Why Is My Toilet Not Filling Up With Water (4 Causes and Fixes)
A toilet that won't fill after a flush usually has a failed fill valve. The $12 part is one of the easiest plumbing swaps in the house. Here is the 20-minute fix.
Quick answer: A toilet that won”t fill after flushing has one of four causes, and three of them are 15-minute fixes. First, the supply valve under the toilet is partially closed (someone bumped it). Open it all the way. Second, the fill valve is stuck or worn out, the most common cause, $12 part, easy swap. Third, the float is set too low so the valve shuts off before the tank reaches the waterline. Adjust the float up. Fourth, the water pressure to the whole house is low (the rare case, see our water pressure article). Check the supply valve first, it solves about 1 in 4 cases for free.
A toilet that flushes but won’t refill is one of those problems that looks expensive but usually isn’t. The actual cause is usually inside the tank, and the part is sold in every hardware store for under $15. Replacing it is one of the easiest plumbing jobs in the house, no soldering, no specialty tools, just the patience to keep a $5 sponge handy and a bucket nearby.
This article works through the four causes in order of probability, with the supply-valve check first because it’s free and takes 30 seconds.
How to Tell What’s Actually Wrong
Lift the tank lid (set it carefully on a folded towel, porcelain cracks easily). Flush the toilet and watch.
- Tank empties, fill valve hisses, water trickles in slowly: Fill valve is partially clogged or worn out. Step 3.
- Tank empties, no hiss, nothing happens at all: Either no water reaching the toilet (Step 1) or the fill valve has failed completely (Step 3).
- Tank fills part-way, then stops well below the waterline mark: Float is set too low. Step 4.
- Tank fills but very slowly, takes 5+ minutes: Could be low water pressure to the whole house, the fill valve, or a partially closed supply valve. Steps 1 to 3.
Inside the tank, there’s usually a horizontal black line or scratch on the back wall labeled “WL” (waterline). The tank should fill to within 1 inch of that line. If yours isn’t, work through the steps below.
What You’ll Need
- A flashlight or headlamp
- A small adjustable wrench or channel-lock pliers
- An old towel and a small bucket
- A sponge or shop towel to soak up tank water
- A new fill valve, only if Step 3 says so (Fluidmaster 400A is the standard, $12)
About a $15 investment if you need the part, $0 if it’s just the supply valve.
Step 1: Check the Supply Valve
The supply valve is the small knob or lever on the wall (or floor) behind the toilet. The pipe coming out of it runs up to the tank.
- Confirm it’s all the way open. Turn it counter-clockwise as far as it goes. Old multi-turn valves should rotate 4 to 6 full turns to fully open. Quarter-turn ball valves should be parallel with the supply line, perpendicular means shut.
- If you find the valve in the middle of its travel, someone closed it part-way (often a guest or kid). Open it fully and flush. If the tank fills normally, you found the answer.
This single check solves more “toilet not filling” cases than any other diagnosis. Always start here.
If the valve is fully open and the tank still won’t fill, continue.
Step 2: Check the Supply Line for Kinks or Clogs
Look at the flexible supply line running from the wall to the bottom of the tank.
- Kinked or pinched: Common after a recent toilet pull or wax-ring replacement. Straighten it. If it’s been crimped hard, replace it. Braided stainless supply lines are $8 and last 10 to 15 years.
- Old and chalky white at the connections: Hard water mineral buildup. The line itself isn’t usually the clog, but the small inlet screen at the bottom of the fill valve is. Step 3 will check it.
- Wet at any joint: A small leak is reducing flow. Tighten the connection a quarter-turn, or replace the supply line.
Shut off the supply valve, unscrew the supply line at the bottom of the tank (small bucket underneath to catch the water), turn the valve briefly into the bucket. Strong steady stream means the wall side is fine. Weak dribble means the issue is upstream of the toilet, not at the toilet itself.
Step 3: Test or Replace the Fill Valve
The fill valve is the vertical column on the left side of the tank with the float attached. When the tank empties, the float drops, which opens the valve, which lets water in. When the tank fills, the float rises and shuts the valve.
Most modern fill valves are the Fluidmaster 400A or similar cylinder-style design. Older toilets have ballcock valves (the classic brass arm with a copper or plastic ball at the end). Both can be diagnosed the same way.
Test the valve:
- Shut off the supply valve.
- Flush to empty the tank.
- Sponge out the remaining inch of water at the bottom.
- Unscrew the cap on top of the fill valve (Fluidmaster: twist and lift the gray cap; older models: unscrew a brass nut).
- Hold a plastic cup upside-down over the valve to catch any spray.
- Open the supply valve for 1 second, then close it.
Water should jet out the top of the valve when you crack the supply. If it sputters or dribbles, the inlet screen at the bottom of the valve is clogged with sediment.
Clean the inlet screen: With the supply valve closed and the cap off, look inside the valve. Use a toothbrush and a paper clip to clear sediment from the screen and the rubber seat at the top. Reassemble. Test.
If cleaning doesn’t fix it, the valve is worn out. Fluidmaster rates their 400A for 5 to 7 years; in hard-water areas, 3 to 4 years is more realistic.
Replace the valve: Same procedure that got the cap off, plus removing the old valve from the tank.
- Shut off the supply. Flush. Sponge out the residual.
- Disconnect the supply line from the bottom of the valve (under the tank). Bucket below.
