How can I tell if the tank is leaking into the bowl?
A few drops of food coloring will tell you. Put them in the tank, don't flush, and wait 15 to 20 minutes. Color in the bowl means the tank is leaking into it. Flush afterward so the dye doesn't stain.
If the water sits below the top of the overflow tube (the open pipe in the tank), it's usually the flapper. Less often it's the rim the flapper seals against, a cracked overflow tube, or a refill tube pushed down inside it.
Run that dye test before you buy a single part.
Another giveaway: the toilet refills for a few seconds when nobody has touched it. That's the tank topping itself up after a slow leak.
Why is the flapper usually the problem?
Because most flappers are rubber, and they sit in water all day. The flapper is the seal at the bottom of the tank. It lifts when you flush, then drops to hold the water in.
Over time the rubber warps, stiffens or collects minerals and stops sealing. Water trickles into the bowl, the fill valve tops the tank back up, and that cycle is your running toilet.
Wiping the flapper and its rim can buy time, but a new flapper is the real fix. Most are 2-inch or 3-inch, so take the old one to the store. Some toilets use a tall cylinder (a canister) instead, with a seal made for that model.
Honestly, we'd skip drop-in tank cleaners, especially bleach tablets. They can wear out rubber parts faster.
What if water keeps running into the overflow tube?
Then the fill valve isn't shutting off. Often the float is set so high that water spills into the tube before the valve gets the signal to stop.
Most fill valves have a screw or clip for lowering the float. Set the water to the line marked in the tank. With no line, go about half an inch to an inch below the top of the overflow tube.
Check the thin refill tube too. It should clip to the top of the overflow tube, not reach down inside, where it can siphon water out of the tank.
If the level is right and it still runs, make sure the float moves freely. If it does, the fill valve is worn or clogged and needs cleaning or replacing.
Can the chain make a toilet run?
Yes. The chain connects the handle to the flapper. Too short, and it holds the flapper slightly open. Too long, and extra links can catch under the flapper.
With the flapper closed, you want 1 or 2 links of slack, about half an inch. A sticky handle can do the same by keeping the chain tight. If you're jiggling the handle to make it stop, check the chain first.
Why does my toilet hiss or fill slowly?
Both usually point to the fill valve. A hiss means water is still getting through it into a full tank. Sometimes the valve isn't closing all the way. Sometimes it's topping up a leak past the flapper, so run the dye test first.
A slow refill is most often sediment in the fill valve, or a valve behind the toilet that's only partly open. Open that valve the rest of the way first, gently. Many fill valves can be flushed clean using the maker's instructions. If that doesn't help, or yours is old, replace it.
Which toilet repairs can I do myself?
The chain, the float and the flapper are fair game. The tank looks more complicated than it is. Lift the lid and it's a handful of parts, all in plain sight.
First, shut off the valve behind the toilet, then flush and hold the handle to drain the tank. Set the lid on a towel, since porcelain cracks if it falls.
A fill valve is a step up, since you disconnect the supply line. Our rule of thumb: take it on only if the valve behind the toilet turns easily and shuts the water off completely.
Follow the new valve's instructions. Set its height where they say, and hand-tighten the plastic nuts. Overtightening can crack a nut that lets go later. Replace an old supply line too, and check for drips the next day.
Here's where we'd draw the line: taking the tank off the bowl or lifting the toilet. Those jobs have more ways to leak, and a slow leak under a toilet can quietly rot the subfloor.
Is a running toilet worth fixing right away?
Soon, yes, though it rarely has to be today. A slow leak wastes water quietly, and the water bill often tells you first. Running nonstop into the overflow tube wastes far more, so fix that one first.
Could it be something bigger than a worn part?
Sometimes. A cracked overflow tube drains water into the bowl no matter how good the flapper is, and replacing it means taking the tank off. A cracked tank or bowl gets replaced, not patched.
If the toilet is old and has several worn parts, it's worth pricing a new one before fixing them one by one. We repair and replace toilets, and we'll tell you which makes more sense for yours.