#### Answer

The diameter of the pine tree is 45.72 centimeters.

#### Work Step by Step

We need to convert 18 inches to centimeters.
To do this, we need to multiply 18 by the ratio that converts inches to centimeters.
The ratio that converts inches to centimeters is $\frac{2.54\ centimeters}{1\ inch}$.
So, 18 inches $\times$ $\frac{2.54\ centimeters}{1\ inch}$ =
18 $\times$ 2.54 centimeters =
45.72 centimeters.
The diameter of the pine tree measures 45.72 centimeters.