximum of 1.28 gallons per flush (compared to the federal maximum of 1.6 GPF), a minimum MaP flush-test score of 350 grams to confirm the toilet actually clears waste at that reduced volume, and third-party testing to verify the claims. A toilet displaying the WaterSense label is certified to both criteria simultaneously, meaning it cannot earn the label by saving water while flushing poorly.

In practical terms, a WaterSense toilet uses at least 20 percent less water than the federal 1.6 GPF standard. For a family of four flushing an average of five times each per day, switching from 1.6 GPF to 1.28 GPF saves roughly 4,000 to 5,000 gallons per year per toilet. Many U.S. states and municipalities offer rebates for WaterSense toilet replacements, and some jurisdictions require WaterSense fixtures in new construction. The full list of certified models is maintained at epa.gov/watersense. Almost every top-rated modern toilet in 2026, including the TOTO Drake II, Kohler Highline, American Standard Champion 4 and Woodbridge T-0001, carries the WaterSense label and achieves MaP scores far above the minimum threshold.

What is the best toilet for preventing clogs?

Clog resistance traces to three factors a toilet either has or does not: the MaP score (measuring actual solid clearance), the trapway width (measuring passage capacity), and the trapway glaze (measuring how easily waste slides through). A toilet with a 1,000-gram MaP score, a 2-inch or wider fully glazed trapway, and a 3-inch or larger flush valve covers all three, which is why the Drake II, Highline and Champion 4 show up consistently in aggregated owner feedback as rarely-plunged toilets despite heavy household use. The Champion 4's oversized passage is so wide that bulk waste does not have the opportunity to bridge and block the way it can in a standard 2-inch trapway, making it the top pick for households with a documented history of clogs.

It is worth separating bowl clogs from line clogs. A bowl clog (waste blocked inside the toilet before reaching the drain) is what trapway width and MaP score address. A line clog (waste blocked in the drain pipe beyond the toilet) is caused by inadequate pipe slope, pipe narrowing or buildup in the line, and a stronger toilet flush helps but does not fix the underlying pipe condition. If your home has repeated line clogs, the toilet is not the root cause, though a 1.6 GPF toilet's higher volume per flush does carry waste farther down a marginal drain line than a 1.28 GPF model, which is worth considering in older homes with aging drains.

How do branded flush technologies compare?

Every major toilet brand uses proprietary names for flush technologies that can make comparison confusing. Here is what each name actually describes, referenced against the underlying engineering.

TOTO G-Max. A gravity siphonic system using a 3-inch flush valve and a wide, fully glazed CeFiONtect trapway. Found on the TOTO Drake (original). Very strong gravity flush; the Double Cyclone update improved bowl-surface cleaning. TOTO Double Cyclone and Tornado Flush. Replaces rim holes with two or three directed nozzles that spin water around the bowl, covering more surface with less water while starting the siphon faster. Found on Drake II, UltraMax II, Vespin II, Entrada and the Aquia IV. All reach 1,000 grams on 1.28 gallons.

Kohler Class Five. A gravity siphonic system centered on a 3-inch canister valve (AquaPiston on some models) that opens fully from the bottom, giving a more even, forceful flush than a traditional flapper that opens from one edge. Found on Highline, Cimarron, Santa Rosa, Memoirs, and Kohler's one-piece lines. The Highline and Cimarron both reach 1,000 grams on 1.28 gallons. American Standard Champion 4. Named for its 4-inch flush valve and 2.375-inch trapway, the widest combination in the residential gravity class. Reaches 1,000 grams, but with a bolder, louder flush character than refined siphonic designs. American Standard Cadet 3. A more conventional siphonic design at a value price, reaching 1,000 grams on 1.28 gallons on most configurations.

Woodbridge T-0001 and T-0019. Gravity siphonic toilets in a modern one-piece skirted design. The T-0001 is a single-flush model; the T-0019 uses a dual-flush mechanism. Both achieve strong MaP scores and carry EPA WaterSense certification. Swiss Madison St. Tropez. A washdown-action dual-flush design (0.8 and 1.1 GPF), popular for its slim modern profile. The washdown action clears effectively at low water volumes but lacks the deep siphon of a TOTO or Kohler. Gerber Viper. A straightforward two-piece gravity siphonic toilet that consistently earns 1,000-gram MaP scores and is among the best-value no-clog options at its price point. Gerber Avalanche. Gerber's pressure-assisted line, using a Flushmate vessel for compressed-air flushing, built for commercial-grade clearance.

Top picks that show how the flush works in practice

Three toilets that each demonstrate a different approach to maximizing flush performance, selected because their published specs and MaP scores directly reflect the engineering principles covered in this guide.

How do tankless and smart toilets flush?

Tankless toilets (also called flushometer-valve toilets or wall-hung toilets with in-wall carriers) bypass the gravity-tank system entirely. Instead of storing water in a tank, they connect directly to the supply line with a diaphragm-operated flushometer that releases a metered burst of water when triggered. This requires adequate supply-line flow (typically 25 PSI minimum and 35 GPM peak) and is why tankless designs are more common in commercial buildings with dedicated water-pressure infrastructure than in residential homes with standard water pressure of 40 to 60 PSI.

Smart toilets, the category that includes Kohler's Veil and Numi, TOTO's Washlet+ systems and products like the Woodbridge B-0750, use a gravity or washdown bowl as the flush mechanism, just as a conventional toilet does. The "smart" features (bidet functionality, heated seat, auto-open lid, deodorizer, auto flush) are additions layered on top of the standard bowl-and-trap design. The auto-flush feature on smart toilets is an electronically triggered flapper or valve, not a different flush mechanism. The bowl still flushes by gravity siphon or washdown; it simply does so when a sensor commands the valve rather than when you press a handle. For a deep look at how toilet configurations affect buying decisions, the complete 2026 toilet-choosing guide covers smart, tankless and conventional options side by side.

How much water does a toilet actually use?

Federal law has capped residential toilet flush volume at 1.6 gallons per flush (GPF) since 1994, down from the 3.5 GPF that was standard through the 1980s. EPA WaterSense certification requires 1.28 GPF or below, and some high-efficiency models go further: the TOTO Aquia IV's half-flush uses 0.8 GPF, and some ultra-high-efficiency designs reach 0.8 GPF on their full flush while remaining WaterSense certified. The practical effect on a household's water use is significant. Moving from a pre-1994 3.5 GPF toilet to a 1.28 GPF WaterSense model saves over 13,000 gallons per year for a single toilet in a four-person home, a figure the EPA's WaterSense program has published based on average flush frequency data.

Within the 1.28 GPF standard, the engineering challenge is preserving flush power at the reduced volume. This is where MaP testing proves its value: it confirms that a toilet's reduced water volume actually produces adequate clearance rather than requiring two flushes (which would eliminate the water savings). The toilets that clear 1,000 grams on 1.28 gallons, including the TOTO Drake II, UltraMax II, Kohler Highline and Cimarron, and American Standard Champion 4 and Cadet 3, pass this test with wide margin. For buyers who want the full picture on toilet water use and efficiency, our coverage of round vs elongated toilet choices also touches on how bowl shape influences water use per flush.