Quick answer
A toilet flush valve is the assembly in the floor of a toilet tank, made of a drain opening, a sealing seat and an overflow tube, that releases the stored water into the bowl when the flapper or seal lifts and catches overflow if the tank overfills.
It is the outlet half of the tank; the fill valve is the inlet. When the seat wears or the gasket beneath it fails, water leaks into the bowl or onto the floor.
Quick facts
The key facts about Toilet Flush Valve, with sources:
- Sizes
- 2-inch and 3-inch openings are common; kits sold for 3-inch 1
- Overflow height
- Below the handle hole, above the water line (one maker) 1
- Overflow vs. fill valve
- Top at least 1 inch below the fill valve’s C.L. mark (one maker) 1
- WaterSense label
- 1.28 gallons per flush or less 2
- Leak check
- Dye in the tank; color in the bowl within 10 minutes means a leak 3
Key takeaways
- The flush valve is the hole, seat and overflow tube the flapper sits on; the fill valve is a different part.
- If a new flapper still leaks, a pitted or warped flush valve seat is the next suspect.
- Replacing a flush valve means separating the tank from the bowl, which is harder than a flapper swap.
- A 3-inch valve moves water faster than a 2-inch one; match the size your toilet was built for.
- In commercial restrooms, "flush valve" can mean a flushometer, a tankless valve on the wall.
The outlet half of the toilet tank
A tank toilet has two valves: the fill valve lets water in, and the flush valve lets it out. Lift the lid and the flush valve is the round opening in the middle of the tank floor, with a tall tube called the overflow pipe standing beside it.
The flapper, or on some toilets a round canister seal, rests on the flush valve’s seat, and lifting it lets the tank’s water drop through the opening into the bowl.
The overflow tube is a safety outlet: if the fill valve fails to shut off, water spills into the tube and down to the bowl instead of over the tank rim onto the floor.
Under the tank, a large nut and a spongy tank-to-bowl gasket hold the flush valve in place and seal it to the bowl. That gasket is part of why flush valve leaks can show up on the floor as well as in the bowl.
What the flush valve does during a flush
- Seal lifts: The handle pulls the chain and the flapper rises off the flush valve seat.
- Water drops: The full tank empties through the opening into the bowl. The flapper’s timing sets how much goes; one maker’s adjustable flapper runs from 0 (quick flush) to 10 (long flush) 1.
- Seal returns: The flapper drops back onto the seat and seals.
- Refill: The fill valve refills the tank and sends a small stream down the overflow tube through the refill hose to top up the bowl 1.
- Overflow standby: If the water level ever rises too high, it spills into the overflow tube instead of onto the floor.
Source: [1]
Flapper, canister, dual-flush and flushometer valves compared
| Type | Where you find it | Notes |
|---|---|---|
| 2-inch flapper flush valve | Many older and standard gravity toilets | Uses a 2-inch flapper |
| 3-inch flapper flush valve | Many newer low-flow gravity toilets | Fluidmaster sells a complete 3-inch valve and flapper kit 1 |
| Canister or tower flush valve | Some newer gravity toilets | A cylinder lifts straight up; uses a round seal instead of a hinged flapper |
| Dual-flush valve | Toilets with two buttons | Gives a reduced flush for liquid waste 2 |
| Flushometer valve | Commercial toilets and urinals with no tank | Exempt from the federal lead-free definition 4 |
Measure the drain opening before you buy parts
A 2-inch flapper will not seal a 3-inch flush valve, and the reverse is also true. With the tank empty, measure across the opening the flapper covers, or look up the toilet’s model number, before buying a flapper or a complete valve kit such as Fluidmaster’s 3-inch 540AKR 1.
Example: what a flush valve has to deliver
The flush valve releases whatever volume the toilet is built for. The chart shows how far that volume has fallen, from EPA WaterSense figures 2; the table shows what the change means for a home.
| Measure | Figure | Source |
|---|---|---|
| Toilets’ share of indoor water use | Nearly 30 percent of an average home’s | EPA WaterSense 2 |
| WaterSense vs. the federal standard | 20 percent less water per flush | EPA WaterSense 2 |
| Cut in toilet water use from replacing older toilets | 20 to 60 percent | EPA WaterSense 2 |
| Yearly savings, old toilets to WaterSense | Nearly 13,000 gallons per home | EPA WaterSense 2 |
A low-flow toilet has less water to work with, so it depends on releasing it fast. That is one reason makers moved to wider valves. It is also why the wrong replacement part, or a flapper set to close too soon, can leave a weak flush.
How to identify yours
Shut off the water, flush, and look into the empty tank. A hinged flapper on two ears at the base of the overflow tube means a flapper-style valve. Measure the opening: about 2 inches or about 3 inches. A tall cylinder that lifts straight up means a canister valve, which needs the maker’s seal.
Write down the model number printed inside the tank. It is the surest way to buy the right part.
On a plumber’s invoice, the job is listed as a flush valve replacement or a "tank rebuild," which often includes a new flapper, fill valve and tank-to-bowl gasket together. That work falls under toilet repair and installation.
