Tool

Seat Match: see the fit before you order

Round and elongated seats differ by about two inches. Everyone says "measure first" — nobody shows you what the mismatch actually looks like. One measurement, and this draws your bowl.

Lid up. Measure from the middle of the two seat-mounting bolts to the front edge of the rim.

Your bowl reads as: round

2. The seat you're considering

Mismatch — about 2.0" of overhang. The front lip hangs past the rim with nothing under it, and that's the exact spot that takes your weight when you stand up. On a bidet seat it also pulls the bumpers and nozzle off their designed positions. Pick a round seat instead.
2.0" past the rimbowl: 16.5" (round)seat: 18.5" (elongated)
Top-down: seat (dashed) over your bowl
no supportside profile
Profile: the front lip is where your weight goes when standing up

Diagrams are schematic — proportions match your measurement, decorative detail doesn't. Bolt spacing is standardized at 5.5" on US toilets; a few seats accept 5–7". Electric bidet seats also need ≥1.5" between the bolt holes and the tank — check your model's fit guide before ordering.

Why the mismatch matters more on a bidet seat

A plain plastic seat that overhangs is annoying. An electric bidet seat that overhangs is a different story: these seats weigh 10–14 pounds, carry their bumpers and mounting geometry where the matching bowl's rim should be, and aim their wash nozzle assuming the bowl length the engineers designed for. Put an elongated seat on a round bowl and the front lip — the exact spot that takes your full weight when you stand up — floats over air. Put a round seat on an elongated bowl and you sit with a crescent of bare porcelain in front of you, and the wash geometry sits further back than intended.

The other two measurements that matter

Bowl shape settled? The next fork is attachment vs full seat, and if you're going TOTO, the Washlet picker sorts their lineup in three questions.