How do I pick the right toilet flapper?

Measure the old flapper across its widest point, from one end to the other. Fluidmaster, a major maker of toilet repair parts, puts the rule plainly: a flapper that measures 3 inches across needs a 2-inch flapper, and one that measures 4 inches across needs a 3-inch flapper.

That sounds backward until you see why. The number in the name is the size of the drain opening the flapper covers, not the width of the rubber. The rubber has to be wider than the hole so it can lap over the edge and seal.

You can also measure the flush valve opening itself, the round hole in the bottom of the tank that the flapper sits on. That is the number that matters.

If the old flapper is missing, warped past measuring, or you simply can't tell, there's an easy way out. Fluidmaster's universal Super Flapper is sold as the one flapper that replaces both 2-inch and 3-inch styles, so you don't have to guess. Buy that, and the sizing question goes away.

Bring the old flapper to the store if you can. Or take a photo with a ruler next to it. Either one beats guessing.

Flappers come in adjustable and non-adjustable versions, and the difference is how long they hold open. Fluidmaster's guide matches them to the toilet's gallons per flush (gpf). Adjustable flappers are made for toilets that use 1.28 or 1.6 gpf. Non-adjustable flappers are made for older toilets that use 3.5, 5 or more gpf.

An adjustable flapper has a dial or slider that sets how long it stays lifted. That matters on a modern low-flow toilet, because it has less water to work with. If the flapper drops too early, the flush is weak. If it stays open too long, water runs into the bowl and wastes gallons.

How do you know which toilet you have? Look for the gpf stamped on the inside of the bowl or the back of the tank, or on the rim under the seat. The federal standard is 1.6 gpf. WaterSense labeled toilets use 1.28 gpf or less, which EPA says is 20 percent below the federal standard of 1.6.

Not sure of the age? Fluidmaster's advice is to guess the age of the home. A house built before the 1990s that never had a bathroom remodel probably still has the original toilet. A newer or remodeled bathroom almost certainly has a low-flow one.

Put simply, if you own a newer toilet, choose an adjustable flapper. That's the safe default for most Phoenix homes built or remodeled in the last thirty years.

The last check is the frame, the plastic ears that hook onto the pegs on the overflow tube. Fluidmaster sells two styles. The flexible frame bends a little, which helps it seat on uneven or slightly scaled valve seats. The solid frame is rigid and holds its shape.

Here's the catch. Fluidmaster says 2-inch flappers are available in both styles, while 3-inch flappers come only in the solid style. So if you have a 3-inch valve, the frame decision is made for you.

On a 2-inch valve, either style works, so pick the one that matches your old flapper and hooks on cleanly. Scale matters here too. Phoenix water is hard, and lime buildup on the valve seat keeps any flapper from sealing. Our page on how hard Phoenix water is explains why the scale forms so fast.

Also look at how your old flapper attaches to the overflow tube, and buy one that attaches the same way.

Sometimes, yes. Many toilet makers sell flappers made for their own models. Those fit exactly and are worth a look on unusual designs, like canister-style flush valves, where a standard flapper won't fit at all.

For a standard two-piece toilet with a regular flapper, a universal flapper from a repair-parts maker does the job. Check that the package lists your size (2-inch or 3-inch) and your flush volume, and you're set.

One more thing that matters more than the brand: skip in-tank cleaning tablets that contain bleach or chlorine. Fluidmaster warns that drop-in bowl cleaners with bleach or chlorine damage tank parts and can void warranties. Those tablets are a common reason flappers go soft and stop sealing early. If your old flapper looked slimy or shed black residue, that may be why.

Fit is only half of it. Three quick checks after the swap tell you it's right.

First, set the chain. It should have only a little slack, about one-half inch, when the flapper is closed. Too tight and it holds the flapper open. Too loose and it can slip under the flapper and jam it.

Second, wipe the flush valve seat. The flapper seals against that ring. Any scale or grit on it will let water past the new rubber. A soft cloth or a non-scratch pad is enough.

Third, run the dye test. Drop a little food coloring in the tank, wait 10 minutes without flushing, and look at the bowl. EPA WaterSense uses this exact test to find toilet leaks. Color in the bowl means water is still leaking past the seal. Clear water means you picked well.

If color still shows up, the flapper may be the wrong size after all, the seat may be pitted, or the chain may be off. Our guide on how to fix a running toilet walks through each one, and the steps in how to replace a toilet flapper or fill valve cover the swap itself.

### When is the flapper not the problem?

A flapper is the right guess for a running toilet, but it isn't the only one. If a new, correctly sized flapper still leaves the toilet running, look at the fill valve and float next. Water that keeps hissing into the tank with the flapper closed points that way. See why a toilet tank won't fill and why a toilet whistles when it fills for those cases.

A flapper also won't fix a weak flush that comes from a partial clog or a blocked vent. If the tank empties fine and the bowl still struggles, read why your toilet won't flush all the way.

A flapper swap is a good DIY job. If the valve seat itself is cracked or badly pitted, or you'd rather not open the tank, HQ Plumbing and Air can take care of it, and we give you the price up front before any work starts. Either way, you'll have picked the right part.