How to Fix a Running Toilet (Without Calling a Plumber)

A running toilet is the single most common plumbing repair in a US home — and one of the easiest to fix yourself. The water you hear flowing into the bowl 24 hours a day isn't a mystery: something in the tank isn't sealing, and water that should be sitting still is escaping through a leak path you can identify in 60 seconds. Once you've identified which of the three common failure modes you have, the fix takes 15–20 minutes with parts that cost $5–$20 at any hardware store.

Below: the three common causes, how to identify which one you have without guessing, the tools and parts needed, the repair procedure for each cause, and the rare cases where it's something else (and a plumber call is actually warranted).

Why a toilet runs continuously

Inside your toilet tank are three components: the flapper (a rubber seal at the bottom that lifts when you flush and falls back to seal the tank), the fill valve (the column on the left side that refills the tank after a flush), and the overflow tube (the vertical plastic tube in the middle that handles excess water). A running toilet is always one of these failing:

Worn flapper (about 70% of running-toilet cases). The rubber seal hardens, warps, or develops mineral deposits and stops sealing properly. Water continuously seeps from tank to bowl. About $5 to fix.
Misadjusted or failed fill valve (about 20%). The fill valve doesn't shut off when the tank is full, so water keeps entering, hits the overflow tube, and drains into the bowl. About $15–$20 to fix if you need a new valve; $0 if it just needs adjustment.
Worn flush valve seat (about 10%). The plastic ring the flapper seals against has eroded or cracked. The flapper can't seal even if it's new. About $15 to fix with a flush valve repair kit.

Diagnose in 60 seconds

Before buying any parts, identify which failure you have. The diagnostic test takes under a minute:

Remove the tank lid and listen. If water is audibly running into the bowl from the flapper area at the bottom of the tank, it's a flapper or flush valve seat issue. If water is running into the overflow tube (the vertical tube in the middle) from above, it's the fill valve.
The dye test. Drop 5–10 drops of food coloring or a dye tablet into the tank water. Don't flush. Wait 15 minutes. If colored water appears in the bowl without flushing, the flapper is leaking. If the tank water level is at or above the top of the overflow tube, the fill valve is overfilling.
Inspect the flapper visually. Lift it gently. If the underside is hardened, cracked, distorted, or covered in mineral residue, it's the flapper. Newer flappers are pliable and uniformly black or red on the seal surface.
Inspect the flush valve seat. With the flapper lifted, look at the plastic ring underneath. If you see cracks, gouges, mineral buildup, or pitting, the seat itself is the problem — and a new flapper alone won't seal against it.

About 90% of the time, this diagnostic lands you on the flapper. Buy a $5 flapper that matches your toilet brand, install it, done. The other 10% is covered below.

Tools and parts you'll need

An adjustable wrench or channel-lock pliers
A sponge and bucket (for clearing the tank water)
Towels
Replacement flapper sized for your toilet (Korky 2001BP universal works for most modern toilets; Fluidmaster 5403 is another reliable option). For Kohler, American Standard, TOTO, or older toilets, get a brand-matched flapper — the seal geometry differs.
Optional: a new fill valve (Fluidmaster 400A is the universal default, about $10) if the fill valve is the failure
Optional: a flush valve repair kit (Korky 481BP for 2" valves, 481-3BP for 3" valves) if the seat is damaged

Repair 1: Replace the flapper

This is the fix for about 70% of running toilets. Total time: 15 minutes.

Step 1 — Shut off the water

The shutoff valve is a small chrome or plastic valve on the wall behind the toilet, on the left side. Turn it clockwise until it stops. If your shutoff is stuck or doesn't fully close (a common issue with valves more than 10 years old), shut off the water at the main supply for the bathroom or the house.

Step 2 — Drain the tank

Flush the toilet to empty most of the tank water. Use a sponge to soak up the remaining water at the bottom — you don't need it bone dry, just enough that the flapper area is accessible.

Step 3 — Remove the old flapper

The flapper is connected by two small ears that hook over the pegs on either side of the overflow tube. Unhook each ear (gently — the ears are flexible plastic but can crack on old flappers). Disconnect the chain from the flush handle's lever. The flapper lifts straight out.

Step 4 — Install the new flapper

Hook the new flapper's ears onto the same overflow tube pegs. Reattach the chain to the flush handle's lever. Leave about 1/2 inch of slack in the chain — too tight and the flapper won't sit fully closed; too loose and the flush won't be powerful enough.

