How To Stop an Toilet Overflow
If your toilet bowl is overflowing or about to, the key is to stop more water from entering the bowl. Water is entering the bowl in two places and both are in the tank portion of the toilet.
The first step is to lift off the lid covering the tank and set it carefully aside. The water in the tank is clean and fresh, you can reach into it. In fact, the water in the tank is safe to drink and is a good source of water in an emergency situation.
In the bottom of the tank is the flapper or stopper. Pushing the flush handle causes the flapper to lift up, allowing water to flow through the hole underneath. The water quickly drains out this hole and creates the sudden flush in the toilet bowl. If the flapper is still in the up position, quickly reach in and push it back down to close the drain. This will stop the main source of water.
The second way water enters the bowl is as part of the refill stage of a flush. The fill valve allows water to flow into the tank to refill it. There is also a tube that connects from the fill valve to the overflow tube. In the picture, the opening in the tube in the upper right is the overflow tube. The small tube connected from the fill valve can be seen positioned to spray water down the overflow tube. The water that flows through the overflow tube is used to refill the toilet bowl and runs as long as the water is running to refill the tank.
To stop the flow of water into the bowl, you can pull this tube out of the overflow and allow it to flow into the tank. You could also lift up the tank float, or the collar float on newer fill valves, to stop the fill valve completely. You could also turn off the water shut-off valve behind the toilet.
Once the water has been stopped, you can work on clearing the clog. If you wait, the water level in the bowl may drop, making it easier to use a plunger or closet auger without making a mess. Follow this link for instructions on how to unclog a toilet.