The short answer
Nine times out of ten a running toilet is one of two parts: a worn flapper (water seeps from the tank into the bowl, then the tank refills) or a failing fill valve (the tank never stops filling and water runs into the overflow tube). Drop a few drops of food coloring into the tank; if color shows in the bowl within 15 minutes without flushing, it is the flapper. If the water level sits above the overflow tube, it is the fill valve or float. Both are $10–$25 parts and a 20-minute job.
Why a running toilet matters
A toilet that runs is the single most common plumbing leak in a house, and the most expensive one that nobody notices. A quiet flapper seep wastes 20 to 30 gallons a day; a toilet that runs continuously into the overflow can pass 200 gallons a day — roughly a full bathtub every hour and a half straight down the drain. On a metered bill in Lynnfield, Peabody or Boston that is real money every month, and in a condo or multi-family it shows up as a mystery jump in the shared water bill.
The two-minute diagnosis
- Lift the tank lid and look at the water level. There is a vertical tube in the middle — the overflow tube. The water should sit about an inch below its top.
- Water above the top of the overflow tube, running into it? The fill valve is not shutting off or the float is set too high. Go to Fix 2.
- Water level looks right but the toilet still refills on its own? Put five or six drops of food coloring in the tank and wait 15 minutes without flushing. Color in the bowl means the flapper is leaking. Go to Fix 1.
- Toilet runs only after a flush and stops if you jiggle the handle? The chain or handle is the culprit. See "Other causes".
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Fix 1: the flapper
The flapper is the rubber disc on the bottom of the tank that lifts when you flush. Chlorine, hard water and age make it warp or stiffen so it no longer seals. Replacement:
- Shut the supply valve behind the toilet and flush to empty the tank.
- Unhook the chain from the flush lever and unclip the flapper's ears from the pegs on the overflow tube (or slide the ring off the tube on older styles).
- Wipe the seat — the rim the flapper sits on — with a rag. Mineral crust on the seat causes as many leaks as the flapper itself.
- Clip on the new flapper, hook the chain with about half an inch of slack, turn the water back on and test-flush twice.
Universal adjustable flappers fit most toilets built since the mid-1990s. If yours is an older 3-inch or a brand-specific canister style (common on newer Kohler and Toto models), match the model number stamped inside the tank.
Fix 2: the fill valve and float
The fill valve is the tall assembly on the left side of the tank that refills after a flush. When water runs into the overflow tube the valve is either set too high or no longer shutting off.
- Try adjusting the float first. On a modern valve there is a screw or clip on the float cup; turn it so the water shuts off an inch below the overflow tube. On an old ballcock, gently bend the float arm down.
- Still running? Replace the valve. Shut the supply, empty the tank, sponge out the rest, disconnect the supply line under the tank, unscrew the plastic nut and lift the old valve out. Set the new one to height (the top should be about an inch above the overflow tube), tighten the nut hand-tight plus a quarter turn, reconnect the supply and adjust the float.
Universal fill valves are $15–$25 and take about 20 minutes. Buy a new supply line at the same time if the old one is braided steel more than ten years old — they are cheap and they fail at the worst time.
Other causes: chain, handle, cracked overflow
- Chain too long or too short. Too long and it gets pinned under the flapper; too short and the flapper never seats. Aim for half an inch of slack.
- Loose or corroded handle. The nut inside the tank is reverse-threaded; snug it and make sure the lever returns to horizontal.
- Cracked overflow tube. A hairline crack below the water line lets the tank drain slowly. That means a new flush valve assembly, which is a bigger job because the tank comes off.
- Water hammer or a hissing valve. Debris in the fill valve seat; some valves can be flushed by lifting the cap while the water is on, otherwise replace.
When to replace the toilet instead
If the toilet is a pre-1994 model using 3.5 gallons or more per flush, a cracked tank or bowl, a flush valve that needs the tank pulled, or a unit that has needed three repairs in two years, replacement is usually the better spend. A modern 1.28-gallon toilet pays for itself in water alone in a family home, and installation is a morning's work. Precision Plumbing Solutions repairs and replaces toilets across Lynnfield, the North Shore and Greater Boston — see toilet repair and installation. If the toilet is clogging rather than running, start with how to unclog a toilet.
Questions people ask
How much water does a running toilet waste?
A silent flapper leak wastes 20–30 gallons a day; a toilet that visibly runs into the overflow can waste 200 gallons or more. On a metered North Shore water bill that shows up within one billing cycle.
How do I know which flapper to buy?
Take the old one to the store or check the toilet brand and model (usually stamped inside the tank). Most are 2-inch or 3-inch; universal adjustable flappers fit the majority of tanks made after about 1995.
Why does my toilet run for a few seconds every hour?
That is a phantom flush: the flapper is letting water seep from the tank into the bowl, and the fill valve tops the tank up when the level drops. A new flapper almost always fixes it.
Is a running toilet an emergency?
Not usually, but shut the supply valve behind the toilet when you are not using it until it is fixed — you are paying for every gallon that runs to the sewer.
