Similar 3D shapes have the same shape but different sizes. Corresponding lengths are measurements in the same position, such as radius with radius or height with height.

The linear scale factor tells you how one length changes into its matching length. From shape A to shape B, we define the scale factor $k$ using the following formula:

$$k=\frac{length\; on\; B}{matching\; length\; on\; A}$$

Always label the direction before calculating, such as small to large. If a radius changes from 6 cm to 15 cm, then the linear scale factor is $k=\frac{15}{6}=\frac{5}{2}$.