You need to specify what you're measuring to determine how many inches are in three and a half.

For example:

* Three and a half feet: There are 12 inches in a foot, so three and a half feet is 3.5 * 12 = 42 inches.

* Three and a half yards: There are 36 inches in a yard, so three and a half yards is 3.5 * 36 = 126 inches.

Please provide the unit of measurement so I can help you calculate the number of inches.