To determine the scale factor for dilating a square from a side length of 6 units to a side length of 18 units, you can use the formula for scale factor:
\[ \text{Scale Factor} = \frac{\text{New Side Length}}{\text{Original Side Length}} \]
In this case:
\[ \text{Scale Factor} = \frac{18 \text{ units}}{6 \text{ units}} = 3 \]
Therefore, the scale factor you should use is 3.