How does a flush toilet work?

Most toilets work through gravity and a change in pressure. Pressing the handle lets stored water rush from the tank into the bowl. That surge fills a curved pipe called the trapway and creates a siphon, which draws the water and waste into the sewer line. When air enters the pipe, the siphon stops, and the tank refills for the next use. [1] [2]

A typical toilet has a tank that stores water, with a fill valve and a flush valve controlling the flow to and from it. The tank is either mounted on the bowl or built into it. At the rear of the bowl, an outlet leads to a wide, curved trapway that carries waste to the drainpipe. Inside the tank, a flapper covers the opening to the bowl, and a float on the fill valve tracks the water level. [1] [3]

Even at rest, some water stays in the bowl. It is held in the curved pipe at the base, known as the S-bend or S-trap, and it seals the sewage pipe so that germs and smells do not come up into the bathroom. [3] [4]

Pressing the handle operates a lever that lifts the flapper. Tank water then flows into the bowl through rim jets and a larger siphon jet. One MIT course report puts the volume at roughly 2 gallons for the toilet it describes, and one home-repair guide describes the flush as lasting about 10 seconds. [5] [2] [6]

The surge quickly fills the trapway and pushes out the air inside it. The resulting pressure change forms a siphon, and water and waste are pulled through the trapway into the sanitary sewer. A siphon is an inverted U-shaped pipe that moves water from a higher reservoir to a lower one. In a toilet the bowl is the higher reservoir and the sewer is the lower one. Normally the rising part of the pipe is not full of water, so no siphoning happens until the flush floods it. [1] [4]

Once the bowl is essentially empty, air enters the pipe and interrupts the siphon. The MIT report links this to the gurgle heard after a flush. Meanwhile the flapper drops back onto its seat, closing the opening between tank and bowl. [5] [4] [6]

As the tank water level falls, the float drops and opens the fill valve, which is connected to the water supply. Water flows in, and a refill tube also sends some down the overflow tube to restore the bowl's water level. As the tank fills, the float rises until it shuts the valve off at a preset level. [1] [2] [3]

There are many designs, but the same process applies to pressure-assisted and pump toilets. In those, supply-line water pressure or an electric pump adds to gravity when delivering water to the bowl. Many commercial toilets have no tank and rely only on supply-line pressure. [1]