If 2 polygons are similar, then the side lengths in one polygon are multiplied by the same scale factor to give the corresponding side lengths in the other polygon.

For these triangles the scale factor is 2:

Here is a table that shows relationships between the lengths of the short and medium sides of the 2 triangles.

The lengths of the medium side and the short side are in a ratio of . This means that the medium side in each triangle is as long as the short side. This is true for all similar polygons: the ratio between 2 sides in one polygon is the same as the ratio of the corresponding sides in a similar polygon.

We can use these facts to calculate missing lengths in similar polygons. For example, triangles and   are similar. 

Since side is twice as long as side , side  must be twice as long as side . Since is 1.2 units long and , the length of side is 2.4 units.