Toilet seats are designed in two standard sizes for the most part, corresponding to standard and elongated toilets. Therefore, you don’t need to measure seat sizes for most toilets. However, if you do wish to measure because you’re not sure which type of toilet you have, measure from the screws at the base of the seat to the front of the bowl – standard seats are 16.5 inches long and elongated seats are 18.5 inches long.