Measure the bowl from bolt hole to bolt hole
The most important measurement is the distance between the two bolts that hold the seat down. Place your measuring tape at the center of one bolt hole and measure straight across to the center of the other bolt hole. This measurement is almost always either 5.5 inches or 6 inches. Write down which one matches your toilet — this single number tells you whether a replacement seat will fit.
The bolt holes are located at the back of the bowl, underneath where the seat hinges attach. If your current seat is still in place, you may need to lift it to see them clearly. If the seat is already removed, the holes are visible on the rim at the back of the bowl.
Key Takeaways
- Measure between the centers of the two bolt holes at the back of the bowl — this distance is either 5.5 or 6 inches and determines whether a seat will fit.
- Measure the bowl length from the front rim to the back rim, and the width at its widest point, to confirm the seat shape matches your toilet.
- Check whether your bowl is round or elongated by comparing length to width — round bowls are roughly equal in both directions, elongated bowls are noticeably longer.
- Bring these three measurements (bolt spacing, length, width, and bowl shape) when shopping so you can verify the replacement seat will fit before purchase.
Measure the bowl length and width
After you have the bolt spacing, measure how long the bowl is from front to back. Place your tape at the very front edge of the rim and measure straight back to the very back edge. Write this number down. Then measure the bowl width at its widest point, usually somewhere in the middle. These two measurements tell you the bowl's overall shape and size.
Most toilets are either round or elongated. A round bowl is roughly the same length and width — typically around 16 to 17 inches long and 14 to 15 inches wide. An elongated bowl is noticeably longer, usually 18 to 19 inches long but still 14 to 15 inches wide. The replacement seat must match your bowl shape, so knowing which type you have prevents buying a seat that will not fit properly.
Check the bowl shape by comparing length to width
Once you have both measurements, compare them. If the length and width are very close to each other — within an inch or so — you have a round bowl. If the length is clearly longer than the width by 2 to 3 inches, you have an elongated bowl. This matters because a round seat will not fit an elongated bowl, and an elongated seat will overhang a round bowl.
The shape is more important than exact measurements. A replacement seat does not need to match your bowl dimensions perfectly — it just needs to be the same shape and have the correct bolt spacing. Manufacturers make round seats and elongated seats in standard sizes, so as long as you know which type your toilet is, you can find a seat that works.
Verify bolt hole spacing on the replacement seat
Before you buy, check that the replacement seat's bolt holes are spaced the same distance apart as your toilet. Most packaging or product descriptions list the bolt spacing — look for "5.5 inch" or "6 inch" in the specifications. If you are shopping in a store, you can measure the replacement seat the same way you measured your toilet: from the center of one bolt hole to the center of the other.
Bolt spacing is the one measurement that cannot vary. If the spacing is off by even a quarter inch, the bolts will not line up with your toilet's holes, and the seat will not install. The bowl shape and overall size have more flexibility — a slightly oversized seat will still work — but bolt spacing must match exactly.
Bring your measurements when you shop
Write down three pieces of information before you go to the store or search online: the bolt hole spacing (5.5 or 6 inches), whether your bowl is round or elongated, and the approximate bowl length and width. These three facts are all you need to find a replacement seat that will fit.
If you are ordering online, use these measurements to filter your search. Most retailers let you sort by bowl shape and bolt spacing. If you are shopping in a store, bring a piece of paper with your measurements so you can compare them to the replacement seats on display. Many stores will let you hold a seat up to your toilet to verify the fit before you buy.
Common mistakes to avoid
The most common error is assuming all toilet seats are the same size. They are not — round and elongated seats are not interchangeable, and bolt spacing varies. Buying the wrong shape or spacing means the seat will not fit, and you will have to return it.
Another mistake is measuring only the bolt spacing and ignoring the bowl shape. You might find a seat with the correct bolt spacing but the wrong shape, and it will not sit properly on your bowl. Always verify both the spacing and the shape before you buy. If you are unsure whether your bowl is round or elongated, measure both the length and width — the measurements will tell you which type you have.
Frequently Asked Questions
What if I cannot find the bolt holes on my current seat?
Lift the seat and lid all the way up. The bolt holes are on the rim at the very back of the bowl, where the seat hinges attach. If the seat is stuck or difficult to lift, try gently rocking it side to side. If the seat is already removed, the holes are visible on the bowl itself.
Do I need to measure anything else besides bolt spacing and bowl shape?
No. Bolt spacing and bowl shape are the only measurements that determine whether a seat will fit. The exact length and width of your bowl matter less than knowing whether it is round or elongated. Manufacturers make seats in standard sizes, so as long as you match the shape and bolt spacing, the seat will work.
Can I use a round seat on an elongated bowl?
No. A round seat will be too small and will not cover the elongated bowl properly. You must match the bowl shape — round seats for round bowls, elongated seats for elongated bowls. The bolt spacing must also match, or the seat will not attach securely.
What if my measurements do not match the standard sizes?
Most toilets use either 5.5-inch or 6-inch bolt spacing, and either round or elongated bowls. If your measurements fall between these standards, measure again to make sure you are measuring from the center of each bolt hole. If your toilet is genuinely unusual, contact the toilet manufacturer or a plumber — they can tell you what seat will fit.
Should I measure the thickness of my current seat?
No. Replacement seats come in different thicknesses and materials, but thickness does not affect whether a seat will fit your toilet. Thickness is a matter of comfort and durability, not fit. Focus on bolt spacing and bowl shape, and you can choose the thickness you prefer.