Failure-to-pay penalty

If you file on time but don't pay everything you owe by the April deadline, the IRS charges 0.5% of the unpaid tax for each month or part of a month it stays unpaid, up to 25%. Interest is charged on top of that. It's separate from the underpayment penalty for missing quarterly estimated payments during the year. Setting up an IRS payment plan cuts the monthly rate to 0.25%.