Why a sound flush valve matters
Sound valve
- Gives the flapper a smooth seat, so the tank holds.
- Releases the toilet’s rated volume for a clean flush.
- Overflow tube protects against a stuck fill valve.
- Gasket underneath keeps the tank-to-bowl joint dry.
Worn valve
- Pitted seat leaks into the bowl even with a new flapper.
- Cracked overflow tube lets the tank drain constantly.
- Failed gasket leaks onto the floor.
- Silent leaks add to the more than 9,300 gallons an average household loses yearly 3.
When the flush valve is not the problem
A running toilet is not always the flush valve. If water is pouring into the overflow tube, the fill valve is set too high or not shutting off.
If the toilet runs on and off, the flapper is the first suspect. Replace the cheap flapper first; move to the flush valve only if the leak continues.
A weak flush also has other causes: a clogged trap or rim holes, a low water level, or a vent problem. A new flush valve will not fix those.
What makes a flush valve repair cost more or less
We do not publish prices, because fair figures need dated local quotes. A flush valve kit is a modest part; labor is the larger share, because the tank must come off the bowl.
Cost drivers: whether the tank bolts are corroded and must be cut; whether the toilet needs a maker-specific valve; whether the fill valve and flapper are replaced at the same visit; the risk of cracking old porcelain; and whether replacing the whole toilet makes more sense. Our cost guide explains how plumbers price fixture repairs.
Water waste is a cost too. EPA says household leaks waste more than 9,300 gallons a year on average 3, and our leak calculator turns a running toilet into a yearly figure.
How a flush valve is replaced
Condensed from Fluidmaster’s 540AKR 3-inch flush valve kit instructions, one maker’s method 1:
- Turn off the water, flush, and sponge the tank dry.
- Disconnect the supply line and unbolt the tank from the bowl.
- Remove the old tank-to-bowl gasket and the large nut under the old valve, then lift the valve out.
- Set the new valve with the overflow pipe toward the back of the tank but not over the bolt holes; hand-tighten the large nut.
- Set the overflow pipe height: below the handle hole, above the full water line, and at least 1 inch below the fill valve’s critical level (C.L.) mark.
- Fit the new tank-to-bowl gasket, set the tank level, and tighten the bolts evenly until the tank does not rock.
- Clip the refill hose to the top of the overflow pipe.
- Reconnect the supply, open the water and test flush; adjust the flapper setting if the flush is too short or doubles.
Porcelain cracks, and cracked tanks flood
Fluidmaster warns not to overtighten nuts or the tank or bowl may crack 1. A cracked tank can let go hours later.
Tighten evenly, stop when the tank is firm, and check for drips under the tank before you leave it.
The kit maker also warns that in-tank cleaners with bleach or chlorine can damage parts and cause flooding 1.
How to keep a flush valve sealing
- Run EPA’s dye test a couple of times a year: color in the bowl within 10 minutes means a leak 3.
- Replace the flapper at least every five years, per EPA 3.
- Wipe scale off the seat when you change the flapper.
- Skip drop-in tank cleaners with bleach or chlorine 1.
- Check under the tank for drips at the bolts and center gasket.
Signs the flush valve is leaking
- The toilet still runs or refills on its own after a new flapper.
- The seat feels rough, pitted or warped under your finger.
- Water drips from the center of the tank bottom.
- The overflow tube is cracked or shorter than the water line.
- The flush is weak even with the tank full.
Call a plumber when the tank bolts are rusted solid, the tank is old and fragile, or a leak under the tank reaches the floor. A slow floor leak can rot the subfloor.
Which federal and code rules apply to flush valves
Flush volume: the federal standard is 1.6 gallons per flush. The 2015 UPC as adopted in Washington (§411.2) sets the same maximum under ASME A112.19.2/CSA B45.1 52. WaterSense labeled toilets use 1.28 gallons or less 2.
Lead-free rule: toilets, fill valves and flushometer valves are exempt from the federal lead-free definition because they do not supply drinking water 4.
Critical level: one maker’s instructions put the overflow top at least 1 inch below the fill valve’s C.L. mark 1. That spacing keeps tank water from siphoning back into the supply through the fill valve.
Permits: under 2018 IRC text published by Idaho Falls, repairing leaks in fixtures is exempt from a permit if no valves, pipes or fixtures are replaced or rearranged 6. Many cities treat a tank rebuild as minor repair, but check locally. Rules checked 2026-10-12.
Flush valve vs. fill valve vs. flapper
| Part | Job | Failure looks like |
|---|---|---|
| Flush valve | Outlet: lets tank water into the bowl | Leak past a worn seat or under the tank |
| Fill valve | Inlet: refills the tank | Water running into the overflow tube |
| Flapper | Seal on the flush valve seat | Dye shows in the bowl; phantom refills 3 |
| Flushometer | Tankless valve in commercial restrooms | Runs on or will not shut off |
Flush valve myths that lead to the wrong repair
- Myth Flush valve and fill valve are the same thing.
