Inside the tank, the fill valve is the tall tower on the left, and it does two jobs: it refills the tank after a flush, and it shuts off when the float riding on it reaches the set water level. Every version of "not filling" maps to that one part, the float, or the water supply feeding them — which is why this is one of the most homeowner-friendly fixes in the house. First, work out which failure you actually have.
| What You See | Likely Cause | Fix |
|---|---|---|
| Tank fills, but stops well below the waterline mark | Float set too low, or float rubbing the tank wall | Raise the float adjustment; reposition so it moves freely |
| Fills, but takes 10+ minutes, sometimes with a hiss | Debris clogging the fill valve | Flush out the valve cap (steps below) |
| Barely a trickle, or nothing at all | Supply valve half-closed, or a failed fill valve | Open the wall valve fully; replace the valve if that fails |
| Fills up, then quietly empties and refills on its own | Leaking flapper — the tank actually fills, then drains | Replace the flapper |
Start with the shutoff valve on the wall behind the toilet. It should be fully open, counterclockwise. These valves get bumped by mops and storage bins, and some get deliberately throttled years ago to quiet a noisy fill and never get opened again. Next, lift the tank lid and watch a fill cycle. If the float — a cup or ball riding the water — hangs up on the tank wall, the refill tube, or the flush handle chain, the valve reads it as "full" and stops early. Free it, then adjust the level: most modern fill valves have a squeeze clip or a screw on the float arm, and the target is the waterline stamped inside the tank, usually about an inch below the top of the overflow tube. A tank that "won't fill" often just needs its float raised half an inch. A weak-flush complaint travels with this one, since a half-filled tank has half the push — if flushing power is your bigger issue, our guide to a toilet that won't flush covers that side.
If the tank fills but slowly, with hissing or sputtering, sediment or a flake of rust is probably lodged in the fill valve's inlet screen and cap — especially if the problem started right after plumbing work nearby or a water-main repair on your street. This five-minute flush clears it, and it's the same valve family behind the noise in our whistling toilet guide.
Shut the water off at the wall valve, then hold the flush handle down to empty the tank as far as it will go. Sponge out what remains if you want a dry workspace.
Remove the fill valve cap. On most common valves, the top cap twists off a quarter-turn or unclips, exposing a rubber seal. Lift the seal out and rinse it under a faucet — grit embedded in it is a frequent culprit all by itself.
Blow the valve clear. Hold your cup upside down over the open valve top to deflect spray, then turn the wall valve on for five to ten seconds. Line pressure blasts water up through the valve and washes the debris out into your cup. Shut the water off again.
Reassemble and test. Reseat the seal, snap the cap back on, turn the water on, and time the fill. A healthy toilet refills in roughly one to three minutes, quietly, and shuts off at the waterline.
If full supply pressure and a debris flush don't restore the fill, the valve itself is done — internal seals harden and the mechanism wears, typically after years of service. Replacement is genuinely a beginner job: shut off and drain the tank, unscrew the supply line and the valve's mounting nut from underneath, drop the new valve in at the marked height, reconnect, and set the float. No soldering, no tank removal, and the part is universal enough that the hardware store's standard model fits nearly any toilet. One related impostor to rule out before you buy anything: if the tank fills fully but the water later vanishes and the toilet refills itself every so often, the fill valve is fine — the flapper at the bottom is leaking tank water into the bowl. That's the same part family as a running toilet, covered in our running toilet fix, and a flapper costs even less than a valve.
Don't bend the float arm on a modern fill valve — older brass arms bent; modern plastic ones adjust with a clip or screw and just snap when forced. Don't overtighten the supply line connection under the tank; hand-tight plus a quarter-turn with a wrench is the standard, and cracked plastic nuts leak impressively. And don't live with a hissing, ten-minute fill for a year: a valve that never quite seals wastes water every hour it runs, and the fix is cheaper than the water.
Tank not filling is a fill valve story in nearly every case. Open the wall valve all the way, free and raise the float, flush the grit out of the valve cap — in that order, because the first two are free and the third takes five minutes. If none of it works, a $10–$15 valve and half an hour ends the problem for years. The only version that isn't the valve is the tank that fills and silently drains, and that's a flapper, which is an even easier Saturday fix.
Typically one to three minutes, depending on your home's water pressure and the valve. A fill creeping past five minutes — especially with hissing — means the valve is clogged or failing, not that you need to be patient.
A sudden change usually means debris — a bit of sediment or rust broke loose (often after nearby plumbing work or a water-main repair) and lodged in the fill valve's inlet. The cap flush in this article is designed for exactly that scenario.
The float is telling the valve to shut off early. Either the float adjustment is set low, or the float is physically catching on the tank wall, the refill tube, or a tangled flapper chain. Free it and raise the setting to the waterline stamped inside the tank.
It can look that way. A leaking flapper lets tank water drain into the bowl as fast as the valve replaces it, so the level never climbs — or the tank fills, then empties itself and refills in a cycle. A few drops of food coloring in the tank will show bowl leakage within 15 minutes. That dye test and the flapper swap are the core of how you fix a running toilet, too — same leaking part, same ten-dollar fix.
Check the wall valve is fully open, then test a nearby faucet. If the faucet runs fine, the toilet's fill valve has failed closed or its inlet is completely blocked — the debris flush or a valve replacement settles it. If the whole house is weak, the problem is upstream of the toilet.