That faint hiss from the bathroom at 2 a.m. isn't your imagination. A running toilet can waste hundreds of gallons a day, and you pay for every one of them on your water bill. The repair almost always comes down to one of three cheap parts inside the tank, and you don't need a plumber or any special tools to replace them.
Here is how to figure out which part is at fault, what it costs, and how to swap it out.
How a Toilet Tank Works (the 60-Second Version)
When you push the handle, a chain lifts the flapper, a rubber seal at the bottom of the tank. Water rushes through the flush valve into the bowl. As the tank empties, the flapper falls back and seals the opening. Then the fill valve, the tall assembly on the left side, refills the tank until a float rises high enough to shut it off. A small refill tube sends some water down the overflow tube to refill the bowl.
A toilet runs when water keeps leaving the tank, or when the fill valve never gets the signal to stop. That's it.
What You'll Need
- Replacement flapper (about $5 to $15) or a universal fill valve (about $10 to $25), depending on the diagnosis
- Large sponge and an old towel
- Bucket
- Adjustable wrench or channel-lock pliers
- Food coloring or a dye tablet
- Rubber gloves
- Your phone, to take a photo of the inside of the tank before you start
Step 1: Take the Lid Off and Watch
Set the tank lid on a towel on the floor, not on the toilet seat where it can slide off and crack. Flush once and watch what happens inside.
Then look at the water level after the tank refills. There's usually a line marked on the inside of the tank or on the overflow tube. Two scenarios tell you most of what you need to know:
- Water is spilling into the top of the overflow tube. The fill valve is not shutting off in time. Go to Step 3.
- Water level is below the top of the overflow tube, but the tank keeps refilling every few minutes (sometimes called "phantom flushing"). Water is leaking past the flapper. Go to Step 2.
To confirm a flapper leak, put a few drops of food coloring in the tank and don't flush for 20 to 30 minutes. If colored water shows up in the bowl, the flapper isn't sealing.
Step 2: Fix the Chain or the Flapper
Check the chain first. It's free. A chain that's too tight holds the flapper slightly open. A chain that's too long can get trapped under the flapper. You want about 1/2 inch of slack when the flapper is closed. Unhook the clip from the handle arm and move it one link at a time until it's right, then trim extra chain so the tail can't fall under the seal.
Inspect the flapper. Run a finger around its rim. If the rubber is warped, slimy, stiff or leaves black residue on your fingers, it's done. Chlorine and in-tank cleaning tablets speed up this decay, so flappers often last only three to five years.
Replace it:
- Turn off the water at the shutoff valve behind the toilet (clockwise). Flush and hold the handle down to drain as much as possible. Sponge out the rest.
- Unhook the chain. Pop the flapper's side ears off the pegs on the overflow tube, or slide the ring off the tube if it's the ring style.
- Check the size. Most American toilets use a 2-inch flapper; many newer high-efficiency models use 3-inch. Measure the drain opening, or take the old one to the store.
- Wipe the valve seat, the ring the flapper sits on, with a soft scrub pad. Mineral crust or a nick there will defeat even a new flapper. Use a non-scratch pad, not steel wool.
- Attach the new flapper, connect the chain with 1/2 inch of slack, turn the water on, and test with dye again.
Step 3: Adjust or Replace the Fill Valve
If water is spilling over the overflow tube, start by lowering the water level.
- Modern float-cup valves: the float is a cylinder that slides up and down the valve body. Squeeze the metal clip on the adjustment rod, or turn the adjustment screw on top, to lower the cup about 1/2 inch.
- Older ball-float valves: a plastic or copper ball sits on a long arm. Gently bend the arm downward, or turn the adjusting screw where the arm meets the valve.
The water should stop about 1 inch below the top of the overflow tube.
If the valve still won't shut off, hisses constantly, or refills slowly, replace it. Universal fill valves fit nearly every standard toilet and come with instructions, but the outline is the same:
- Shut off the water, flush and sponge the tank dry.
- Put a bucket and towel under the tank. Unscrew the supply line nut under the tank, then the large plastic locknut that holds the valve. A little water will drip out.
- Lift out the old valve. Insert the new one and adjust its height so the critical level mark sits at least 1 inch above the overflow tube opening, as the package directs.
- Hand-tighten the locknut, then give it a quarter turn more with pliers. Overtightening can crack the tank, which is a much more expensive problem.
- Reconnect the supply line, clip the refill tube to the overflow tube (it should not reach down into it below the water line), turn on the water and check for drips under the tank.
Step 4: Check the Handle
Sometimes the handle sticks in the down position, so the flapper never fully drops. If the handle feels loose or sticky, tighten the nut inside the tank. Note that it's usually reverse-threaded, so you turn it clockwise to loosen. A new handle costs about $5 to $10.
What This Usually Costs
At a typical US hardware store, here's the ballpark:
- Chain adjustment: free
- Flapper: roughly $5 to $15
- Universal fill valve: roughly $10 to $25
- Complete flush valve and fill valve kit: roughly $25 to $45
- A plumber's visit for the same job: often $150 or more, depending on your area
When It's More Than a Flapper
If you replace the flapper and dye still seeps into the bowl, the valve seat may be pitted or cracked. You can buy a seat repair kit that glues a new surface on top, or replace the whole flush valve. Replacing the flush valve on a two-piece toilet means removing the tank from the bowl, which is still a DIY job, but plan on two hours and new tank bolts and gasket.
Also check outside the tank. Water on the floor, rather than running inside, points to a supply line, tank bolt or wax ring problem, which is a separate repair.
Tips
- Take a photo of the tank interior before you remove anything. It's a free reference if you forget how the chain was hooked.
- Skip in-tank bleach or chlorine tablets. They shorten the life of rubber parts.
- Keep a spare flapper in the garage. When it fails, you can fix it in ten minutes.
- If the shutoff valve won't turn or drips when you turn it, don't force it. That valve may need replacing, and that is a good moment to call a plumber.
Wrapping Up
Most running toilets are fixed with a chain tweak or a $10 flapper, and the rest with a new fill valve. Work through the diagnosis in order, test with dye, and you'll know for certain the problem is solved, rather than hoping the hiss doesn't come back. While the lid is off, it's a good time to check every toilet in the house the same way.
