To measure a toilet seat’s length you must first lift up the existing toilet seat (if there is one).
Place your tape measure directly in the middle of the seat post holes and stretch it out until it reaches the end of the bowl – this is your toilet seat length.
The width of a toilet seat is determined by measuring the width of the toilet bowl.
Again, just like the length, to correctly measure the width you must measure from the toilet bowl’s outside edge on one side to the other outside edge.