Step 5 — Restore water and test

Turn the shutoff valve back on. Wait for the tank to fill. Flush once and watch the flapper — it should lift completely, then drop back fully to seal. Listen for any continued running. Do the dye test again after 30 minutes to confirm the leak is gone.

Repair 2: Adjust or replace the fill valve

If the dye test showed the leak is at the overflow tube (water entering it from above) rather than at the flapper, the fill valve is the issue. First try adjustment; if that doesn't fix it, replace the valve.

Adjustment first (free fix)

Most modern fill valves have a clip or screw that adjusts the water level. The target water level is about 1 inch below the top of the overflow tube (most overflow tubes have a marked fill line). To lower the water level: on Fluidmaster-style valves, pinch the clip on the float rod and slide the float downward by about an inch. On older ballcock-style valves, gently bend the float arm downward.

Flush the toilet, let it refill, and check the new water level. Repeat until water sits about 1 inch below the overflow tube top. About 60% of the time, this adjustment alone fixes the running toilet.

If adjustment doesn't fix it: replace the fill valve

If the valve still runs continuously even with the water level set correctly, the valve itself has failed and needs replacement. The Fluidmaster 400A is the universal default; it fits virtually any US toilet, costs about $10, and takes 20 minutes to install.

Shut off the water at the toilet's shutoff valve and flush to drain the tank.
Disconnect the water supply line from the bottom of the fill valve. Have a towel ready — there's always some residual water.
Unscrew the locknut underneath the tank that holds the fill valve in place. Lift the old valve out.
Install the new valve, following the manufacturer's height adjustment instructions (the valve top should sit about 1 inch above the overflow tube).
Secure the locknut underneath, reconnect the supply line, and turn the water back on.
Adjust the float to set the water level 1 inch below the overflow tube top.

Repair 3: Replace the flush valve seat

If you've installed a new flapper and the toilet still runs, and the dye test still shows water seeping from tank to bowl, the flush valve seat (the plastic ring the flapper seals against) is the issue. The fix is a flush valve repair kit — about $15 — which usually consists of a new sealing ring that adheres to the existing seat plus a new flapper.

The Korky 481BP kit is the most common solution: a soft rubber gasket with adhesive backing that sticks onto your existing flush valve seat, restoring a smooth sealing surface. Installation:

Shut off water and drain the tank.
Remove the old flapper.
Clean the flush valve seat thoroughly — a wire brush and white vinegar removes mineral buildup. The surface needs to be clean and dry for the adhesive to bond.
Peel the backing off the new sealing ring and press it onto the flush valve seat. Apply firm pressure for 30 seconds.
Install the new flapper (included in the kit) onto the overflow tube pegs.
Turn water back on and test with the dye method.

For severely damaged flush valves where even an adhesive kit won't seal, the whole flush valve assembly can be replaced. This is more involved — it requires removing the tank from the bowl, which means lifting the tank, replacing the spud washer, and remounting. About $20 in parts and 60–90 minutes of work, or $200–$350 for a plumber. At that point, given the labor involved, it's often worth considering a new toilet entirely — see our best toilets list for current picks.

Less common causes

If you've checked the flapper, fill valve, and seat and none of them is the issue, look at these rarer failure modes:

Chain too tight or tangled

The chain connecting the flush handle to the flapper can be too short (preventing the flapper from sealing fully) or tangled around itself or the overflow tube. Loosen the chain to about 1/2 inch of slack. If it's tangled, untangle it or replace with a new chain (about $2).

Flush handle stuck or sticking

Sometimes the flush handle itself sticks in the partially-depressed position, holding the flapper slightly open. Check that the handle returns fully to its resting position after each flush. If it's sticky, the handle mounting nut may be over-tightened — loosen it by 1/8 turn.

Tank-to-bowl gasket leak

If water is leaking between the tank and the bowl (not into the bowl through the flapper), the spud washer between them has failed. This causes water to drip onto the floor rather than into the bowl — but it's worth knowing about because it can look similar from a distance. Fix involves removing the tank, replacing the spud washer ($5), and reseating.

Crack in the overflow tube

Rare, but possible on toilets more than 15 years old. If the overflow tube has a crack below the water line, water continuously drains through the crack. Visible inspection identifies it. Fix requires replacing the entire flush valve (more involved repair).

What it costs (DIY vs. plumber)

DIY: $5 for a flapper, $10 for a fill valve, $15 for a flush valve repair kit. Total time: 15 minutes for the flapper, 30 minutes for the fill valve, 45 minutes for the seat repair. Even if all three need replacement, the parts total is about $30 and the total time is under 90 minutes.

