It happens to every toilet sooner or later: The seat breaks. Whether it's the bolts that secure it to the bowl or a crack in the lid, a toilet seat has a limited lifespan and will probably need to be replaced several times over the lifetime of your toilet. Fortunately, changing a toilet seat is a speedy chore that can be accomplished in minutes, and even if it's not broken you can change the seat when redecorating your bathroom for a custom look. If you need to change your toilet seat, measuring is at a minimum and choices for a new one are vast.
Locate the bolts at the back of the seat that secure the seat to the toilet base.
Mark a spot midway between the two bolts.
Measure from the mark to the front of the toilet base. The measurement will be either 16.5 inches or 18.5 inches, which are the two sizes in which toilet seats are produced.
The two toilet sizes are called "round" (16.5 inches) and "elongated" (18.5 inches).