How to Measure a Toilet Seat
When you're shopping for a toilet seat, the decision about whether it's round or elongated may already be made for you. If you're not sure what type of toilet seat you need by looking at your toilet, just measure its length.
Grab a tape measure or a long wooden ruler (at least 20 inches).
Locate the holes where the seat bolts to the toilet.
Start with these bolts and measure straight down from the center of the drum.
This is how you measure the length of the toilet. Below are the dimensions to help you decide if you need a slim or round toilet seat.
Round Seat = 16.5 inches
Extended Seat = 18.5 inches