An easy cheat is:
Make the decimal seperator a single quote (feet). Then user type exactly as you speak.. "I'm 6 foot 3" and you type it 6'3
Then what ever you do with the value which is 6.3 (six, with a decimal 3) is you choice. (Split the value on the ".", and then you know its 6'3" and convert and store it in an STANDARD INTERNATIONAL format. The conversion can be done easy in your SQL stored procedure.
I have a numeric textbox with a dropdown next to it where the person choose m, cm, mm or " ' and then that goes to the stored procedure and I handle it there... I store everyone's height in my choice (cm in my case). My select stored procedure, look at the users user preference (lenght measurement) and then know how to transform the value.
When the dropdownbox builds, I set group separator, decimal separator, max. min, etc, based on the users' choice of lenght measurement.
Similar I do for weight, volume, temprature.