**Similar** figures are the same shape, but not necessarily the same size.

**Scale factor** is the ratio of corresponding side lengths

(new image to old image) of similar figures.

For figures which differ by a scale factor, the scale factor is the ratio of corresponding side lengths (new image to old image).

$\begin{array}{rcl}\text{scale factor}& =& \frac{\mathrm{new\; length}}{\mathrm{old\; length}}\\ & =& \frac{\mathrm{12\; ft}}{\mathrm{4\; ft}}\\ & =& 3\end{array}$