: will the toto warmlet fit my american standard? the top of the opening measures 12" front to back- 10" side to side.

Jeff,
The Warmlet seat will fit on "Elongated" toilet bowls. Your measurements indicate the standard "Round Front: bowl.
You may want to consider upgrading to an elongated bowl if you like the features of a heated seat.
Regards,
Terry Love

• Toto Warmlet seat