A toilet that keeps running wastes between 200 and 4,000 gallons of water per day depending on the severity, which translates to an extra $70 to $200 on your monthly water bill. In almost all cases, the fix costs $5 to $30 in parts and takes under an hour to do yourself. The five most common culprits are a worn flapper, a faulty fill valve, an incorrectly adjusted float, a cracked overflow tube, or a bad flush valve seat.
This guide walks you through diagnosing your specific problem and fixing it step by step, plus tells you exactly when the repair is worth calling a plumber.
Why This Matters More Than You Think
That gentle hissing sound coming from your bathroom might seem harmless, but running toilets are one of the biggest sources of wasted water in American homes. According to the EPA’s WaterSense program, a single running toilet can account for up to 30 percent of a household’s total water use.
At average US water rates around $0.0015 per gallon, a toilet running at 200 gallons per day costs roughly $90 per month in wasted water. A severe internal leak running at 4,000 gallons per day can spike your bill by $1,800 per month.
Beyond your water bill, constant water movement in the tank can wear down internal components faster, eventually leading to a full tank replacement that costs $150 to $400.
Quick Diagnosis: The Dye Test
Before diving into individual parts, do this 30-second test to confirm your toilet is actually leaking into the bowl versus just refilling normally:
- Remove the tank lid and set it aside safely.
- Drop one or two dye tablets (available at hardware stores for under $3) or a few drops of food coloring into the tank water.
- Wait 15 to 20 minutes. Do not flush.
- Check the toilet bowl water. If you see color in the bowl, water is leaking from the tank through the flapper. If the bowl water stays clear, the issue is with the fill valve or water supply side.
The 5 Most Common Causes (and How to Fix Each)
1. Worn or Misaligned Flapper
Likelihood: 60 to 70 percent of running toilets
The flapper is the rubber seal at the bottom of your tank. It drops down to seal the tank after each flush. Over time, rubber flappers warp, crack, or build up mineral deposits that prevent a tight seal.
How to diagnose it: Put your hand in the tank and press down firmly on the flapper. If the running sound stops immediately, a bad flapper is your problem.
How to fix it:
- Turn off the water supply valve (the knob behind the toilet near the floor) by turning it clockwise until it stops.
- Flush the toilet to empty the tank.
- Unhook the flapper chain from the flush handle arm.
- Slide the flapper off the two pegs on the sides of the flush valve.
- Take the old flapper to a hardware store or snap a photo of the toilet model number (usually stamped inside the tank) to find an exact match. Universal flappers work for most toilets.
- Snap the new flapper onto the pegs and reattach the chain. Leave about half an inch of slack in the chain. Too much slack means the flapper won’t open fully; too little means it won’t seal properly.
- Turn the water back on and test with another dye test.
Cost: $5 to $15 for a replacement flapper. This is the easiest DIY plumbing fix in the house.
What to watch for: If you have a Kohler, American Standard, Toto, or Mansfield toilet, always buy brand-specific flappers. Generic flappers sometimes don’t create a proper seal with proprietary flush valve designs.
2. Faulty Fill Valve (Ballcock)
Likelihood: 20 to 25 percent of running toilets
The fill valve controls water flowing into the tank after a flush. When it fails, it either doesn’t shut off at all (constant running) or cycles on and off repeatedly, a sound sometimes called “phantom flushing.”
How to diagnose it: Remove the tank lid and watch what happens as the tank fills. If the water level rises above the overflow tube and spills into it, your fill valve isn’t shutting off when it should.
How to fix it:
Replacing a fill valve takes about 20 minutes:
- Turn off the water supply valve and flush to drain the tank.
- Use a sponge or towel to soak up remaining water in the tank.
- Disconnect the water supply line from the bottom of the tank (have a small bucket ready for drips).
- Unscrew the locknut holding the fill valve in place from underneath the tank.
- Pull the old fill valve out from the top.
- Install the new fill valve, following the instructions on the package. Fluidmaster 400A is a widely trusted universal fill valve that fits nearly all toilets and costs around $12 to $18 at any home improvement store.
- Reconnect the supply line, turn the water back on, and adjust the water level using the valve’s adjustment screw or clip.
Cost: $12 to $25 for a quality fill valve. Plumber labor if you hire out: $100 to $175 for this repair.
3. Float Set Too High
Likelihood: 10 to 15 percent of running toilets
Many toilets, especially older ball-float designs, have the float set so high that the water level reaches the overflow tube before the fill valve shuts off. Water then continuously drains through the overflow tube into the bowl.
How to diagnose it: Look inside the tank while it’s running. If water is spilling into the tall vertical tube in the center of the tank (the overflow tube), your float is set too high.
How to fix it:
On newer toilets with a vertical fill valve (like the Fluidmaster 400A), there is a small adjustment clip on the side of the valve. Slide it down or turn the adjustment screw counterclockwise to lower the water level. The correct water level should sit about 1 inch below the top of the overflow tube.
On older toilets with a large ball float on a metal or plastic arm, gently bend the arm downward slightly. This lowers the position at which the float signals the valve to shut off.
Cost: $0 if it’s just an adjustment. If the float itself is cracked and filling with water (causing it to sit lower in the tank), a replacement ball float costs $6 to $12.
