Interest

irs.gov -

Interest

Interest is charged on taxes not paid by the due date, even if an extension of time to file is granted.

Interest is also charged on penalties imposed for failure to file, negligence, fraud, substantial valuation misstatements, substantial understatements of tax, and reportable transaction understatements. Interest is charged on the penalty from the due date of the return (including extensions). The interest charge is figured at a rate determined under section 6621.