You will have to change the input to accept plain text.
Then, when done, convert the expressions to decimals.
To convert fractions (inches, I guess) you can use my functions found at
GitHub (too extensive to post here):
VBA.Round
Browse for the paragraph:
Converting between meters and inches
The functions provided will handle very large and very small values for inches:
 from ±7922816299999618530273437599

to 1/2097152 or the decimal value 0.000000476837158203125
The format of the imperial output covers a very wide range:
 Feet and inches, or inches only
 No fraction for a numerator of zero
 No fraction at all
 Dash or no dash between feet and inches
 Only feet if total of inches is 12 or more
 Zero feet if total of inches is smaller than 12
 No inches if feet are displayed and inches are zero
 No units
 Units spelled out as ft, ft., or foot/feet and in, in., or inch/inches
It is the module
Imperial.bas to study.