4. Overflow Tube That’s Too Short
Likelihood: Rare but easy to miss
If a previous repair or replacement used an overflow tube that is too short for your tank, the water level may exceed the tube even when the fill valve is working perfectly. This one is easy to spot but requires replacing the flush valve assembly.
How to diagnose it: Measure from the bottom of the tank to the top of the overflow tube. Then compare it to the water level. The water should naturally stop at least 1 inch below the overflow tube’s top opening. If the tube is shorter than the fill valve’s minimum water level, the tube is the problem.
How to fix it: This requires replacing the entire flush valve, a moderate DIY repair that takes 45 to 60 minutes. The flush valve costs $15 to $35. For most homeowners, this is a reasonable project if you’re comfortable with basic plumbing. If not, it’s a straightforward plumber call.
5. Corroded or Damaged Flush Valve Seat
Likelihood: Less common, but causes persistent leaks even after flapper replacement
If you’ve replaced the flapper and the toilet still runs, check the flush valve seat, the plastic or brass ring that the flapper presses against to create a seal. Over time, mineral deposits or physical damage create pits and ridges that prevent the flapper from sealing completely.
How to diagnose it: After draining the tank and removing the old flapper, run your finger around the rim of the flush valve seat. If you feel roughness, grooves, or visible pitting, that’s your problem.
How to fix it: For minor buildup, try scrubbing the seat with fine steel wool or a Scotch-Brite pad. For a damaged seat, you can install a seat saver kit (a thin rubber ring that adheres over the damaged seat) for around $8 to $12. These are sold by Fluidmaster and other brands. For a severely damaged seat, the entire flush valve assembly needs replacement.
Repair Cost Summary
| Problem | DIY Part Cost | Plumber Cost |
|---|---|---|
| Worn flapper | $5 to $15 | $100 to $150 |
| Faulty fill valve | $12 to $25 | $100 to $175 |
| Float adjustment | $0 to $12 | $75 to $125 |
| Flush valve replacement | $15 to $35 | $150 to $250 |
| Full tank rebuild kit | $20 to $40 | $150 to $250 |
A complete toilet tank rebuild kit (flapper, fill valve, and handle) from brands like Fluidmaster or Korky costs $20 to $40 and gives you new versions of every moving part. If your toilet is over 10 years old and has been running intermittently, replacing all three parts at once is often smarter than replacing them one at a time.
When to Call a Plumber Instead of DIYing
While most running toilet repairs are genuinely beginner-friendly, here are the situations where you should call a licensed plumber:
Water pooling at the base of the toilet. This usually means a failing wax ring seal, not an internal tank issue. A plumber needs to remove the toilet to replace it. Cost: $150 to $300.
Cracks in the porcelain tank or bowl. Cracked porcelain cannot be reliably repaired and the toilet should be replaced. A plumber can handle removal and installation. Full replacement including the new toilet: $300 to $800 depending on model.
Low water pressure in the whole house. If other fixtures are also affected, the issue is upstream of the toilet. This may indicate a failing pressure regulator or main line issue.
Sewage smell or slow draining. These point to a drain or vent issue, not a tank problem. Don’t delay on these.
You’ve replaced the flapper and fill valve and it still runs. At that point, there may be a crack in the tank or a problem with the flush valve seat that’s easier for a professional to assess quickly than to diagnose yourself.
How to Find a Reliable Plumber
If you do decide to hire out, here’s how to protect yourself:
- Verify the plumber holds a current state plumbing license. You can check your state’s contractor licensing board online. Most states publish a license lookup tool.
- Get at least two written estimates for any repair over $150.
- Ask if they charge a flat rate or by the hour. Running toilet repairs at an hourly rate of $75 to $150 per hour can add up quickly.
- Avoid any plumber who insists on replacing the entire toilet without showing you why internal parts won’t fix it.
- Check reviews on Google or the Better Business Bureau before booking.
For a full breakdown of what plumbers charge, see our guide on how much does a plumber cost.
Water Savings: Why Fixing It Now Pays Off
According to the EPA WaterSense program, fixing household water leaks including running toilets can save the average household 10 percent on their water bills.
At an average US water bill of $70 per month, that’s $84 per year saved from a $15 flapper repair. Most running toilet repairs pay for themselves within one to two months.
Many local water utilities also offer rebates for upgrading to WaterSense-certified low-flow toilets, which use 1.28 gallons per flush or less (compared to 1.6 GPF for standard models and up to 3.5 GPF for toilets made before 1994). Check your local utility’s website or EPA’s rebate finder for available incentives in your area.
A Word About Older Toilets
If your toilet was manufactured before 1994, it uses 3.5 to 7 gallons per flush compared to today’s 1.28 to 1.6 GPF models. Even if you fix the running issue, upgrading to a modern toilet saves significant water long term.
Replacing a toilet typically costs $300 to $500 installed. Many homeowners recover that cost within two to three years in reduced water bills alone. For our full guide, see toilet replacement cost.
Official Resources
- EPA WaterSense: Find the Source of Leaks - Guide to locating and fixing household water leaks
- Consumer Product Safety Commission: Plumbing Products - Product safety recalls and standards
- WaterSense Rebate Finder - Search for water efficiency rebates in your state
- HUD Healthy Homes - Resources on home maintenance and healthy living conditions
- NAHB: Home Maintenance Guides - National Association of Home Builders resources for homeowners