...Right, the strange 'blank post' bug on this forum cropped up again, so I'll try again:
So you have the Feet and Inches stored?
If so, here's how you can convert them to millimeters.
I'll pretend your two variables so far are called vHeightFeet and
vHeightInches, so change my code to suit what you've actually called them.

Dim vHeightmm As Double

'We prepared a new variable in which to hold the converted length in mm


vHeightmm = (vHeightFeet + (vHeightInches/12)) * 304.8

'We just converted Feet+Inches into mm and stored it in the vHeightmm variable

EXPLANATION:
Kadghar said that
1ft =
304.8mm
So we want to know how many feet and inches there are, in feet only.
Since there are
12 inches in a
foot, if we divide the
inches by 12, we get that equivalent length in
feet (i.e. a decimal value less than 1).
This, plus the number of feet, equals the number of feet 'in decimal'.
Example:
1 foot 6 inches
6 inches divided by 12 =
0.5 feet
1 foot +
0.5 feet = 1.5 feet =
1 foot 6 inches 'in decimal'
Then we multiply this final value  1.5  by 304.8 to convert it to milimeters, and store it in variable vHeightmm.
This variable is declared as a 'Double' data type, meaning a number which can get very big, and have very accurate decimal points.