Toilet Seat Sizes and Compatibility Guide for Bidet Seats
If you are new to bidet seats, the most common worry is whether one will actually fit your toilet, and that is a fair question when toilets come in so many shapes. The reassuring part is that fit comes down to just a couple of simple measurements, and once you understand them you can shop with confidence. At Majestic Flush we put this guide together for first time buyers, using the official fitment standards published by TOTO, so the numbers here come from the manufacturer rather than guesswork.
In short, you only need to match two things: the shape of your toilet bowl and the spacing of the seat mounting holes. Almost every standard American toilet uses the same bolt spacing, so for most people the only real question is whether the bowl is round or elongated.
The two measurements that decide fit
Two numbers determine almost everything about compatibility, and the brand of your toilet matters far less than either of them.
- Bowl shape. Whether your bowl is round or elongated decides which version of the seat you order.
- Mounting hole spacing. The distance between the two bolts that hold the seat in place, which on standard toilets is remarkably consistent.
Get these two right and the seat will fit. The diagram below shows the three things worth measuring before you buy.
Round versus elongated bowls
The first thing to confirm is your bowl shape, because a bidet seat is made to match one shape or the other. To check, measure from the center of the seat mounting holes to the front edge of the bowl. According to TOTO, about 18.5 inches indicates an elongated bowl, while roughly 16.5 inches indicates a round bowl. These are the standard reference figures, and they can vary slightly between brands and models, so it is always worth measuring your own toilet rather than assuming.
This shape match is not optional, because an elongated seat is designed for an elongated bowl and a round seat for a round bowl, and the two are not interchangeable. Order the version that matches what you already have.
The mounting hole spacing, about 5.5 inches
The two bolts that hold a standard toilet seat in place sit about 5.5 inches apart, measured from the center of one hole to the center of the other. This spacing has been a long standing standard on American toilets, and it is the main reason most bidet seats fit most toilets without any trouble. To confirm yours, measure between the centers of the two holes where your current seat bolts down.
Check the clearance behind the seat
Beyond shape and bolt spacing, a bidet seat needs a little flat room to mount and to open and close freely. TOTO advises that on an elongated toilet you want at least 2 inches of flat space from the front of the tank to the center of the mounting holes. Round bowls vary a bit more depending on the model, so it is best to check the spec sheet for the exact seat you are considering. TOTO also calls for a minimum of 8 inches of open space measured from the center of the toilet to the edge of the bowl's flat surface. When your toilet meets these, the baseplate sits flat and the lid clears the tank.
French curve and skirted toilets, the main exception
This is the one area where beginners get caught out, so it is worth a careful look. Most toilets have a tank with a flat front face, which leaves a clean flat area behind the seat bolts for the bidet to mount onto. A French curve toilet is built differently, because the front of the tank sweeps down in a smooth concave curve to blend into the bowl. The result is elegant, but it leaves less flat space behind the bolts, and TOTO notes that a French curve toilet may prevent a Washlet from installing. If you think you have one, check that there is enough flat width across the back of the bowl for the seat's mounting plate before you order.
Skirted toilets, where the trapway is hidden behind a smooth side panel, are a smaller consideration, since some need an alternate mounting bracket, although many current bidet seats include one in the box.
If you are not sure which kind you have, look at your toilet from the side. When the tank meets the bowl with a continuous flowing curve rather than a distinct flat step, treat it as a French curve and measure the flat area carefully before buying.
On a standard toilet the tank meets the bowl with a flat step that leaves room for the seat. On a French curve toilet the tank sweeps down in a curve, which can leave too little flat space behind the bolts.
How to measure your own toilet
You only need a tape measure and a minute or two. Working through these four checks tells you everything you need before you order.
- Mounting hole spacing. Measure center to center between the two seat bolt holes, where you should find about 5.5 inches.
- Bowl length. Measure from the line of the mounting holes to the front edge of the bowl, where about 16.5 inches means round and about 18.5 inches means elongated.
- Tank clearance. Measure the flat space from the front of the tank to the center of the mounting holes, aiming for at least 2 inches.
- Side profile. Look from the side to see whether the tank meets the bowl flat or in a French curve.
Quick reference
| What to measure | Round bowl | Elongated bowl |
|---|---|---|
| Mounting hole spacing | About 5.5 in | About 5.5 in |
| Length, holes to front edge | About 16.5 in | About 18.5 in |
| Seat version to order | Round | Elongated |
These figures follow TOTO's published Washlet standards and make a reliable starting point, though you should always confirm against your own toilet and the spec sheet for the seat you choose.
What about Brondell and HOROW seats?
The same principles carry across brands. American toilets share that 5.5 inch bolt spacing, and seats are still sold in round and elongated versions, so the shape match and the clearance checks apply just as they do with TOTO. The one thing to remember is that the exact clearance a given seat needs can differ a little between brands, and even between models from the same brand, so check the spec sheet for the specific model you want. At Majestic Flush we carry TOTO, Brondell, and HOROW, and each product page lists the measurements you need.
Frequently asked questions
Can I put an elongated bidet seat on a round toilet?
No. Bidet seats are shape specific, so a round bowl needs the round version and an elongated bowl needs the elongated version. They are not interchangeable, which is why confirming your bowl shape first matters.
How do I know if my toilet is round or elongated?
Measure from the center of the seat mounting holes to the front edge of the bowl. About 16.5 inches points to a round bowl, and about 18.5 inches points to an elongated bowl.
What is the distance between toilet seat mounting holes?
On standard American toilets the two mounting holes sit about 5.5 inches apart, measured center to center. Because this spacing is so consistent, most bidet seats fit most toilets.
What is a French curve toilet, and will a bidet seat fit it?
A French curve toilet has a tank that sweeps down into the bowl in a smooth curve rather than a flat step. According to TOTO, this can leave too little flat space behind the bolts and may prevent a bidet seat from fitting, so it is important to measure the flat area carefully before buying.
Do I need a plumber or electrician to install a bidet seat?
Most non electric attachments install with simple hand tools and connect to your existing water supply. An electric bidet seat needs a nearby grounded outlet, and if there is no outlet already in place, it is best to have a licensed electrician add one.
Still not sure which seat fits?
If you have measured your toilet and still are not completely certain, send us your numbers and we will confirm the right fit before you spend anything. You can browse the full bidet seat collection at Majestic Flush, where every model lists its required measurements, or reach our team at (980) 452-3079 and we will help you choose.
Measurements in this guide reflect TOTO's official Washlet fitment standards.
Leave a comment