John Walkenbach describes a method he

http://j-walk.com/ss/excel/usertips/tip045.htm

Actually, his approach is the opposite of what you want, in that he
colours a cell that contains a formula, but perhaps you can modify the
approach.

An alternative is to lock the cells with formulae in and protect the
sheet (with or without a password) to prevent accidental deletion/
overwriting of the cells containing formulae.

Hope this helps.

Pete

On Apr 14, 2:59*pm, Neil Pearce wrote:
Dear all,

Cell A1 contains a formula. *

How would I go about conditonally formatting A1 such that the cell will be
filled if the formula is overwritten by a hard number?

Thanks in advance,

Neil