=IF(A1="Y",5,IF(A1="N",2,""))

If you have a lot of letters you need to convert, then
I'd suggest creating a table and using VLOOKUP. See here
for a tutorial:

http://www.contextures.com/xlFunctions02.html

HTH
Jason
Atlanta, GA

-----Original Message-----
How do I enter a formula in a cell so that letters= a
number i.e Y=5, N=2.
.