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Local Service Directory · Repair

Running Toilet Repair: How to Fix It and What It Costs

A running toilet wastes 2,000–10,000 gallons per month and adds $20–$100 to your water bill. The fix is usually under $20 in parts and 30 minutes of DIY work. Here is how to diagnose the cause and fix it.

Toilets Updated May 2026
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How to confirm you have a running toilet

A running toilet either: (1) won't stop refilling the tank (you hear water running constantly or in cycles), (2) drains the tank slowly between flushes (the water level drops over hours), or (3) wastes water silently (no audible flow but the water bill is climbing).

Test for silent running: add 5–10 drops of food coloring to the tank water. Don't flush. Wait 15 minutes. If colored water appears in the bowl without flushing, you have a slow leak from the tank into the bowl.

The four causes of a running toilet

Cause 1: Worn flapper (most common, ~70% of cases)

The flapper is the rubber disc at the bottom of the tank that seals the flush valve opening. It rises when you flush (letting tank water flow into the bowl) and reseats to close. Over 3–7 years, the rubber degrades — it warps, develops mineral buildup, or hardens — and stops sealing. Result: tank water continuously leaks into the bowl, and the fill valve continuously runs to replace it.

Diagnostic: Lift the tank lid, push down on the flapper manually. If the running stops, the flapper is the issue. Also check for visible warping, mineral deposits, or hardening of the rubber.

Fix: Replace the flapper. $8–$15 at any hardware store. Korky 528 is the most universally-compatible replacement. Installation:

  1. Turn off water at shut-off valve
  2. Flush to empty tank
  3. Unhook chain from flush lever
  4. Slip old flapper off the flush valve overflow tube ears
  5. Slip new flapper on, attach chain to flush lever (leave 1/2 inch slack)
  6. Turn water back on, test flush

Cause 2: Failed fill valve (~20% of cases)

The fill valve is the tall component on the left side of the tank that refills water after a flush. It has an internal float mechanism that's supposed to shut off the water when the tank is full. When this mechanism fails — usually from mineral buildup or worn rubber seals — the valve never fully closes, and water continuously flows.

Diagnostic: Listen to the fill valve. If you hear a constant slow hiss even after the tank is full, the fill valve isn't shutting off.

Fix: Replace the fill valve. Fluidmaster 400A is the universal replacement ($12–$18). Installation is 30–45 minutes and requires an adjustable wrench:

  1. Turn off water at shut-off valve
  2. Flush to empty tank, sponge out remaining water
  3. Disconnect supply line from bottom of tank
  4. Unscrew old fill valve nut from inside the tank (turn counterclockwise)
  5. Pull old fill valve out the top of the tank
  6. Install new fill valve, adjusting height per the package instructions
  7. Reconnect supply line, turn water on, test

Cause 3: Misadjusted float (~5% of cases)

The float controls the fill valve shut-off. If the float is set too high, water rises above the overflow tube and continuously drains into the overflow. If the float is misaligned, the fill valve never fully closes.

Diagnostic: Look at the water level in the tank. If it's at or above the top of the overflow tube (the vertical tube in the middle of the tank), the float needs adjustment.

Fix: Adjust the float per the fill valve's instructions. On Fluidmaster valves, the float is a cylinder that slides up and down a metal rod — slide it down to lower the shut-off water level. Target water level: 1/2 to 1 inch BELOW the top of the overflow tube. 5-minute fix, no cost.

Cause 4: Cracked overflow tube or flush valve (~3% of cases)

Less common but does happen — the overflow tube or flush valve develops a crack and continuously leaks water from the tank.

Diagnostic: Visually inspect the overflow tube and flush valve. Look for hairline cracks or weeping water.

Fix: Replace the flush valve assembly. $20–$35 in parts, 45–60 minutes of work. More complex than flapper or fill valve replacement — requires removing the tank from the bowl on a two-piece toilet. This is often the point where DIY users opt for a plumber.

What it costs by route

DIY route: $8–$35 in parts, 30–60 minutes of your time, no labor cost. Total: under $40.

Plumber route: $75–$200 service call + $15–$50 in parts. Total: $90–$250. The plumber typically diagnoses and replaces the failed component while on-site.

Doing nothing route: $20–$100/month in extra water bill, plus continued frustration. Over a 6-month delay, that's $120–$600 in wasted water.

How long it takes to break even on the fix

If your running toilet is wasting 5,000 gallons per month (typical for a moderately running toilet), and your combined water-and-sewer rate is $10 per 1,000 gallons, that's $50/month in waste. Fixing it for $20 (DIY) pays back in less than two weeks. Fixing it for $200 (plumber) pays back in 4 months. Either way, the math heavily favors immediate repair.

When to suspect a bigger problem

If you replace the flapper AND the fill valve and the toilet still runs, the issue is likely the flush valve or a more complex internal failure. At that point, a plumber visit is justified. If the same toilet keeps developing the same running issue every 6–12 months, the toilet itself may be at end of life — porcelain scale buildup that prevents seals from sealing, or trapway issues that affect water level. Replacement of the entire toilet ($500–$950 all-in) may be more cost-effective than recurring repairs.