Measure your mirror glass. Measure horizontally from left mirror edge to right mirror edge. 

Measure your mirror glass. Measure vertically from top mirror edge to bottom mirror edge. 

Measure your mirror's longest diagonal measurement. This mirror's longest measurement happens to be from the inside upper to outside lower. 

Measure your mirror's encasement/ rim. Measure horizontally from left encasement edge to right encasement edge. 

Measure your mirror's encasement/rim. Measure vertically from top encasement edge to bottom encasement edge. 

Measure your mirror's longest diagonal measurement. Be sure to include your mirror's encasement/rim. This mirror's longest measurement happens to be from the inside upper to outside lower.