The late payment penalty is one-half of one percent of the tax (0.5%) owed for each month, or part of a month, that the tax remains unpaid after the due date, not exceeding 25 percent.
You will not have to pay the penalty if you can show reasonable cause for the failure.
The one-half of one percent rate increases to one percent if the tax remains unpaid after several bills have been sent to you and the IRS issues a notice of intent to levy.
Currently, if you filed a timely return and are paying your tax via an installment agreement, the penalty is one-quarter of one percent for each month, or part of a month, that the installment agreement is in effect.