Match the flange to the drain pipe under it. A 3" x 4" flange covers both common sizes: it’s cemented over 3" pipe (3-1/2" across) or set inside 4" pipe (4-1/2" across).
What’s under the toilet?
Measure across the outside of the pipe from a basement or crawl space below.
The flange that fits
A flange that fits over 3" pipe
3" PVC is 3.5" across. A 3" x 4" flange is cemented over it, and so is one sold as 3" hub.
On the package: 3 in. x 4 in. closet flange, or 3 in. hub
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A flange that fits inside 4" pipe, or over it
4" PVC is 4.5" across. A 3" x 4" flange goes inside it, and a 4" hub flange fits over it.
On the package: 3 in. x 4 in. (inside 4 in.) or 4 in. hub
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A cast iron repair flange
Repair flanges either slide over the cast iron pipe and seal with a gasket as their bolts tighten, or go inside it and seal as they’re turned or tightened.
On the package: Cast iron repair flange, over or inside the pipe
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A flange spacer kit
Spacers stack on the existing flange to bring it up to the finished floor, with longer closet bolts.
On the package: Closet flange spacer kit
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An offset flange
It moves the toilet’s connection up to 2" forward, back or sideways, and keeps the same opening inside.
On the package: Offset closet flange, 3 in. x 4 in.
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Cast iron takes a repair flange made for it.
Flange types
| Sold as | Fits |
|---|---|
| 3" hub / inside 4" (3" x 4") | Cemented over 3" pipe, or inside 4" pipe |
| Hub (3" or 4") | Over the pipe: the pipe goes into the flange’s socket |
| Inside (3" or 4") | Inside the pipe |
| Spigot | Its tail goes into a fitting’s socket, the way a pipe end does |
| Offset | Moves the toilet up to 2" off the drain’s center |
| Cast iron repair | Over or inside cast iron pipe, sealed by a gasket |
What the code says
The IPC sets 3" as the smallest building drain that can serve a toilet. The codes don’t count a 4" x 3" flange, a closet bend with a 4" inlet and 3" outlet, or an offset flange as shrinking the drain. The IRC also accepts a 3" quarter bend under a 4" x 3" flange.
The flange has to be fastened to the drain and anchored to the floor structure, and the closet bolts that hold the toilet are a copper alloy, such as brass.
Flange height
The bottom of the flange rests on top of the finished floor and is screwed down through it into the subfloor, before the toilet goes on. If new flooring has left the flange below the surface, a spacer kit stacks on it to make up the height, as long as the old flange isn’t broken or badly rusted.
Before you lift a toilet, turn off its water supply and flush to empty the tank. If the drain will stay open for more than an hour or two, cover it to keep sewer gas out.
Not sure where the toilet sits?
Toilet rough-in