Seat sizing: elongated vs. round is a measurement, not a guess

The two measurements

Measurement A — length (the one that matters)

Find the two bolts that hold your current seat, at the back of the bowl. Measure from the midpoint between them — the center of the bolt line — straight forward to the front outer edge of the porcelain. 18 to 19 inches: elongated. 16 to 17 inches: round. Manufacturers phrase it identically (Bio Bidet's guidance, for instance: measure from the center of the bolt holes to the edge of the bowl). If you land in the gap around 17.5″, you have a “compact elongated” or an unusually shaped bowl — buy the elongated size and confirm the retailer's fit checker agrees, because a too-long seat overhangs and a too-short one perches.

Measurement B — bolt spread

Center to center between the two seat bolts: 5.5 inches on the overwhelming majority of North American toilets. Bidet seats and attachment plates are slotted to forgive small deviations. If yours measures something exotic, that is the first honest signal you own a non-standard fixture — read the shape section below before ordering anything.

The clearances nobody prints

• Bolt line to tank: attachments and seat brackets need roughly 1.5 inches or more of flat deck between the bolt holes and the tank wall. Less than that, and the mounting plate cannot sit flat — the single most common “it doesn't fit my toilet” report in the category.
• Side clearance: seats with side-arm controls (rather than remotes) need a hand's width beside the bowl. In tight water closets, remote-controlled models win by geometry.
• Behind the bowl: electric seats route cord and hose from the rear; a few inches of access behind the toilet saves real frustration during install.

The special cases

One-piece toilets

Possible, with care. The integrated tank removes access to the underside of the bolt holes, so seats need top-mount hardware (expanding anchors fitted from above) — many manufacturers sell the kit or include it; verify before purchase, not during install.

Skirted toilets

The smooth-sided designs that hide the trapway also hide the bolt access. Same top-mount answer, plus longer install times. Budget an extra half hour and zero patience for improvisation.

French curve toilets

A small family of design-forward bowls curves the rear deck where the seat mounts. Some seats genuinely cannot fit these; manufacturers publish exclusion lists, and for once the honest move is to believe them.

Wall-hung & compact toilets

Measure A still governs, but concealed carriers and non-standard bolt patterns make this professional-consultation territory. Bring the measurements to the manufacturer before bringing a seat home.

Attachments vs. seats on fit

Attachments forgive more: they occupy only the bolt line and the bowl's underside rim, and work on either bowl length — though on round bowls the nozzle geometry sits slightly differently, and some brands sell round-specific variants. Seats are strict: the shell is molded to the bowl, and elongated-on-round looks and feels wrong in a way you will notice every day. When in doubt between the tape measure and the product name — “fits most elongated toilets” — believe the tape measure. See seat vs. attachment for the full decision, and take these measurements before the cost ledger, so the project you price is the project that fits.