Is this it!


Cell A7 is the cell for entering the required Inches

Cell B7 is the cell for entering the required mm

Enter the following formula in cell C7 to get the resultant formula.

=(IF(A7>0,A7*25.4,(IF(B7>0,B7*0.03936996,""))))

Place labels in the cell above each. just copy into your spreadsheet and change the references for A7 and B7 to suit