- Reality The flush valve lets water out; the fill valve lets it in.
- Myth A running toilet always needs a new flush valve.
- Reality A worn flapper is the most common cause, per EPA 2.
- Myth The overflow tube can be any height.
- Reality One maker sets it below the handle hole and at least 1 inch below the fill valve C.L. mark 1.
- Myth Tighter tank bolts stop leaks.
- Reality Overtightening can crack the tank or bowl 1.
- Myth Bleach tablets keep valve parts clean.
- Reality Fluidmaster says they damage parts and can cause flooding 1.
Water and local programs
Hard water leaves scale on the flush valve seat, which stops a flapper from sealing; hard-water regions see this more.
Many utilities also offer toilet rebates or leak programs aimed at moving homes from older toilets, which used up to 6 gallons per flush, to WaterSense models at 1.28 or less 2.
Check your water utility for current programs; we do not list amounts.
When to rebuild the flush valve and when to replace the toilet
Rebuild when the toilet is sound, the bowl flushes well and only the seat or gasket is worn.
Consider a new toilet when the old one uses well over 1.6 gallons per flush, the porcelain is cracked, or parts are hard to find.
EPA estimates WaterSense toilets can save nearly 13,000 gallons a year compared with older ones 2.
Next step: replace the flapper and run the dye test, then use the guide below, which sorts each symptom by where the water ends up.
- If Dye reaches the bowl within 10 minutes 3
- Then Replace the flapper first
- If Dye still leaks after a new flapper
- Then Flush valve seat is worn or scaled: clean it or replace the valve
- If Water spills into the overflow tube
- Then Fill valve set too high or failing, not the flush valve
- If Water drips under the tank
- Then Tank-to-bowl gasket or bolts
- If Bowl is cracked or toilet uses far more than 1.6 gallons 2
- Then Price a new toilet instead of a rebuild
Questions about Toilet Flush Valve
Why does my toilet keep running?
A running toilet is either leaking water out of the tank or overfilling it. Put food coloring in the tank and wait: EPA says color in the bowl within 10 minutes means a leak past the flapper or flush valve seat.
If the bowl stays clear but water trickles into the overflow tube, the fill valve is set too high or not shutting off. Fix the flapper first, since it is the cheapest part, then look at the valve seat.
What is the difference between a flush valve and a fill valve?
The flush valve lets water out of the tank into the bowl. The fill valve lets water into the tank from the supply line and shuts off when the tank is full. A running toilet can be either one, so check where the water is going.
How do I know if my flush valve is bad?
If a new flapper still lets dye into the bowl, feel the seat. A rough, pitted or warped seat is bad. Water dripping from the center under the tank points to the gasket below the valve. A cracked overflow tube lets the tank drain constantly.
Is a 3-inch flush valve better than a 2-inch?
A wider opening lets water reach the bowl faster, which helps low-flow toilets. But the right size is the one your toilet was built for. Measure the opening or use the model number; makers sell complete kits for each size.
Can I replace a flush valve myself?
Many people can, but it is harder than a flapper. You must unbolt the tank from the bowl, swap the valve and gasket, and set the tank back without cracking it. Fluidmaster warns that overtightening the nuts can crack the tank or bowl.
How high should the overflow tube be?
Fluidmaster’s instructions say the top should be below the handle hole, above the full water line, and at least 1 inch below the critical level mark on the fill valve. Cut or adjust it to fit those limits.
What is a flushometer?
A flushometer is a tankless flush valve used on commercial toilets and urinals. It uses line pressure to deliver a measured flush. EPA lists flushometer valves among products exempt from the federal lead-free definition, since they do not supply drinking water.
How much water does a toilet use per flush?
Under the federal standard, new toilets use no more than 1.6 gallons per flush. WaterSense labeled models use 1.28 or less. EPA says older toilets can use up to 6 gallons per flush.
Sources
- Fluidmaster, 540AKR Complete 3-inch Flush Valve and Flapper Repair Kit installation instructions (Part 5-3094), retrieved .
- U.S. EPA WaterSense, Residential Toilets, retrieved .
- U.S. EPA WaterSense, Fix a Leak Week, retrieved .
- U.S. EPA, Use of Lead Free Pipes, Fittings, Fixtures, Solder and Flux for Drinking Water, retrieved .
- Washington State Building Code Council, WSR 17-04-089 amending WAC 51-56 (2015 Uniform Plumbing Code), retrieved .
- City of Idaho Falls, ID, Work Exempt from a Permit (2018 IRC text), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Replacement steps, overflow height, C.L. spacing, adjustable flapper and warnings are from Fluidmaster’s 540AKR instructions (single maker, labeled). Flush volumes, savings and flapper as main leak source from EPA WaterSense; dye test and household leak totals from EPA Fix a Leak Week.
Flush limit also from the 2015 UPC as adopted in Washington. Lead-free exemptions from EPA. Permit exemption from 2018 IRC text published by Idaho Falls. Canister and 2-inch rows are general trade description. No prices.
Suggest a correction. We fix errors and say what changed.