There are certain direct formulas to convert inches to millimeters in excel
You'll have to use it directly in Excel
for converting inches to millimeter - =CONVERT(A2,"in","mm")
for converting inches to centimeter - =CONVERT(A2,"in","cm")
And in excel sheet u can find an option KUTOOL>CONTENT>UNIT conversion option
thereafter selecting unit conversion, select the unit category from Units drop-down list, and specify the units you want to convert between two lists if you want to display the result in a comment, check to Add results as a comment
select ok and now you can see the converted values