Tank Fit

Seats

Elongated or round, slow-close, and leaving room for a bidet

Seats are the easiest part on this site to buy wrong, because the two shapes look similar in a box and identical online. Two measurements settle it. A third one decides whether you can add a bidet seat next year without buying a new toilet.

The two measurements

  1. Bowl length: from the center of the seat bolt holes at the back to the outer front edge of the bowl rim. About 16½ in → round. About 18–18½ in → elongated. There is no meaningful in-between in the US market; if you measure 17½, measure again from the bolt centers.
  2. Bolt spread: center to center of the two mounting holes. 5½ in is the near-universal standard. If yours is not 5½, you likely have a specialty or older European fixture and need the manufacturer's seat.

Also note bowl width at the widest point if the bathroom is tight, and whether the holes are open holes in the china (normal) or captive inserts (some one-piece designs).

Materials, honestly

  • Enameled wood: heavier, feels solid and warmer, finish can chip and then swell if water gets in. Good in a low-traffic bathroom.
  • Polypropylene / plastic: lighter, cheaper, does not chip, easier to clean around the hinges. The practical choice for a busy family bathroom.
  • Molded/composite premium: heavier plastic with better hinges. The hinge quality is what you are paying for and it is what fails first on cheap seats.
Slow-close elongated, 5½ in bolt spreadSoft-close elongated seat with quick-release hingestypically $25–$45
Fits
18–18½ in elongated bowls with standard 5½ in bolt spread. Quick-release hinges are worth the extra few dollars — you can lift the seat off in one motion to clean under the hinges, which is where seats get gross.
Does not fit
Round bowls (it will overhang the front and look wrong), and bowls with non-standard bolt spread. Slow-close hinges also add height at the back — measure your tank-to-hinge clearance if the tank lid overhangs.
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Slow-close round, 5½ in bolt spreadSoft-close round seattypically $22–$40
Fits
16½ in round bowls, standard bolt spread. Common in powder rooms, older homes, and compact baths.
Does not fit
Elongated bowls — a round seat on an elongated bowl leaves exposed rim at the front, which is both uncomfortable and a cleaning problem.
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Leaving room for a future bidet seat

Electronic bidet seats — TOTO Washlet and equivalents — are much easier to add if you plan for them before you buy a regular seat. What they need:

  • Rear clearance. A bidet seat's control body sits behind the bowl, between the seat hinges and the tank. Some tank shapes, especially skirted one-piece toilets with a forward-sloping tank front, leave too little room. Measure from the bolt holes back to the tank face and compare with the model's published requirement.
  • A GFCI outlet within reach. Electronic seats need power at the toilet. If there is no outlet, that is an electrician's visit and worth doing while a wall is open for anything else.
  • A working, accessible shutoff. Bidet seats tee off the toilet supply. A stop that will not close cleanly makes the install miserable. See supply line and shutoff.
  • Bowl shape match. Bidet seats are sold in round and elongated the same as regular seats, so the same measurement applies.

Non-electric bidet attachments — a thin plate that sits under your existing seat with a side control lever — are the cheap way to test whether you like the idea. They need nothing but the supply tee, and they raise the seat by a few millimeters.

Seat bolts, the one annoyance

Old seat bolts corrode. Plastic bolts you can often cut through with a utility knife. Metal ones may need a nut driver from underneath while holding the top, or a hacksaw blade with cardboard protecting the china. Do not attack a seized bolt with a drill next to the bowl — a slip cracks vitreous china and turns a $30 job into a new toilet.

What a new seat will not fix

  • A wobbly seat that keeps loosening on a bowl with worn holes — that is the mounting, and rubber washers plus proper hinge nuts fix it.
  • Odor. Persistent smell near the floor is a seal issue, not a seat issue. See wax ring and flange.
  • Any tank noise or leak. Start at the diagnostic.

Next, in the tank