The short version: check your bowl shape, your bolt hole spacing, and the flat space between the bolt holes and the tank. Get those three right and the seat will fit. Everything else here is detail for when one of them comes back odd.
Almost every toilet in North America will take a bidet. The exceptions are real but narrow, and you can identify them in about five minutes with a tape measure.
On this page
Why bidet seats are fussier than regular toilet seats
A standard toilet seat is a hinged lid. It tolerates a loose fit because nothing depends on it sitting precisely.
A bidet seat is different in three ways. It carries a mounting plate that has to sit flat on the porcelain behind the bolt holes. It houses a nozzle that has to arrive at a specific position over the bowl. And on electric models it holds a water heater, a pump and control electronics, which makes it heavier and less forgiving of a bowl that doesn’t support it evenly.
Bio Bidet is blunt about this in their own compatibility documentation: the bowl supports the seat and distributes the load across the fixture, so a mismatch can break the seat outright. Match the shape. Don’t try to make an elongated seat work on a round bowl because it was on sale.
The three measurements
You’ll want a tape measure and about five minutes. It’s easier if you take your existing toilet seat off first, though you can usually manage without.
Bolt spacing, tank clearance, and bowl length
1. Bowl length — round or elongated?
Find the two bolts at the back of your seat where it hinges. Measure in a straight line from the centre of those bolt holes to the front edge of the bowl. Around 16½ inches is a round bowl. Around 18½ inches is elongated.
Manufacturers publish slightly different bands around those figures. Brondell calls 16 to 17¾ inches round and 18 to 19½ inches elongated. Coway’s ranges are 16.59 to 17.85 for round and 17.98 to 19.24 for elongated. If your number lands cleanly in one band you’re done. If it falls in the gap between roughly 17.9 and 18 inches, check the spec sheet for the specific seat rather than assuming.
2. Bolt hole spacing
Measure between the centres of the two mounting bolt holes. Nearly every American toilet is 5.5 inches, so for most people this is a formality. Measure it anyway — older toilets, imported fixtures and some commercial-style bowls vary, and this is the measurement that quietly disqualifies a seat after you’ve already ordered it.
3. Clearance behind the bolt holes
This is the one that catches people out, and it’s the reason a toilet can pass the first two checks and still not take a seat.
Measure from the centre of the bolt holes back to the front face of the tank. You are looking for flat, level porcelain — the mounting plate has to sit on it. Minimums vary quite a bit: Bio Bidet asks for at least 1.5 inches, Coway for 1.8, and Brondell will work with as little as ¾ inch. If you have 2 inches of flat space you have options across every brand. Under 1.5 inches, your choices narrow sharply.
Tolerances by brand
Here’s what each manufacturer publishes. These are compiled from the brands’ own fitment documentation, and they don’t agree with each other — which matters if your toilet is near a limit, because a bowl one brand rejects may be fine for another.
| Brand | Bolt spacing | Clearance | Round | Elongated |
|---|---|---|---|---|
| Brondell | 5.25″–8″ | 0.75″ min | 16″–17.75″ | 18″–19.5″ |
| Bio Bidet | 5.5″–7.5″ | 1.5″ min | ~16.5″ | ~18.5″ |
| Coway | 5.23″–7.9″ | 1.8″ min | 16.59″–17.85″ | 17.98″–19.24″ |
The practical takeaway: Brondell is the most forgiving on both bolt spacing and tank clearance. Coway is the strictest on clearance. If your toilet is marginal on one of these numbers, that’s your ordering hint — and it’s worth phoning the retailer with your measurements before you buy rather than after.
The toilets that cause trouble
If your three measurements came back normal, you can stop here and go choose a seat. The rest of this page covers the awkward cases. Look at your toilet from the side.
Skirted toilets
A skirted toilet has a smooth, filled-in side panel that conceals the trapway. They cause two specific problems, and both are solvable with the right part.
The bolt holes are hidden from underneath. Standard mounting hardware assumes you can reach up under the bowl rim. On a skirted toilet you can’t. The fix is a top-mount kit, which secures the seat entirely from above. Many current seats include one; some don’t. Check before ordering.
The tank fill valve may be hard to reach. If the skirt blocks access, you’ll want an alternate T-valve that connects at the wall supply instead. Standard part, easy to source, better to know in advance.
French curve toilets
This is the one that genuinely stops some installations. On a conventional toilet the tank meets the bowl at roughly a right angle, leaving a flat shelf. On a French curve the tank sweeps down in a smooth concave curve, leaving very little flat porcelain for a mounting plate.
TOTO states directly that a French curve may prevent a Washlet installing at all, and Bio Bidet says their seats don’t fit well on deep curves. Some French curve toilets take a seat; some cannot under any circumstances. Look at yours from the side — that concave sweep is unmistakable in profile — then measure the flat width across the back of the bowl carefully, because a seat with a narrow mounting plate may still work. The Infinity Eaze z3 is known for fitting a number of them.
With a French curve, call the retailer with your toilet’s make and model before you order. This is the one case where measuring alone isn’t enough.
One-piece and wall-hung toilets
Most one-piece toilets are fine, but they’re more likely to combine features — a skirted base and a curved tank junction — so run all three measurements rather than assuming.
Wall-hung toilets use an in-wall tank, so there’s no exposed fill valve to tee off. You’ll need a plumber to provide an accessible ½-inch water supply connection. Doable, but it’s plumbing work rather than a DIY afternoon.
A note on electrical requirements
Fit is only half of it. Electric seats also need a grounded GFCI outlet within about four feet, and the cord exits from a specific side — which catches people out, because TOTO Washlets exit left while most other brands exit right. We cover all of that in whether you need an outlet next to the toilet.
If your toilet won’t take a bidet seat
You still have options.
A bidet attachment is the most common answer. Attachments sit between your existing seat and the bowl rather than replacing the seat, and they fit a much wider range of fixtures, including many curved and one-piece toilets that reject full seats. They’re also far cheaper. What you give up is warm water on most models, plus the heated seat and dryer.
A handheld sprayer mounts to the wall or the side of the tank and connects to the supply line. It doesn’t touch the seat at all, so bowl shape is irrelevant. It requires more hand control to use.
Replacing the toilet is worth mentioning honestly. If you have a French curve toilet you dislike anyway, or you’re already planning a renovation, changing the fixture opens up every seat on the market. It’s the expensive answer, but for anyone doing an aging-in-place retrofit it may be on the list regardless.