I have searched the forums and couldn't find the answer to my question, so
here goes.

I have a spreadsheet that calculates Bittering Units (beer brewing) by
specifying AA% of the hops (not variable), and the weight and it will
determine the Bittering Units.

Is there any way I can allow the user to either specify the IBU, or the
mass, and for Excel to automatically calculate the other one without deleting
the formula from the cell which has been specified?

i.e. in the mass cell: If IBU is specified, calculate mass, however if mass
is specified, calculate IBU

Cheers.