How to calculate federal penalty and interest ?

There is interest accruing from the due date of the tax return in 2021, plus late filing and late payment penalties:

The penalty for late filing is 5% per month for up to 5 months. If your return is over 60 days late, there's also a minimum penalty for late filing; it's the lesser of $485 (for tax returns required to be filed in 2024) or 100 percent of the tax owed .

There is also a failure-to-pay penalty of half of one percent per month that the tax remains unpaid after the due date. If both a Failure to File and a Failure to Pay penalty are applied in the same month, the Failure to File penalty is reduced by the amount of the Failure to Pay penalty for that month, for a combined penalty of 5% for each month or part of a month that your return was late.

Generally, interest accrues on any unpaid tax from the due date of the return until the date of payment in full. The interest rate is determined quarterly and is the federal short-term rate plus 3 percent. Interest compounds daily. Visit Newsroom Search for the current quarterly interest rate on underpayments.

See IRS Topic No. 653 for more information.