Hi - I am working on a mortgage spreadsheet.

I need the spreadsheet to work out interest only calculations, principle &
interest calculations, and remain empty if no loan amount or interest rate is
entered.

I currently use this formula that preforms step 1 and step 3 perfectly

O21: =IF(OR(L21="",N21=""),"",L21*N21)

Cell definition is L21 = loan amount, N21 = Interest rate

The cell that will have either IO (Interest Only) or PI (principle and
Interest) is I21.

The principle and interest formula I want to use (I know it's crude, but
does the job required) is =PMT(0.09/12,30*12,-L21)

To sum up:

When IO is placed in I21 I require the 'interest only' formula to work, when
PI is place in I21 I require the 'Principle and Interest' formula to work,
and if nothing is placed in L21 or N21 when nothing is placed in O21.

Any help would be appreciated.

Thank you in advance.

Cheers Scott