In cell B1 enter =A1*1.2 and copy down the length of the list. You will then
have the values you want in column B, you could then copy & paste special
'values and source formatting' column B to column A then delete column B, but
if you add more values or have to change some values then you will have to
re-perform the operation on these altered values.

" wrote:

I am sure this will be fairly straightforward but not immediately
obvious to me. I think it should be!

How would I increase the values of a whole column of cells by a
percentage?

e.g. I want to change the values in cells A1 .. A200 by increasing them
all by 20%?

Thanks