If the bowl is rising,
close the valve.
Most toilet overflows can be stopped with a valve that is already in the room. The repair starts after the floor is no longer receiving water.
Look behind the bowl, low, where the supply comes out of the wall. The small valve there is the angle stop. Turn it clockwise. The tank will stop refilling, and a toilet that is overflowing because its flapper or fill valve failed will stop. If you cannot reach it, or it will not turn, shut the house main. A few towels matter less than the shutoff.
If you close that valve and the bowl still rises, the water is not coming from the tank. It is coming back from the drain. That happens when the line downstream is closed and a fixture above this one is still being used. Stop water in the rest of the house. Do not keep flushing.
The bowl, the wax, the line
A toilet that rocks, or a ceiling stain directly under it, is often the seal at the floor, not a clog in the street. A toilet that overflows only after a long flush, then slowly recedes, may be a partial clog in that branch. A toilet that overflows together with a tub or a floor drain is the building drain or the sewer. Those three look similar in a panic and they are different jobs.
Do not pour a chemical opener into a bowl that is already full. You will be asking someone to pull a fixture full of it. A plunger is reasonable when this is the only fixture and the water level is falling, not rising. It is the wrong tool when sewage is appearing in a second fixture.
What to say when you call
Tell us whether the valve stopped it, whether any other fixture is involved, and whether this toilet was just installed or just snaked. A new toilet that leaks at the floor is not a sewer emergency. A toilet that backs up every Friday night might be. We would rather hear the plain sequence than a theory about the city main.
A toilet that will not stop running
An overflow and a running toilet are neighbors, not the same failure. If the bowl is calm and you hear water, look in the tank. A flapper that no longer seats lets the tank trickle into the bowl all day. A fill valve that will not shut off can raise the water until it spills down the overflow tube and, if the tube or the valve is wrong, over the rim. Dye from a tank tablet, and the chlorine in it, is a common reason a flapper warps. Replacing that flapper is a small repair. It is not a sewer problem, and it does not need a cable.
Jiggle the handle only to see whether the flapper drops. Do not keep holding it. If the water in the bowl is rising while the angle stop is still open, close the stop. Then you can take the lid off without making the flood worse.
One bathroom, or the whole side of the house
A toilet that overflows alone, and then slowly recedes, is often paper, a toy, or a partial clog in that branch. A toilet that overflows when the shower above it runs is the line those fixtures share. A toilet on the lowest floor that overflows when any water is used upstairs is the building drain or the yard line, and every extra gallon from a sink becomes sewage in that room. Tell the rest of the house to stop. That includes the laundry, which people forget because it is behind a door.
If this started the day a new toilet was set, say that. A poor seal, a jammed wax ring, or a flange that sits too low will leak at the floor without any clog at all. We would rather hear “it was installed Tuesday” than arrive ready to cable a line that was never the problem.
Related service: Toilets and fixtures. For the visit itself, call 818-631-7296 or send the address.