Lift the toilet seat and lid
Measure the distance from the center of the left mounting hole to the center of the right mounting hole
Measure the bowl length from the center of the mounting holes to the front edge of the bowl
Measure the bowl width at the widest point
Check the shape of the bowl: round or elongated
Measure the distance between the mounting holes and the back of the tank if clearance is tight
Note whether the seat uses top-mount or bottom-mount hardware
Compare your measurements to the seat size specifications before buying