Measure a toilet seat by checking the bowl shape length and bolt spread. Round bowls measure approximately 16.5 inches from front to mounting bolts while elongated bowls measure about 18.5 inches. The bolt spread is the distance between the two mounting holes which is standard at 5.5 inches for most toilets. Measure the bowl width at its widest point. Note whether your bowl is round or elongated before purchasing a replacement seat for proper fit.