- Hold the bottom of the valve with one hand and unscrew the large plastic locking nut on the underside of the tank. Pull the old valve out the top.
- Put the new valve in, adjusting its height per the package (most are adjustable, the top of the valve should be 1 to 1.5 inches above the overflow tube).
- Hand-tighten the locking nut from below, then a quarter-turn with channel-locks. Don’t overtighten, the tank is porcelain and can crack.
- Reconnect the supply line.
- Open the supply valve. Watch for leaks at the locking nut and the supply connection.
- Flush. The tank should fill correctly to the waterline.
About $12 for the valve, 20 minutes of work, no special tools.
Step 4: Adjust the Float
If the valve is working but the tank stops filling well below the waterline mark, the float is set too low.
On a Fluidmaster 400A, the float is the cup-shaped piece that slides up and down on the valve column. A small thumb screw on the side of the float lets you slide the float higher (more water in tank) or lower (less water). Turn the screw, slide the float up about a half inch, flush, and check the new water level. Repeat until the water stops about 1 inch below the top of the overflow tube.
On older ballcock valves with a horizontal arm and a brass float ball, the arm bends. Carefully bend the arm upward, and the ball rises further before shutting off the valve. Don’t bend too aggressively, the arm fatigues and snaps after a few bends.
Modern code (per the EPA’s WaterSense program) sets the high-fill mark about 1 inch below the top of the overflow tube. Filling beyond that puts water down the overflow and into the bowl, wasting 1 to 3 gallons per flush.
The Rare Case: House Water Pressure
If you’ve ruled out the supply valve, the supply line, the fill valve, and the float, and the toilet still fills very slowly, the issue may be your home’s overall water pressure. Check by running another fixture nearby (a bathtub tap on full). If it’s also weak, you’ve got a house-wide pressure issue, see our low water pressure guide for that diagnostic.
A pressure regulator failure usually causes either too-high or too-low pressure across the whole house. Most residential regulators last 7 to 12 years, per Watts Water Technologies. If yours is older than that and pressure is weak everywhere, the regulator is the likely culprit.
Common Mistakes to Avoid
Buying a new toilet. A fill valve failure looks like the toilet is dead. It isn’t. The toilet itself (the porcelain) lasts 50+ years. The fill valve is a $12 wear part that’s expected to be replaced a few times during the toilet’s life. Don’t replace a $300 toilet to solve a $12 problem.
Bending the float arm on a Fluidmaster 400A. That’s the fix for old brass-arm ballcocks. Modern cylinder-style valves don’t have a bendable arm, they have a screw or a slide. Trying to bend a Fluidmaster column will break it.
Overtightening the locking nut on the new valve. The tank is porcelain. Hand-tight plus a quarter-turn is enough. Cranking it down cracks the tank, and a cracked tank means a new toilet anyway.
Skipping the inlet screen check. A clogged inlet screen looks exactly like a dead fill valve from the outside. Five seconds with a toothbrush solves it. Always check the screen before buying the part.
Pouring chemicals in the tank to “clean” it. Drop-in tank tablets (chlorine bleach pucks) destroy the rubber seals on the fill valve and flapper. Licensed plumbers on /r/Plumbing universally warn against drop-in tank tablets, they cut valve life in half. Clean with white vinegar in a spray bottle if the tank is gunky.
FAQ
How long does a fill valve last? Fluidmaster rates their 400A for 5 to 7 years. In hard-water areas, 3 to 4 years is typical, sediment builds up faster and the rubber seat wears out. In soft-water areas, 8 to 10 years is normal. The pattern is: it works fine, then one day it doesn’t, no slow warning.
My tank fills, but really slowly. Is it the valve or the supply? Test the supply by closing the supply valve, disconnecting the supply line under the tank, and opening the supply valve into a bucket. If you get a strong flow into the bucket, the supply is fine and the issue is the valve (Step 3). If the flow into the bucket is also weak, the supply or the supply line is the issue (Step 2 or house-pressure).
Can I replace just the rubber parts of the fill valve instead of the whole thing? Some valves have replaceable seals. The cost of the seal kit is usually $8 vs. $12 for a whole new valve, and the labor is the same. Replace the whole valve. The other internal parts are due to fail soon anyway.
Why does my toilet fill correctly but then drain back into the bowl? That’s a flapper problem, not a fill valve problem. The flapper at the bottom of the tank isn’t sealing, so the tank water trickles into the bowl and the fill valve has to kick on to top it up. See our running toilet guide for that one.
My toilet whistles or makes a high-pitched noise while filling. Same problem? Different symptom. Whistling is usually a partially-closed supply valve or a stuck diaphragm in the fill valve. Open the supply valve fully first. If it still whistles, replace the fill valve, the diaphragm is worn.
Do I need to call a plumber for this? No, fill valve replacement is in the easiest tier of plumbing work. No soldering, no glue, no specialty tools. If you can shut off a valve and turn a wrench, you can swap a fill valve. A plumber charges $125 to $250 for the visit, and they install the same $12 part you can buy at any hardware store.
The supply-valve check at the top of this article solves more cases than the actual valve replacement does. Always start there. If the supply is fine, the fill valve is the next stop, and replacing it is the easiest plumbing job in the house. For the opposite problem, a toilet that fills constantly, see our running toilet walkthrough, the flapper is usually the culprit.