Lift the toilet seat and lid
Measure the distance between the two hinge bolt centers
Measure the bowl length from the center of the hinge bolts to the front edge of the bowl
Measure the bowl width at the widest point
Check whether the bowl is round or elongated
Compare your measurements to the seat size specification
Measure the hinge bolt spacing if replacing only the seat hardware
Verify the seat shape matches the bowl shape
Confirm the seat cover length matches the bowl length
Recheck all measurements before buying a replacement seat