How to Fix a Running Toilet

Drop 5–10 drops of food coloring into the tank (the back of the toilet, not the bowl). Don't flush. Wait 15 minutes. If color appears in the bowl, your flapper is leaking — water is seeping past it from tank to bowl. If the fill valve keeps running even when the tank is full, that's a fill-valve or float-arm problem. If the water level is too low to flush properly, also a fill-valve problem. This 5-minute test tells you which part to buy.

Behind or below the toilet there's a small shutoff valve on the supply line. Turn it clockwise to close. Flush the toilet to drain the tank — hold the handle down to let as much water out as possible. Sponge or wet-vac the last inch of water in the tank so you can work dry. Have a couple of old towels on the floor for spills.

The flapper is the rubber/silicone disc at the bottom of the tank that seals the flush valve. Over time the rubber degrades, loses its seal, and water seeps continuously into the bowl. Replacement is one of the easiest plumbing repairs you'll ever do: unhook the chain, lift off the old flapper, install the new one in reverse. ~$8 part, 5-minute job.

If you hear water hissing or trickling even when the tank is full, the fill valve (the tall vertical assembly on the left side inside the tank) is no longer shutting off. Disconnect the water supply line, loosen the locknut underneath the tank, lift out the old assembly, drop in the new one, reconnect. ~$15 part, 15-minute job. The Fluidmaster 400A is the universal replacement.

If the toilet runs because the flapper closes too early (weak flush, then water keeps running to refill), the chain between the handle and the flapper is too long or too short. Adjust the chain so there's ½ inch of slack when the flapper is seated. Also check the flush valve (the larger assembly under the flapper) for cracks or mineral buildup — if cracked, the whole valve needs replacement, which means removing the tank.

After installing new parts, the tank water level needs to be set correctly. Mark on the inside of the tank is the 'water line' — fill to that mark, no higher (overflow risks) and no lower (weak flush). The float arm or float-cup on modern fill valves is adjusted by clip or screw. Test 3 flushes; the level should refill to the same mark every time.

Braided stainless supply lines have a service life of about 10 years. Plastic ones fail sooner. A burst supply line floods bathrooms; this is one of the top home-insurance claim categories. While you have the water off for the fix, replace the supply line if you can't remember when it was last changed. $8 part, 30 seconds of work, saves a $5,000 claim later.

Turn the supply valve back on. Let the tank fill. Flush 3 times in a row. Check for leaks at every connection: under the tank, at the supply-line nut, at the supply valve. Watch for ~10 minutes — slow leaks don't show up immediately. Re-do the dye test: 15 minutes after a flush, no color in the bowl means the new flapper is sealing.

The blue/green chlorine tablets sold for 'cleaning while you flush' destroy rubber flappers and gaskets, void manufacturer warranties (Kohler, Toto, and American Standard all explicitly call this out), and cause exactly the running-toilet problem you just fixed. If you want a clean-bowl product, use one that goes IN the bowl, not in the tank.

If you smell sewer gas after fixing the run, the wax ring between the toilet and floor has dried and cracked. This requires removing the entire toilet (two bolts, disconnect supply, lift), scraping off old wax, setting a new ring, re-setting the toilet. 90-minute job, $5 part. The smell goes away immediately.