May be this is what you are looking for:

=IF(B10,B1-A1,"")

Hope this help, let us know!

"drk442" wrote:

I have a a spreadsheet where I substract column B from A and show results in
C. Then C is sent to column A one row down. When I drag the formulas down
so that they repeat I get an entry in column A & C. Is there a way to not
have the formula value show up until an entry is entered in Colum B?