How do you calculate interest on an amended return?

[BobKamman]

I give the clients an estimate of how much it will be and tell them to wait for a bill.  IRS doesn't charge interest on interest, so they might as well know the exact amount.  

[George4Tacks]

I don't. Like Bob says, give them a guess but let IRS send a bill. 

Here is an estimator https://www.irscalculators.com/interest-calculator

[IRonMaN]

I don't even go that far.  I just tell them that they might get a bill for some interest down the road.  Nobody has ever came back and complained to me about not giving them an estimate.

[BobKamman]

But I have had clients call me to say IRS billed them for $100 when my estimate was $40.  And it turned out I was right.  Of course, this results in work that builds goodwill but produces little revenue.  

[IRonMaN]

Did I forget to mention that I do very few amended returns so they don't tend to be revenue or goodwill producers.

[BobKamman]

The last amended return I did was for a client who received a corrected statement for unemployment compensation.  It was for about $2,000 and they were in a 24% tax bracket, so the amount owed was about $480.  (No credits or other complications.)  They paid the $480 and just received a bill from IRS for another $450.  No explanation of the change.  I think IRS is catching up with the backlog with a new computer program that uses Artificial Stupidity.