A flush toilet stores a measured dose of water in a tank and releases it in one rush. Use the buttons to look inside and follow the water from the handle to the drain.
A flush toilet is a bowl connected to a drain by a bent pipe called an S-trap, with a tank of water sitting above it. Press the handle and the whole tank empties into the bowl in a couple of seconds. That single rush of water is what carries the waste away.
The design is clever because it needs no pump and no electricity. Gravity does the work: the water is stored high in the tank, so it arrives at the bowl with enough energy to cross the bend of the trap and start a siphon.
From the outside a toilet is a smooth white shape. Almost everything that matters — the fill valve, the float, the flapper and the trapway — is hidden inside, so hiding the outer shell shows you where the water actually goes and what each part does.
Two ideas explain the whole sequence: a seal that opens and closes (the flapper) and a bend that empties the bowl by siphon (the S-trap). Everything else exists to deliver water to those two parts at the right moment.
The tank holds a measured dose of water above the bowl. A float on an arm rests on the surface, and while the tank is full the fill valve stays shut. Every flush starts from this standing water.
Pressing the handle turns a lever inside the tank and lifts the chain. The chain pulls the rubber flapper up off the flush valve seat, breaking the seal at the bottom of the tank.
With the seal open, the stored water drops through the flush valve and rushes down into the bowl. Because most of the flush is this sudden release, the bowl water rises quickly and picks up the waste.
When the rising water fills the bend of the S-trap and flows over its weir, a siphon forms. The falling column of water in the trap pulls the bowl contents along with it until air reaches the bend and breaks the siphon.
As the tank empties, the float drops with the surface and reopens the fill valve. Fresh water refills the tank and a little is routed to the bowl, restoring the water seal. When the float rises to the set level, the valve shuts off and the toilet is ready again.
Modern toilets use about 6 litres (1.6 gallons) per flush, while older models can use 13 litres (3.5 gallons) or more. The tank is simply sized to deliver that single dose of water.
The fill valve refills the tank up to the set level and, on most toilets, also tops up the bowl through the overflow or a small refill tube. Once the float rises far enough, the valve shuts off.
The flapper is the rubber seal sitting on the flush valve seat at the bottom of the tank. Lifting it releases the stored water; when it drops back it seals the tank so the tank can refill.
The S-trap. When enough water pours into the bowl, it fills the bend, and once the flow passes over the weir a siphon forms that pulls the bowl contents out until air breaks it.
It is a safety drain. If the fill valve ever fails and the water keeps rising, the overflow tube lets it run into the bowl instead of spilling onto the floor.
That is the water seal. The S-trap holds water in its lowest bend, which blocks sewer gases from rising back into the room.
No. A gravity toilet uses only the weight of the water stored in the tank. The water flows downhill into the bowl and the siphon does the rest.