The penalty for failing to file on time is 5% per month (or part of month) then 0.5% per month after. That would not apply to you if you have an extension.
The penalty for late payment is 0.5% per month or part of month. So if you pay 1 day late or 29 days late, 0.5%. If you pay between 31 and 60 days late, 1%. And so on.
Then, the entire balance (interest plus penalties) is assessed interest which is a variable rate and I think is currently 8% APR.
So on a debt of $20,000 for 6 months, penalty and interest should be around $1400, using this calculator.
https://www.calculator.net/interest-calculator.html?cstartingprinciple=20%2C000&cannualaddition=&cmo...