Professional plumber: $150–$350 for the same repair, depending on region. The parts cost is the same; you're paying for the labor and trip charge. For a confident DIY-er, this is one of the highest-ROI repairs in the house — both in terms of savings per hour of work and in terms of water-bill impact.

A continuously running toilet wastes roughly 200 gallons per day. At average US water rates ($3–$8 per 1,000 gallons depending on region), that's $20–$50 per month, every month, until it's fixed. Six months of running adds up to enough water-bill savings to pay for a plumber even if you weren't going to DIY it.

Preventing running toilets

Replace the flapper preventively every 3–5 years. They're a $5 wear item. Doing it on a schedule prevents the slow-leak phase that's easy to miss.
Avoid drop-in toilet tank cleaners with bleach. The bleach is harsh on the rubber flapper and shortens its life significantly. Use bowl cleaners that are applied to the bowl, not the tank.
If you have hard water, the failure window is shorter. Mineral deposits accelerate flapper and seat wear. Consider a whole-house water softener — toilet components last roughly twice as long on softened water.
Set a calendar reminder to do the dye test every 6 months. A leak caught early is the $5 flapper fix; a leak ignored for a year can erode the flush valve seat, turning a $5 repair into a $15 repair.

When to call a plumber

Call a licensed plumber if any of the following apply: you can't get the shutoff valve closed (the valve itself is failing); the toilet's tank or bowl has a visible crack (replacement, not repair); water is seeping from underneath the toilet onto the floor (wax ring failure, requires removing the toilet); or you've replaced the flapper, fill valve, and flush valve seat and it's still running (rare but possible — at that point it's usually a tank that's no longer worth saving on a toilet 20+ years old).

For everything else, this is genuinely one of the most rewarding 20-minute repairs you can do. Once you've fixed one running toilet, the next one will take you 10 minutes start to finish — and you'll save a few hundred dollars and a few hundred gallons of wasted water every time.

Frequently Asked Questions

How do I know if it's the flapper or the fill valve?

Open the tank lid and listen for where the water is entering. If water is running from the bottom of the tank into the bowl, it's the flapper. If water is running into the overflow tube (the vertical tube in the middle), it's the fill valve. The dye test confirms — drop food coloring in the tank, don't flush, wait 15 minutes; colored water in the bowl means flapper, water above the overflow tube top means fill valve.

Will a universal flapper work, or do I need a brand-specific one?

For modern toilets (most made after 2000) with a standard 2-inch flush valve, a universal flapper like the Korky 2001BP fits the majority of US toilets. For older toilets, low-flow models with a 3-inch flush valve, or brand-specific designs (Kohler Class 5, American Standard Champion, TOTO with their specific flapper shapes), get a brand-matched flapper — the seal geometry differs and universal flappers often don't seat properly. The packaging on most universal flappers lists which brands they fit.

My fill valve makes loud noises when filling — does it need to be replaced?

Not necessarily. Whistling, water hammer, or vibration during fill is usually caused by water pressure that's too high (above 80 psi), partially-closed shutoff valves, or air in the lines. Open the shutoff valve fully, then partially close it (turn one full turn clockwise from open) to reduce flow rate slightly. If that doesn't quiet it, the valve's internal diaphragm may be wearing out — a $10 replacement fixes it.

How much water does a running toilet actually waste?

A continuously running toilet wastes roughly 200 gallons per day, sometimes much more for a heavy leak. Across a month that's 6,000 gallons. In a high-water-rate region (Northeast, California, Southwest), the wasted water alone can cost $30–$60 per month — and it adds up surprisingly quickly. A six-month-undetected leak can run several hundred dollars in water bills alone.

Can I use a flapper from a different brand?

Yes, if the flush valve diameter matches. Most US toilets use a 2-inch flush valve, and 2-inch flappers from any manufacturer (Korky, Fluidmaster, Kohler, American Standard) interchange. Higher-capacity toilets use a 3-inch flush valve and need a 3-inch flapper. Measure the diameter of the opening at the bottom of the tank before buying.

My new flapper leaks too. What went wrong?

Three common causes: the chain is too short or too tight (preventing the flapper from sealing fully — leave 1/2 inch of slack); the flapper is the wrong size or shape for your flush valve (use a brand-matched flapper); or the flush valve seat itself is damaged (cracks, gouges, or mineral buildup on the plastic ring the flapper seals against — fix with a flush valve repair kit, about $15).