The toilet is overflowing. Stop it in ten seconds.

Lift the lid off the tank and push the rubber flapper at the bottom down with your hand. That stops the water leaving the tank immediately. Then reach behind the toilet and close the small valve on the supply line, turning it clockwise.

Both take seconds and either one alone will stop the bowl filling. Do them before you look for a mop, and do not flush again to see what happens.

Why the flapper first

The bowl overflows because the tank is emptying into a bowl that cannot drain. The flapper is the rubber seal at the bottom of the tank that lifts when you flush. Pushing it back down seats it and the tank stops discharging, which is faster than finding the valve.

Then close the supply valve, because otherwise the tank simply refills and you are one accidental flush away from doing it again. If the valve is seized, lift the float in the tank instead and prop it up, which stops the refill valve. Old supply valves under toilets seize often, so do not force one that will not move. If it will not turn at all, there are other ways to stop the water.

Work out which kind of water you are standing in

This is the decision that changes everything afterwards, and it is not about how it looks.

Blockage in the toilet itself, past the bowl but before the drain: the water that came over the rim is tank water plus whatever was in the bowl. If the bowl was clean, this is close to clean water and it is mostly a drying problem.

Blockage further down the drain line, or coming up when you run a washing machine or a shower, or coming up in more than one fixture: that is not a toilet problem. That is a drain or sewer backup, the water is contaminated, and it needs to be handled as a health problem rather than a spill.

The tell is simple. One fixture affected, and everything else drains normally, means the blockage is local. More than one fixture, gurgling in a nearby drain, or water rising in the shower when the toilet is flushed, means the blockage is downstream and shared.

Contain it before you clear it

The water is already spreading. Bathrooms usually have hard floors, which means it runs to the edges of the room and under the flooring at the threshold rather than sitting where you can see it.

• Towels or old bedding along the doorway first, to keep it in one room.
• Then lift anything absorbent off the floor: bath mats, laundry baskets, the cardboard box under the sink.
• Get standing water up with a wet vacuum if you have one, or towels wrung into a bucket.
• Check the ceiling of the room below if this is an upper floor. Water that reaches the gap around a toilet flange goes straight through.

Then clear the blockage

Once the water is off and the floor is contained, the blockage itself is usually simple.

Use a flange plunger, the kind with a soft extended sleeve that folds out of the cup, not the flat cup style meant for sinks. Seat it fully over the exit hole, push gently to expel air first, then work it firmly a dozen times without breaking the seal. The pull matters as much as the push.

If the bowl is full to the brim, take some water out with a jug into a bucket before you start, or the plunge puts it on the floor.

A toilet auger (a closet auger) is the next step: a short cranked cable with a protective sleeve, made to turn the trap of a toilet without scratching it. A drain snake from the hardware store will scratch the porcelain.

Do not use chemical drain cleaner in a toilet. It rarely clears the kind of blockage a toilet gets, it sits in the bowl as a caustic pool, and it makes the job dangerous for whoever puts a plunger or a hand in there next.

If plunging and augering do not clear it, stop. Repeated hard plunging against a solid blockage can unseat the wax ring under the toilet, which turns a blockage into a leak at the floor that you cannot see.

What actually caused it

Worth knowing, because it changes whether it happens again:

• Something that should not have been flushed. Wipes labeled flushable are the most common single cause. They do not break up.
• A partial blockage that has been building. If the bowl has been draining slowly for weeks, this was coming.
• A blocked vent. If the toilet gurgles and drains slowly and there is a smell, the vent stack on the roof may be blocked, often by a nest.
• A problem in the main line. Tree roots, a collapsed section, or a blocked sewer connection. This is the one that comes back, and it usually shows in more than one fixture.

The drying part

If the water was clean and the area is small, air movement and time will usually do it. Pull up the bath mat, open the window, and move air across the floor rather than at it.

The thing to check is what is under and behind the toilet, and what is at the threshold. Water that gets under vinyl or laminate has nowhere to escape from, and the window before that becomes a mold problem is 24 to 48 hours from when it got wet.

If the water was contaminated, or it reached carpet, or it went through to the floor below, that is past the point where drying it yourself is the right call.

The short version

Flapper down, supply valve closed, do not flush again. One fixture affected means a local blockage and mostly clean water. More than one means a drain backup and contaminated water. Plunge with a flange plunger, never chemicals, and stop before you unseat the toilet.