Why Is My Toilet Running? Causes & How to Fix It

Why a Running Toilet Is a Big Deal

A toilet that won't stop running isn't just annoying — it's expensive. A single running toilet can waste over 200 gallons of water per day, adding hundreds of dollars to your water bill over the course of a few months. In Oakland and Macomb County, where water rates have climbed steadily, that's money straight down the drain.

The good news is that most running toilets come down to one of two parts: the flapper or the fill valve. Both are inexpensive, and in many cases you can fix the problem yourself with basic tools. Below, we'll walk through each cause and show you exactly what to check.

Step 1: Shut Off the Water

Before you touch anything inside the tank, turn off the water supply. Look behind the toilet near the floor — you'll see a small valve called an angle stop connected to the supply line. If it's a quarter-turn valve, rotate the handle 90 degrees. If it's an older multi-turn valve, turn it clockwise until it stops.

Here's the important part: after you shut it off, slowly disconnect the supply line at the bottom of the tank and confirm the water actually stopped. The last thing you want is to pull a hose loose and discover the valve didn't fully close. Keep a towel underneath to catch the small amount of water that drains from the fill valve when you disconnect.

Step 2: Check the Flapper

The flapper is the rubber seal at the bottom of the tank that lifts when you flush and drops back down to let the tank refill. Over time, the rubber warps, cracks, or collects mineral buildup — and when it can't seal properly, water slowly leaks from the tank into the bowl. Your fill valve keeps running to compensate, and you hear that constant hissing or trickling sound.

To check it, flush the toilet with the water off so the tank drains, then inspect the flapper. Run your finger along the sealing edge and look for cracks, warping, or a slimy deteriorated surface. Also check the flush valve seat — the ring the flapper sits on — for nicks or mineral deposits that could prevent a tight seal.

Replacing a flapper is one of the easiest plumbing repairs there is. Unhook the old one from the mounting ears on the overflow tube, disconnect the chain from the flush handle lever, and snap the new one into place. Make sure the new flapper is designed for your specific toilet model — a universal flapper doesn't always seal correctly. When installed, the flapper should sit flush against the valve seat and lift cleanly to the top when you trip the handle.

Step 3: Check the Fill Valve

If the flapper looks fine, the fill valve is your next suspect. The fill valve is the tall assembly on the left side of the tank — it controls the water flowing in after each flush and shuts off once the tank reaches the proper level. When the internal diaphragm wears out or gets clogged with debris, the valve won't shut off completely.

Most modern fill valves can be partially disassembled without removing them from the tank. Lift the top cap and turn it to release it — the mechanism varies by brand, but it usually lifts and twists off. Inside, you'll find a small diaphragm (a rubber disc) and sometimes a tiny pin or screen. Pull the diaphragm out and inspect it for tears or buildup.

Rinse the diaphragm thoroughly under clean water. If it's grimy, wash it with a little dish soap. Then look down into the valve body itself — sediment and debris from your water supply can accumulate in there and prevent the valve from sealing. A quick trick: reconnect the supply line, hold a rag over the top of the valve, and briefly crack the angle stop open. The water pressure will flush debris up and out. Close the valve again before reassembling.

Put the diaphragm and pin back exactly as they came out, then snap the cap back on with a lift-and-turn. Reconnect the supply line, turn the water back on, and watch. If the valve shuts off cleanly when the water reaches the fill line, you've fixed it.

Step 4: When to Replace the Fill Valve

If cleaning the diaphragm doesn't solve the problem, the fill valve itself needs to be replaced. This is still a manageable DIY job. With the water off and the tank drained (use a sponge or wet-vac to get the last bit out), unscrew the mounting nut under the tank and lift the old valve out. Keep a towel underneath — residual water will drip.

When installing the new fill valve, adjust it to the same height as the old one before you put it in — you already know that height worked. Make sure the rubber washer is seated on the bottom, slide the valve into the tank hole, and tighten the nut from underneath until snug. Reattach the fill tube to the overflow pipe.

One critical detail: the top of the fill valve should sit about a half-inch to one inch below the top of the overflow tube. If it's too high, the tank can overfill and run continuously through the overflow — which mimics a flapper problem but is actually a fill valve height issue.

Step 5: Reassemble and Test

Reconnect the supply line to the fill valve and snug it up — hand-tight plus a quarter turn with channel-lock pliers is usually enough. Don't overtighten or you'll crack the plastic fitting.

Turn the angle stop back on and watch the tank fill. The water should rise steadily and the fill valve should shut off once the water reaches the fill line marked on the inside of the tank (or about an inch below the overflow tube). Flush once or twice to confirm everything works and there are no leaks at the supply connection.

Finally, set the tank lid back on carefully. Porcelain is brittle and a dropped lid will crack or shatter. Lower it straight down — don't slide it.

When to Call a Professional

Most running toilets are a flapper or fill valve fix, and either one is a $10–$25 part. But there are situations where calling a pro makes sense:

The flush valve itself is cracked or corroded — replacing it means pulling the entire toilet off the floor, which involves resetting the wax ring and bolts. If the angle stop won't shut off the water, you've got a valve problem that goes beyond the toilet. If you've replaced both the flapper and fill valve and the toilet still runs, something less obvious is going on — a hairline crack in the tank, a warped flush valve seat, or an issue with the overflow tube.

At The Toilet Guys, running toilet repairs start at $100. We diagnose the actual problem, carry the parts on our truck, and fix it on the spot. If your toilet is old enough that repairs aren't worth it, we'll talk to you honestly about replacement options — no pressure, just straight talk.