How to Fix a Running Toilet — Replace the Flapper and Fill Valve

Learn how to fix a running toilet by replacing the flapper, adjusting the fill valve, or swapping out the entire valve assembly — no plumber needed, under $25 in parts.

That faint, periodic trickling sound from your toilet isn’t just annoying — it’s wasting up to 200 gallons of water a day. A running toilet is one of the most common (and easiest) home repairs you’ll ever face. Here’s how to diagnose the problem and fix it yourself in under 30 minutes.

What Causes a Toilet to Keep Running?

A toilet runs when water continues flowing from the tank into the bowl after flushing. The toilet tank should refill and then stop. If it never stops, or if it starts trickling minutes after a flush, one of three parts is usually to blame:

1. A worn flapper — The rubber flapper at the bottom of the tank degrades, curls, or no longer seals against the flush valve. Water seeps past, so the fill valve keeps running.
2. A misadjusted or faulty fill valve — The fill valve (on the left side of the tank) doesn’t shut off at the correct water level, or the float is stuck.
3. A loose or corroded flush handle/chain — The chain connecting the handle to the flapper is too tight (catching and holding the flapper open) or too loose (failing to lift it properly).

What You’ll Need

• New flapper (universal, about $5–$8)
• New fill valve (if needed, universal, about $10–$15)
• Adjustable wrench or pliers
• Flathead screwdriver
• Clean rag or sponge
• Bucket (to catch water)

Step 1: Diagnose the Problem

Before touching anything, flush the toilet and watch what happens inside the tank:

Scenario A — Water runs constantly and never stops: The flapper isn’t sealing or the fill valve isn’t shutting off. Start with the flapper.

Scenario B — Water stops briefly, then starts trickling 30–60 seconds later: The flapper seal is leaking slowly. Replace the flapper.

Scenario C — Water flows but the tank is slow to fill: The fill valve is partially clogged or failing. Replace the fill valve.

Scenario D — Runs only when the handle is held down: The chain is too short and the flapper is being held open. Lengthen the chain.

The dye test: Put a few drops of food coloring in the tank. Wait 15 minutes without flushing. If colored water appears in the bowl, the flapper is leaking.

Step 2: Fix the Flapper (The Most Common Fix)

Turn off the water at the shutoff valve behind the toilet (turn clockwise).

Flush the toilet to drain the tank. Sponge out the remaining water at the bottom.

Unhook the old flapper from the ears on the overflow tube. Slide the chain off the handle arm.

Install the new flapper:

• Slide the rubber ears onto the overflow tube pegs.
• Attach the chain to the handle arm with about ½ inch of slack — not tight, not dragging.
• Press down on the flapper to ensure it seats evenly over the flush valve opening.

Turn the water back on, let the tank fill, flush once, and check. Did the running stop? If yes, you’re done. If not, move to Step 3.

Step 3: Adjust the Fill Valve

The fill valve controls when the tank stops filling. Locate it on the left side of the tank — it’s the tall plastic column with a float.

For a float-cup fill valve (the most common type, like Fluidmaster):

• Pinch the adjustment clip and slide the float up to raise the water level, or down to lower it.
• The water level should sit about ½ inch below the top of the overflow tube.
• Each adjustment changes the shutoff point. Flush and check after each tweak.

For an old-style float-ball fill valve:

• Gently bend the brass float arm downward to lower the water level (causes earlier shutoff), or upward to raise it.
• If the float ball is waterlogged (it has water inside), replace the entire fill valve.

Step 4: Replace the Fill Valve (If Needed)

If adjusting the fill valve doesn’t fix it, or if the valve is visibly corroded or leaking, replace it entirely.

Drain the tank (shutoff valve off → flush → sponge remaining water).

Disconnect the water supply line from the bottom of the fill valve using an adjustable wrench.

Remove the fill valve:

• Unscrew the large plastic lock nut on the underside of the tank (inside the tank, behind the toilet).
• Pull the old fill valve out through the top of the tank.

Install the new fill valve:

• Slide the new fill valve through the tank hole. The rubber gasket goes under the tank (outside).
• Tighten the plastic lock nut hand-tight, then about ¼ turn with pliers — don’t overdo it.
• Reconnect the water supply line.

Adjust the new fill valve height so the critical water level mark on the valve body sits about 1 inch above the overflow tube top. Trim the valve tube if needed (most have a twist-and-cut section).

Turn the water on, flush, and make final height adjustments.

Step 5: Check the Flush Handle and Chain

A simple adjustment can fix many running toilets without replacing any parts:

• Chain too short: Lengthen by moving the hook to a higher link. You want a gentle droop, not tension.
• Chain too long: Shorten to prevent tangling or the hook slipping off the handle arm.
• Handle sticking: Tighten or loosen the mounting nut behind the handle. WD-40 on the handle mechanism can free up corrosion.
• Handle arm hitting the lid: Bend the arm slightly so the lid fits without pushing the arm down.

When to Call a Plumber

Most running toilet fixes are dead simple. Call a pro if:

• The porcelain tank is cracked (replace the toilet).
• The shutoff valve is seized or leaking when you turn it.
• The toilet is severely corroded inside and parts won’t budge.
• You’ve replaced both the flapper and fill valve and it’s still running — there may be a crack in the flush valve or overflow tube.

Bottom Line

A running toilet is rarely a serious problem — 90% of the time it’s a $7 flapper replacement that takes 5 minutes. The remaining 10% is a $15 fill valve swap that takes about 20 minutes. Your water bill will thank you, and that phantom trickling sound will be gone for good.