Your toilet flushes just fine, but it doesn't know when to quit. Perhaps it stops running and then starts up again suddenly. Either way, it's wasting a lot of water and making that noise that keeps you up at night. Fortunately, it's usually not difficult or expensive to repair a toilet if you know a bit about how they work.

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[edit] Steps

Inside the tank, left to right: valve and float, fill tube, overflow tube and flapper. Handle connects to lever and chain. Click to enlarge.

Get to know what's in your toilet. Mechanisms vary, but they all work on the same principles. Flush a couple of times while you watch in the tank with the tank lid off and notice the process.
- When you push the handle, the chain lifts a flapper, letting a tankful of water fall through the opening in the bottom, into the bowl. As the water level drops, the flapper drops and closes the opening.
- A plastic float drops as the water drains. The float is connected to a valve that lets water into the tank when the float is down and stops (or should stop) when the float is up.
- In the middle, there's also an overflow tube that drains water out into the bowl if it gets too high.
Catch it in the act. If you've waited long enough after flushing and the toilet hasn't quit running, lift the tank lid and look in.
The lever, chain and flapper must all work together. Adjust them so they do not catch or tangle.

Close the flapper. If the tank is not full and it is not filling, chances are that the flapper is stuck open. Reach in and close it with your hand. If it sticks repeatedly, look for the cause. Make any necessary adjustments.
- Is the chain catching on something?
- Is the flapper catching on the chain?
- Is the flapper wedged open on its hinge?
- Is the flapper aligned with the opening?
- Is the flapper simply just old and stiff (and needs replacing)?
- If you have a ball seal instead of a flapper, is the wire that lifts the ball straight and does it move freely?
Adjust the valve and float. If the tank is full and the flapper is closed, and water is running over the top of the overflow tube, but the toilet hasn't stopped running, first try adjusting the valve and float. Pull up on the float with your hand. If this action stops the flow, then adjust the level of the float. No matter how you adjust the float, adjust it so the tank stops filling when the water is about an inch (2.5cm) below the top of the overflow tube. A leaking flapper can be caused by excess pressure if the tank level is too high, even with a brand new replacement.
- Pinch this clip to adjust the float height.
  
  If the float is around the valve post, pinch the metal clip and slide the float down on the wire.
- On this style valve, tighten (clockwise) the blue screw or try bending the rod down.
  
  If the float is a ball on an arm, try turning the small screws on top of the valve. Sometimes, you can also bend the arm further down.
- Make sure the float ball isn't touching anything else. Adjust it so it isn't dragging against the side of the tank, the overflow tube, or anything else.
- Make sure the float ball isn't leaking or filling with water. If you unscrew the float ball and hear water inside when you shake it, replace the float ball.
Replace the flapper. If the toilet stops filling and then starts again intermittently, you have a slow leak. Try this to be certain. Place a dye tablet or a few drops of food coloring in the tank. Your local hardware store may have free dye tablets for this purpose. If, after an hour or two without flushing, you see this dye in the bowl, you have a slow leak, a small amount of water running into the bowl. The most common cause of slow leaks is a leaky flapper. Over time, this inexpensive rubber part decays, or minerals build up on it. Cleaning may work to remove mineral buildup, but it's usually best just to replace the whole part. There are a few standard kinds. Take your old one with you to the hardware store if you want the comparison to get the right size.
- Close this valve by turning it clockwise, as you would a faucet.
  
  To perform the replacement, simply close the valve sticking out of the wall under your toilet tank.
- Flush the toilet. If the valve is completely closed, the tank will not refill and you will not hear water running after the tank empties.
- Pop the old flapper off its hinges, disconnect it from the chain, and pop the new one into place.
- Run a finger carefully around the rim where the flapper seats. Remove any uneven buildup of minerals that might cause a leak.
- Don't forget to open the valve all the way when you're ready for water again.
- Try flushing a few times to make sure the chain is the right length for the new flapper. It should open when you push the handle and then drop closed all the way when the tank empties. You may have to trim and adjust the chain by trial and error. Also, make sure that the flapper aligns properly with the opening.
Troubleshoot other possible problems. Occasionally, something else will cause water to drain slowly into the tank.
- The small rubber fill tube leading from the valve to the overflow tube and sometimes the valve itself can act as a siphon. In that case, adjust the valve height or tube height up, or adjust the water level down.
- The valve itself will not stop the water completely. Some valves can be opened and the rubber seals replaced. If not, you may need to replace the whole valve.
- One or more of the non-rubber components may break in the toilet's water valve mechanism, such as the lever connected to the plastic ball that shuts off the water by pressing down on a button as the water level rises. If this happens, the best course of action is to buy a replacement, but super glue can work temporarily in some situations.

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[edit] Tips

If you notice the leak in the middle of the night or some other time you can't get to working on the problem right away, close the shutoff valve to avoid using excess water. Post a note nearby that the water is shut off temporarily, and can be turned back on to fill up the tank if needed, to avoid panicking your guests.
Be systematic about tracking down the problem. There are only so many things that can go wrong inside a toilet tank.
Watch in the tank as the toilet flushes a couple of times and it won't be too hard to figure out what parts do what.
If pulling the float up gently to the top of its travel does not stop the toilet running, and you've tried adjusting those screws on top of the valve, you may have to replace the whole valve assembly. Replacing the whole valve is a bit more of a project, so check the other possible causes and remedies thoroughly first. If you think you need to replace the valve, it is manageable by one person and not too expensive. Ask for advice at your hardware store, and read the directions carefully that come with the replacement valve.
If the ball valve and assembly are covered in limescale then why not descale them while you have it disassembled. It only takes minutes and is well worth the effort. If you have the ball valve out but cannot get it apart to get at the valve washers its often the limescale that's gluing it together.
Try threading the flapper chain through a plastic soda straw to prevent a long chain from getting stuck on things and preventing the flapper from seating properly.
Rinse off the flapper if the flapper is still in good shape. Sometimes this is all it takes to make it work.
If you must replace the valve post, first shut off the main inlet valve, and then flush the toilet fully, which will *almost* empty the tank. Keep an old towel handy and a large cup to catch any residual water in the tank when you unscrew the valve post from its hole in the bottom of the tank. Obviously not heeding these precautions will produce a big mess on your bathroom floor.
Depending on the design of the float mechanism and how it relates to the fill tube, the fill tube can occasionally go over the float mechanism and hold it down. Don't move the fill tube while the toilet is filling; you may be in for a wet surprise.

[edit] Warnings

The water in a toilet tank is clean and hasn't been through the bowl yet, but it doesn't hurt to wash your hands after working in there, just on general principles.
If you live in an apartment or other rented residence, get permission from the management before performing any major repairs. Replacing a flapper or untangling a chain is no big deal, but replacing the valve could be.
Do not use the toilet cleaning pellets that drop or hang in the tank and turn the water blue. Having all those chemicals in the tank degrades the mechanism much faster. If a bowl brush doesn't suit you, look for the sort of in-tank cleaning system that dispenses directly into the fill tube.
These instructions are appropriate for most ordinary household toilets. While they are rarer, pressure tank toilets and other designs exist^[1]. Don't try to service a pressure tank yourself.
Tank lids are heavy, ceramic objects. Take care not to drop them.