Toilet Fill Valve: Fluidmaster 400A vs Korky 528MP (2026)

A $10 part sits inside every toilet tank, it gets replaced more often than any other component in the bathroom, and Home Depot carries over 13,000 ratings for one model of it. Yet most people buy a fill valve blind, install it, and end up right back on a forum asking why the tank still runs.

The Fluidmaster 400A and the Korky QuietFILL Platinum 528MP are the two valves hardware stores stock at eye level. They cost within a few dollars of each other and both claim to fit almost any toilet. The choice between them is not brand loyalty. It is the age of your toilet.

First, Ask How Old Your Toilet Is

The US government phased toilet water use down twice, and each valve is built around one era of that timeline:

Korky states plainly in the 528MP spec sheet that the valve is ideal for toilets made since 1994, running 1.6 GPF or less. Fluidmaster positions the 400A as a universal replacement with no era in mind, and its own marketing leans on range: the company invented the fill valve in 1958 and the 400A is, by its claim, the best selling fill valve in the world.

That single sentence difference is the whole matchup. Korky tuned the 528MP for modern low-water tanks, with a refill adjuster that meters how much water returns to the bowl. Fluidmaster tuned the 400A to replace anything, including the ballcock your house came with in 1987.

What Each Valve Actually Is

Fluidmaster 400A ($9.98 at Home Depot). Anti-siphon, code approved, height adjusts from 9 to 14 inches, fits one-piece and two-piece toilets. It is the default because it installs in ten minutes and works in nearly everything. Home Depot shoppers rate it 4.6 stars across more than 13,000 reviews, and Fluidmaster backs it with a 5-year warranty.

Fluidmaster 400H PerforMAX ($18.99 direct). The 400A's louder sibling in reverse: same 9 to 14 inch height range, but with what Fluidmaster calls vortex refill technology to speed up bowl refill, precision fill tech to cut refill noise, and a water-saving dial. It fits 2-inch and 3-inch flush valves across 1.28 to 3.5 GPF, and the seals are rated against chlorine and hard water. If your complaint is a weak flush or a tank that hisses all night, this is the Fluidmaster you want.

Korky 528MP QuietFILL Platinum (about $15.97 at Home Depot). Korky claims it is the quietest fill valve in lab testing, and the design backs the intent: a refill adjuster on the valve body, a strainer that catches sediment and can be pulled out and cleaned, a 5-year warranty, and USA manufacturing. Home Depot rates it 4.4 stars across about 3,100 reviews, on fewer reviews than the 400A simply because it is the enthusiast pick rather than the default.

Three Rules That Settle It

Rule one: the toilet's decade picks the valve. Replacing a genuine old ballcock, or servicing a pre-1994 tank with big water volume? The 400A is the safest universal swap ever made. Working on a 1.28 GPF tank built after 2011? The 528MP's refill adjuster exists precisely because those tanks punish overfilling, and the 400H's GPF-range tuning covers the same ground from Fluidmaster's side.

Rule two: noise is a real spec, not a luxury. A tank refilling at 2 a.m. wakes the house. Both brands sell quiet as a feature, but only Korky puts a lab-testing claim on the box. Independent roundup Best Flushing Toilets notes the same pattern reviewers report: the 528MP operates more quietly than the standard 400A, particularly during the refill phase. The 528MP's refill adjuster also lets you trim bowl-refill water to the minimum the trap needs. If the bathroom shares a wall with a bedroom, the quiet tier (528MP or 400H) is worth the extra six to nine dollars over the 400A.

Rule three: hard water pushes you to Korky. The 528MP's strainer catches sediment before it jams the valve seal, and Korky designed it to be removed and rinsed instead of replaced. Fluidmaster's 400H answers with chlorine-resistant and hard-water-resistant seals, but the 400A's own documentation is blunt about what kills valves early: in-tank drop-in cleaners containing bleach or chlorine, which void the warranty on top of eating the seals.

One caveat from the trade side: a r/Plumbing thread on this exact matchup points out that many plumbers default to Fluidmaster partly because it installs faster on service calls, and the same thread carries the recurring owner complaint that the 400A has a shorter service life than the Korky. Neither point changes the fit rules above, but if you are paying a plumber by the hour, the install-time argument explains the pro preference better than part quality does.

The Tank Still Runs After the Swap. Now What?

This is the most common follow-up question, and it is almost never the valve's fault. Water trickling into the bowl means the tank is overfilling and spilling down the overflow tube. Fix the height, not the brand: set the float so the water level stops about an inch below the top of the overflow pipe. The r/DIY thread on exactly this problem is long, and the answer is almost always float height or a flapper that never seated after the repair, siphoning water past the flush valve.

Water leaking at the base of the shank under the tank is different. That is the shank washer or an overtightened locknut, and hand-tight plus a quarter turn is the whole trick. If the valve runs in short bursts with no flush in between, that is the tank flapper leaking, which the fill valve was never going to fix.

One more habit worth dropping: drop-in bleach tablets. Fluidmaster's own warning says bleach-based in-tank cleaners destroy tank components and void the 400A warranty. Their Flush 'n Sparkle system keeps the bowl clean without dosing the tank water.

So Which One

Old tank, ballcock replacement, rental property you want fixed cheap and forever: the 400A at ten dollars is the right answer, and its 4.6 stars over 13,000 Home Depot reviews say it is a safe one. Tank built this decade, bathroom beside the bedroom, or hard water on a well: the Korky 528MP's tuning and cleanable strainer earn its premium, and the 400H is the play if you would rather stay in Fluidmaster's catalog for a night-quiet refill with more flush power.

Narrow it to your two finalists and run them side by side before you buy. Versa AI's comparison tool reads the real owner reviews and spec sheets for any Amazon pair you feed it, so a $16 plumbing decision gets the same treatment as a $1,600 laptop.

Frequently Asked Questions

Are toilet fill valves universal?

Mostly, yes. Both the 400A and the 528MP adjust vertically (the 400A spans 9 to 14 inches) to match tank height, and both fit one-piece and two-piece toilets with a standard 2-inch flush connection. The 400H adds 3-inch flush valve compatibility. The rare exceptions are proprietary tanks from one-piece European-style toilets, where OEM parts are the only reliable fit.

Why does my toilet keep running after I replaced the fill valve?

Because water is still escaping somewhere else. The usual culprit is the float set too high, so the tank overfills and drains down the overflow tube; lower the float until the water stops about an inch below the tube's top. The second suspect is the flapper not seating after the repair. Roto-Rooter's plumbing guide lists the flapper, the fill valve, and the float as the three causes behind virtually every running-tank call.

How long should a fill valve last?

Four to five years is typical in average water, and both brands warranty theirs for five. Hard water, sediment, and chlorine-containing in-tank cleaners shorten that dramatically, which is why Korky added a cleanable inlet strainer to the 528MP.

Is the Fluidmaster 400H worth the extra money over the 400A?

If your toilet is a modern 1.28 GPF model, or if refill noise bothers you, yes. The 400H's vortex refill speeds up bowl refill, its precision fill cuts noise, and its dial lets you trim water use across the 1.28 to 3.5 GPF range. For an older tank with no noise complaint, the 400A does the same job for ten dollars less.

Can I use bleach tablets in the tank?

Not with these valves. Fluidmaster explicitly warns that in-tank cleaners containing bleach or chlorine damage tank components, cause flooding risk, and void the 400A warranty. The manufacturer's suggested alternative is a bowl-cleaner system that keeps the cleaning agent out of the tank entirely.