Converting Feet & Inches to Decimal Inches

I have a spreadsheet that has feet & inches in Column D, with the data starting from cell D2.

Data is written like so - 6'-2-1/2"

I have a formula in Column E that I found elsewhere, but it doesn't work for all of the cells. It also doesn't convert it down to the exact decimal.

Is there a way to convert the feet & inches to an exact decimal so I can determine the total amount of inches per cell?

I am currently using Excel 2010 & it is an English version.