How can I calculate RMD?

Most RMD's were suspended for 2020.  If you are asking about 2021, see this:

IRA Required Minimum Distribution Worksheet (irs.gov)

Thanks for the reply.  Am doing taxes for my Dad who passed away in May...  I called administrator and was told IRS pub 721 would be where to find out how to calculate... should I assume zero for 2020?

Yes 0.  When it asks if this is a RMD say NO.  There is no RMD for 2020.  Or was this from a pension?

I looked up Pub 721.  That seems to be for retirees.  If this was a pension then say Yes it was all the RMD.  Anything a pension pays you is considered to be the RMD.

@1940ceramics 

The rules for RMD calculation are found in IRS Pub 590B.