Six and a half what?
You need to define your desired units of measurement for the six and a half.
For example:
6.5 centimeters = 65 millimeters
6.5 inches = 165.1 millimeters
6.5cm = 65mm (there are 10 millimeters per centimeter).
65 mm
6.5 inches is 165.1mm
5 millimeters are equal to a half of a centimeter.
Take any number of millimeters, divide it by 25.4 to get the same length in inches. You can do this.
152.4 millimeters
1 centimeter is 10 millimeters. 4 and a half centimeters are 45 millimeters.
500 millimeters
5.6mm is 'five and six-tenths millimeters'. OR five point six millimeters
38 millimeters
1397 millimeters
There are 25.4 millimeters in an inch. That works out to be 63.5 millimeters.