Standard toilet seats come in two shapes and sizes. Round seats measure 16.5 inches, and oblong or elongated seats measure 18.5 inches. While the width of toilet bowls can fluctuate slightly, these two sizes are standard and fit most bowls.
Toilet bowl sizes include three measurements. The first measurement is between the post holes at the back of the bowl where the seat and lid attach. The second measurement is the width, which is taken at the widest point of the bowl. The third measurement is taken from the front of the bowl to the back, ending between the two post holes.
A replacement toilet seat and lid can be purchased directly from the toilet manufacturer or at a home improvement center that offers a variety of seats in the two standard sizes.